Chapter 16—Formal Taxonomy

Generic Description

Arceuthobium M. Bieb.
Dwarf Mistletoe

Razoumofskya Hoffman, Hortus Mosquensis, unpaged, 1808. Arceuthobium M. Bieb. Flora Taurico-Caucasica 3(IV) Supplement, p. 629, 1819. Nom. Cons. 2091. [Orthographic variants listed by Farr and others (1979) are: Arcenthobium Presl (1825), Arceuthobion Sprengel (1826), Arceutholobium Furnrohr (1850), Arceutobium Grenier & Gordon (1851), Razumowskia Presl (1825) and Razoumoeskya Jussieu (1826).]

Herbs or shrubs from 0.5 cm to approximately 70 cm high; parasitic on Pinaceae and Cupressaceae; plants glabrous, variously colored from greenish yellow to orange, reddish, or black; dioecious; stems with variant (anomalous) patterns of secondary growth; leaves reduced to minute, opposed, connate scales; internodes angled (at least when young); flowers generally decussate or rarely whorled on young shoots, 2–4 mm across; staminate flowers with a central nectary, perianth segments usually 3–4 (rarely 2 and up to 7) bearing a sessile, uniloculate, circular anther on each perianth segment; pollen spherical with 6 alternating spiny and smooth sections; pistillate flower manifestly epigynous with 1 style, perianth segments persistent, adnate to ovary, 2-merous; ovary 1-chambered; fruit an ovoid berry, 1-seeded, mucilaginous and bicolored (distal and basal portions of different shades), explosive at maturity; seeds without true integuments, usually 3–5 mm long, ovate-lanceolate, containing 1 (rarely 2) distal, cylindrical embryo, with copious endosperm. Basic chromosome number n = 14.

A genus of 42 species, mostly in the United States and Mexico; 8 species in the Old World.

Type species: Arceuthobium oxycedri (DC.) M. Bieb.

Artificial Key to Arceuthobium

An artificial key to Arceuthobium Species is available as a Word97 document.

New World Taxa

Arceuthobium abietinum
Fir Dwarf Mistletoe

1. A. abietinum Engelmann ex Munz, Manual Southern California Botany: 114, 1935. TYPE COLLECTION : CALIFORNIA: Sierra County: Sierra Valley, on Abies concolor, Lemmon in 1875 (Lectotype MO! Isotype GH). A. abietinum Engelm. in Gray, Proceedings American Academy of Arts and Sciences 8: 401, 1872, nomen nudum. A. douglasii Engelm. var. abietinum Engelm., in Watson, Botany of California 2: 107, 1880. A. occidentale Engelm. var. abietinum Engelm. in Watson, Botany of California 2: 107, 1880. Razoumofskya douglasii (Engelm.) Kuntze var. abietina (Engelm.) Howell, Flora Northwest America 1: 609, 1902. Razoumofskya douglasii (Engelm.) Kuntze var. abietina (Engelm.) Piper, Contributions U.S. National Herbarium 11: 223, 1906. Razoumofskya abietina (Engelm.) Tubeuf forma parvula Tubeuf. Naturwissenschaftliche Zeitschrift für Forst- und Landwirtschaft 17: 219, 1919, nomen nudum. Razoumofskya abietina (Engelm.) Tubeuf forma magna Tubeuf, loc. cit: 220, nomen nudum. Razoumofskya abietina (Engelm.) Abrams, Illustrated Flora Pacific Coast States 1: 530, 1923. A. campylopodum Engelm. forma abietinum (Engelm.) Gill, Transactions Connecticut Academy Arts and Sciences 32: 195, 1935. A. abietinum (Engelm.) Hawksworth & Wiens, Brittonia 22: 68, 1970.

Description: Mean shoot height ca. 8 (max. 22) cm. Shoots yellow green to yellow, flabellately branched (fig. 16.1). Basal diameter of dominant shoots 1.5-6.0 (mean 2) mm. Third internode 4–23 (mean 14) mm long, 1.5–4.0 mm (mean 2) mm wide; length/width ratio ca. 7:1 to 9:1. Staminate flowers ca. 2.5 mm across; perianth 3-merous, sometimes 4-merous, apex acute; same color as shoots; segments ca. 1.2 mm long, 1.0 mm wide. Mean anther diameter 0.4 mm, centered 0.7 mm from tip of segment. Pollen polar diameter 18–23 (mean 20) µm; equatorial diameter 19–25 (mean 22) µm; polar/equatorial diameter ratio 1:1.07; spine height ca. 1.5-2.0 x wall thickness (7 collections). Mature fruit ca. 4 x 2 mm; proximal portion ca. 2.5 mm long. Seeds 2.8 x 1.2 mm. n = 14.

Phenology: Meiosis in July. Anthesis usually August–September. Fruits mature in September or October of the year following pollination; maturation period averages 13–14 months.

Hosts: Abies spp.

Discussion: Parmeter and Scharpf (1963) first reported that, on the basis of field evidence and inoculation studies, the dwarf mistletoe on Abies concolor will not infect associated A. magnifica. Conversely, the parasite of A. magnifica will not parasitize associated A. concolor. Our observations in several mixed fir stands in the Sierra Nevada confirm Parmeter and Scharpf’s conclusions. However, we have been unable to find morphological differences between the two mistletoes, and they are also similar phenologically. In the same locality, however, seed dispersal begins about 1 week earlier for the plants infecting A. magnifica (Scharpf and Parmeter 1967). Greenham and Leonard (1965) studied the amino acids of the two dwarf mistletoes and their respective hosts, and they concluded that amino acid composition probably could not explain the restriction of the two dwarf mistletoes to their respective hosts.

Munz and Keck (1959) report that there are up to 7 perianth segments in this species, but this is in error.

Because the host affinities of these two dwarf mistletoes are distinct, and inasmuch as they are of considerable importance in forest management, we have designated them as formae speciales in accordance with recommendation 4B of the International Code of Botanical Nomenclature.

 

Key to the formae speciales

1. Parasitic principally on Abies concolor or A. grandis; known in two areas in Chihuahua on A. durangensis. The primary distribution is from southern Washington southward through the Cascade and southern Coast Ranges in Oregon, and the North Coast and Cascade Ranges, Sierra Nevada to southern California. Isolated populations occur in southern Utah, northern and southern Arizona, and Chihuahua, Mexico

1a. A. abietinum f. sp. concoloris.

1. Parasitic principally on Abies magnifica from southwestern Oregon (Josephine County) to the southern Sierra Nevada, California

1b. A. abietinum f. sp. magnificae.

 

Arceuthobium abietinum f. sp. concoloris
White Fir Dwarf Mistletoe

1a. A. abietinum Engelmann ex Munz f. sp. concoloris Hawksworth & Wiens, Brittonia 22: 267, 1970.

Description: Mean shoot height ca. 10 (max. 22) cm. Basal diameter of dominant shoots 1.5–6.0 (mean 2) mm. Third internode 4–23 (mean 13.3 ± 3.8) mm long, 1.5–4.0 (mean 2.0) mm wide (15 collections), length/width ratio 6.7:1. Pollen polar diameter 18–23 (mean 20) µm; equatorial diameter 19–25 (mean 22) µm; polar/equatorial diameter ratio 1:1.07; spine height (ca. 1.5 µm) slightly greater than the wall thickness (ca. 1.1 µm) (5 collections). n = 14.

Phenology: Peak anthesis usually from mid-July to mid-August, with extremes from early July to late September (fig. 16.2). Fruits usually mature in September or October, with extremes from late August to early November; maturation period averages 13–14 months. Seeds germinate from February through June (Scharpf and Parmeter 1967).

Hosts: The principal hosts of this dwarf mistletoe are Abies concolor, A. grandis, and A. durangensis. Abies concolor var. concolor (Nevada, Utah, and Arizona) and var. lowiana (California) are about equally susceptible, although the dwarf mistletoe is much more widely distributed on the latter. The rare Brewer spruce, Picea breweriana, is associated with infected Abies concolor on Flat Top Mountain, west of Grants Pass, Oregon, and Arceuthobium abietinum produces heavy infections on Brewer spruce in this area (Hawksworth and others 1967). On the North Rim of Grand Canyon, Arizona, Abies lasiocarpa is occasionally parasitized where this tree grows in association with infected A. concolor. Abies amabilis is a rare host of this dwarf mistletoe; the only known instance of this host–parasite combination is in the vicinity of Crater Lake, Oregon. Pinus lambertiana, P. monticola, P. ayacahuite var. brachyptera, and P. contorta var. murrayana are rare hosts for Arceuthobium abietinum f. sp. concoloris.

Distribution: United States (Washington, Oregon, California, Nevada, Utah, and Arizona) (fig. 16.3), and Mexico (Chihuahua) (fig. 16.4). Arceuthobium abietinum f. sp. concoloris is widely distributed from southern Washington (Skamania, Wenatchee, and Klickitat Counties) south through the Cascade Range and Sierra Nevada to the San Bernardino Mountains of southern California. Reports of the dwarf mistletoe on Big Pine Mountain and Reyes Peak in Santa Barbara County have not been confirmed (Smith 1976). A single, probably relictual, population is known in the Willamette Valley of Oregon (at Helmick State Park, Polk County, on Abies grandis). It also occurs along the coast ranges from Mendocino County, California, to Curry County, Oregon. Three isolated populations are known in Nevada (Spring, Sheep, and Groom Mountains [= Bald Mountain of Beatley 1976]) and two in southwestern Utah (northwestern Kane County). The parasite has long been known in the Grand Canyon area of northern Arizona, and small populations have since been found some 400 km to the south in the Chiricahua Mountains (Cochise County) (Mathiasen 1976) and in the Santa Catalina Mountains (Pima County) (Mathiasen and Jones 1983). This dwarf mistletoe was recently found on A. durangensis in two localities in Chihuahua—near Tomochic in the central part of the state (Hawksworth and Wiens 1989) and at Cerro Mohinora in southern Chihuahua. The latter area is about 1,000 km south of the previously known southern limits of the taxon in southern Arizona.

This dwarf mistletoe occurs on Abies concolor throughout most of its range in California, Nevada, Arizona, and southern Oregon. Abies grandis is the principal host in central Oregon, southern Washington, and in the coastal ranges of Oregon and California. In the Siskiyou Mountains, Del Norte County, California, just south of the Oregon boundary, we found the parasite commonly infecting trees that exhibit characteristics of both Abies concolor and A. grandis in mixed stands, although A. grandis is not considered to be present in this area (Griffin and Critchfield 1972). This tree is an apparently undescribed taxon (R. S. Hunt, personal communication).

Several localities for Arceuthobium abietinum recorded by Gill (1935) in other parts of the western United States have since been found to be based on the occasional parasitism of Abies by other species of Arceuthobium:

Arceuthobium abietinum f. sp. concoloris occurs from near sea level along the coast of northern California and southern Oregon to over 2,650 m in the Spring (Charleston) Mountains of southern Nevada.

Discussion: In the Northwest, two other species of Arceuthobium occur on Abies: (1) Arceuthobium tsugense on Abies amabilis, A. lasiocarpa, and A. grandis, and (2) Arceuthobium laricis on Abies lasiocarpa and A. grandis. However, insofar as we are aware, neither of these dwarf mistletoes is sympatric with Arceuthobium abietinum, although A. tsugense occurs within about 0.5 km of A. abietinum on the east side of McKenzie Pass in central Oregon. Arceuthobium laricis and A. tsugense rarely infect pure stands of Abies, but they may parasitize Abies secondarily in stands where the principal hosts of these dwarf mistletoes are parasitized (for example Larix for A. laricis and Tsuga for A. tsugense). Arceuthobium laricis is readily distinguished from A. abietinum by its shorter, darker shoots (4 cm versus 10 cm) and shorter (in summer) staminate spikes (2–3 mm versus 5–7 mm). Arceuthobium tsugense differs from A. abietinum by its shorter (7 cm), green to purple shoots compared with the longer (10 cm), yellowish shoots of A. abietinum.

Dying branches (flagging) are one of the most conspicuous field symptoms of Abies concolor infected by dwarf mistletoe, particularly in California. Scharpf (1969c) has shown that flagging of dwarf mistletoe-infected fir branches is typically associated with the fungus Cytospora abietis.

 

Arceuthobium abietinum f. sp. magnificae
Red Fir Dwarf Mistletoe

1b. A. abietinum Engelmann ex Munz f. sp. magnificae Hawksworth & Wiens, Brittonia 22: 268, 1970.

Description: Mean shoot height ca. 6 (max. 12) cm. Basal diameter of dominant shoots 1.5–3.0 (mean 2) mm. Third internode 10–22 (mean 15.0 ± 3.1) mm long, 1.5–2.0 (mean 1.7) mm wide (5 collections), length/width ratio 8.8:1. Pollen polar diameter 19–23 (mean 21) µm; equatorial diameter 19–24 (mean 22) µm; polar/equatorial diameter ratio 1:1.07; spine height (ca. 2 µm) approximately twice the wall thickness (ca. 1 µm) (2 collections). n = 14.

Phenology: Peak anthesis usually from early August to mid-September, with extremes from mid-July to late September (fig. 16.5). Fruits mature from early September to late October; maturation period averages 13–14 months.

Hosts: Abies magnifica

Distribution: United States (Oregon and California) (fig. 16.6). Arceuthobium abietinum f. sp. magnificae is distributed from Josephine County in southwestern Oregon south to Kern County in the southern Sierra Nevada in California. The reports of it on Abies procera in southern Oregon are probably the result of confusion with Arceuthobium tsugense, which occasionally parasitizes Abies procera in this region. We have not confirmed the presence of any populations of this dwarf mistletoe in Oregon outside of Josephine County, and we are unable to establish the basis for the reports of it in the vicinity of Crater Lake (Hawksworth and Wiens 1972, Weir 1917). It may occur on Abies magnifica in Nevada in the vicinity of Lake Tahoe (Kartesz 1988), and Guyon and Munson (1991) record it within 3 km of the Nevada border in Sierra County, California. Elevational range is 1,500–2,400 m.

Discussion: Arceuthobium abietinum f. sp. magnificae is a very common and serious disease agent of the Abies magnifica forests of the Sierra Nevada. In a series of sample plots in California, this dwarf mistletoe was present on 46% of 103 Abies magnifica plots and on 36% of the trees (California Forest Pest Control Action Council 1968). Scharpf (1969b) discusses infection in young Abies magnifica stands and gives recommendations for control of the dwarf mistletoe.

Just as in Arceuthobium abietinum f. sp. concoloris, flagging is one of the most conspicuous field symptoms of Abies magnifica infected by dwarf mistletoe (fig. 16.7). Also as in f. sp. concoloris, such flagging is often associated with the fungus Cytospora abietis (Scharpf 1969c).

Arceuthobium abietis-religiosae
Mexican Fir Dwarf Mistletoe

2. A. abietis-religiosae Heil, Zentralblatt für Bakteriologie Abteilung 2: 28, 1923. TYPE COLLECTION: MEXICO: Mexico: Between Amecameca and Paso de Cortez, near km-78, below Popocatepetl and Ixtaccihuatl National Park, on Abies religiosa, Hawksworth & Wiens 3339 in 1963 (NEOTYPE COLO! Isotypes: F, FPF, INIF, MEXU, MO, US). (See Hawksworth and Wiens, Brittonia 17: 231, 1965.)

Description: Mean shoot height ca. 10 (max. 16) cm. Shoots olive green, older shoots typically with black variegations, occasionally with verticillate branching (fig. 16.8). Basal diameter of dominant shoots 2–10 (mean 4) mm. Third internode 8–24 (mean 15.4 ± 5.3) mm long, 1–4 (mean 2.8) mm wide (3 collections), length/width ratio 5.5:1. Staminate buds 2–4 per node. Staminate flowers 2 mm long,
2–4 mm across; perianth mostly 3-merous, sometimes 4-merous; apex obtuse-acute; same color as shoots on outer surface, reddish on inner surface distal to anther; segments ca. 1.2 mm long, 0.9 mm wide. Mean anther diameter 0.4 mm, centered 0.8 mm from tip of segment. Pollen polar diameter 17–23 (mean 20) µm; equatorial diameter 22–26 (mean 24) µm; polar/equatorial diameter ratio 1:1.22; spine height approximately equal to wall thickness (1.5 µm) (2 collections). Pistillate flowers ca. 1.0 mm long, 0.5 mm across. Mature fruit 3.5 x 2 mm; proximal portion ca. 2.5 mm long. Seeds 2.2 x 1.0 mm. n = 14.

Phenology: Meiosis in September. Anthesis poorly known but apparently exhibiting flowering periods in March–April and September–October. We have found that Arceuthobium nigrum also has two flowering periods, and A. hawksworthii may have three. Likewise, A. juniperi-procerae has multiple flowering periods in Kenya, and A. aureum subsp. aureum appears to flower continuously during the dry season in Guatemala. Heil (1923) stated that the fruits of A. abietis-religiosae mature in September. The fruits we observed in mid-September 1969, near Mexico City, however, were at least a month from maturity, and this discrepancy may be due to the two flowering periods. Fruits probably mature in October or November, but we have not determined when fruits pollinated from the two flowering periods are mature.

Hosts: Known only on Abies spp., and A. religiosa (including var. emarginata) is by far the most common host. In the Sierra Madre Oriental this dwarf mistletoe also frequently parasitizes A. vejari. It does not parasitize Pinus or Pseudotsuga, even where these trees are closely associated with infected firs. The parasite probably occurs on other Mexican and Guatemalan species of Abies. For example, an illustration of A. guatemalensis at Copainala, Chiapas, Mexico (Martínez 1963, p. 124) seems to exhibit witches’ brooms that may be caused by this mistletoe.

Distribution: Mexico (Distrito Federal, Hidalgo, Jalisco, Mexico, Michoacán, Nuevo León, Puebla, Tamaulipas and Tlaxcala) (fig. 16.4). This dwarf mistletoe is common in the Abies religiosa forests of central Mexico (Madgiral 1967) and also occurs in the fir forests of the Sierra Madre Oriental. Elevational range is 2,500–3,350 m.

Discussion: The nomenclature of this distinctive Mexican dwarf mistletoe is discussed by Hawksworth and Wiens (1965). It is characterized by its large shoots, occasional verticillate branching (a feature shared in the New World only with Arceuthobium americanum), and exclusive parasitism of Abies. With the exception of the rare occurrence of Arceuthobium abietinum in Chihuahua, this is the only dwarf mistletoe that typically parasitizes Abies in Mexico.

Additional items of interest in Arceuthobium abietis-religiosae include the occurrence of systemic witches’ brooms and verticillate staminate floral buds. Systemic infections are not common in this species, but on the few that we observed, the dwarf mistletoe shoots had formed only at the girdles of the host branches (fig. 2.13). The shoots on the systemic brooms are only about half as high as those on non-systemic infections. The staminate flowers are occasionally verticillate. The only other New World species with verticillate staminate flowers is A. verticilliflorum, in which the character is consistent and conspicuously developed.

 

Arceuthobium americanum
Lodgepole Pine Dwarf Mistletoe

3. A. americanum Nuttall ex Engelmann in Gray, Boston Journal Natural History 6: 214, 1850. TYPE COLLECTION: OREGON: Blue Mountains, on Pinus contorta, Douglas in 1826 (Lectotype MO!). Razoumofskya americana (Nutt. ex Engelm.) Kuntze, Revision of Genera of Plants 2: 587, 1891.

Description: Shoot height 5–9 (max. 30) cm. Shoots yellowish to olive green, with verticillate branching (fig. 16.9 and 16.10). Basal diameter of dominant shoots 1–3 (mean 1.5) mm. Third internode 6–23 (mean 12.1 ± 3.0) mm long, 1–2 (mean 1.2) mm wide (20 collections), length/width ratio 10.1:1. Staminate flowers borne on pedicel-like segments, ca. 2 mm long, 2.2 mm across; perianth mostly 3-merous, sometimes 4-merous; same color as the shoots; segments ca. 1.1 mm long, 1.0 mm wide. Mean anther diameter 0.6 mm, centered 0.7 mm from tip of segment. Pollen polar diameter 19–28 (mean 21) µm; equatorial diameter 23–30 (mean 25) µm; polar/equatorial diameter ratio 1:1.16; spine height (mean 1.8 µm) equal to or slightly greater than wall thickness (mean 1.5 µm) (8 collections). Pistillate flowers verticillate; ca. 1.5 mm long, 1.0 mm across; 2-merous. Mature fruit 3.5–4.5 (mean 4) mm long, 1.5–2.5 (mean 2) mm wide; proximal portion ca. 2.5 mm long. Seeds 2.4 x 1.1 mm. n = 14.

Phenology: Meiosis in August. Anthesis usually from early April to early June, with extremes from late March to late June (fig. 16.11). Fruits mature in late August or September of the year following pollination; maturation period averages ca. 16 months. Germination begins in May in Colorado.

Hosts: The principal hosts are Pinus contorta (var. latifolia and murrayana) and P. banksiana; all are about equally susceptible. Pinus contorta var. contorta is infected east of Squamish in southern coastal British Columbia, the only known area where this pine occurs within the range of Arceuthobium americanum (Smith and Wass 1979). Pinus ponderosa var. scopulorum is frequently parasitized in Colorado, Utah, and Wyoming, primarily where this tree is associated with infected P. contorta, but sometimes also in pure stands of P. ponderosa (Hawksworth 1968b). However, P. ponderosa var. ponderosa is less susceptible and is only occasionally infected (Kuijt 1953). Other occasional hosts for A. americanum include P. albicaulis, P. flexilis, and P. jeffreyi. Spruces are rare hosts—Picea glauca (Smith and others 1972), P. mariana (Baker and others 1988), P. engelmannii and P. pungens (Hawksworth and Graham 1963, Kuijt 1960b, Molnar and others 1968). When P. mariana and P. pungens are parasitized, witches’ brooms are formed, but no dwarf mistletoe shoots have been observed. The presence of the dwarf mistletoe was confirmed by the occurrence of the endophytic system in the host branches.

>Pinus aristata is a rare host in Colorado and Pseudotsuga menziesii is an extremely rare host in Alberta (Muir 1973b) and in northern Utah. In Utah, a Pseudotsuga menziesii with several small witches’ brooms was observed in an infested Pinus contorta stand. The witches’ brooms produced no dwarf mistletoe shoots, but dissection of infected twigs again revealed the presence of the endophytic system and confirmed infection by Arceuthobium americanum. Weir (1917) reported A. americanum on Pinus attenuata and P. jeffreyi near Oregon Mountain, Josephine County, Oregon. His specimens are unquestionably A. americanum, but we and others have visited this locality several times and have been unable to confirm the presence of A. americanum.

Weir (1918a) successfully inoculated seedlings of the European Pinus mugo (as P. montana) with Arceuthobium americanum, and this dwarf mistletoe has been found on planted P. sylvestris in Washington (Graham and Leaphart 1961) and in Alberta (Powell 1968).

Distribution: Canada (British Columbia, Alberta, Saskatchewan, Manitoba, and Ontario) (fig. 16.12 and 16.13) and the United States (Washington, Idaho, Montana, Oregon, California, Utah, Wyoming, and Colorado) (fig. 16.14). It probably occurs in Nevada in the vicinity of Lake Tahoe, but no specific locations are known (Kartesz 1988). Arceuthobium americanum has the most extensive distribution of any North American dwarf mistletoe (fig. 16.12). Its latitudinal range of about 2,800 km is exceeded only by A. douglasii. It occurs from Lake Athabasca in northern Saskatchewan south to the southern Sierra Nevada in California (Kern County) and to southern Colorado (Saguache County). Its longitudinal distribution of nearly 2,400 km extends from western Ontario to western British Columbia (Elliot and others 1967, Kuijt 1963, Larsen and Gross 1970, Zalasky 1956). Arceuthobium americanum extends to nearly latitude 60°N. Arceuthobium tsugense in Alaska and Viscum album in Sweden (Wallden 1961) reach similar latitudes, but both of these mistletoes occur in maritime environments that are much less extreme than the continental climate in which A. americanum is distributed.

There is one report of Arceuthobium americanum on Pinus banksiana in the Northwest Territories at about latitude 63°N, about 190 km north of Fort Providence (Moody and Cerezke 1985). However, this has not been confirmed (Y. Hiratsuka, personal communication and our observations). We observed brooming to be common in P. banksiana south of Fort Providence, but the brooms were not caused by dwarf mistletoe, and we suspect the report by Moody and Cerezke (1985) is in error.

The distribution of Arceuthobium americanum is centered within that of its principal host Pinus contorta, particularly varieties latifolia and murrayana. However, variety murrayana in southern California and Baja California is not infected. The latter is about 600 km south of the known range of the parasite in the southern Sierra Nevada.

Arceuthobium americanum rarely occurs within the distribution of Pinus contorta var. contorta (shore pine), but it has been observed on this host in coastal British Columbia (Smith and Wass 1979). As previously mentioned, Weir (1917) reported this variety as a host in the Oregon Mountains in southwestern Oregon, and he indicated it was associated with infected P. jeffreyi and P. attenuata. Although P. contorta var. contorta is common in this area, the only dwarf mistletoe occurring on it is A. siskiyouense, and then only rarely. Finding A. americanum in the Oregon Mountain area would extend the western limit of distribution by approximately 100 km from its known distribution in the Cascade Mountains.

Arceuthobium americanum occurs in outlying populations of Pinus contorta var. latifolia in the Cypress Hills of southeastern Alberta and several isolated mountain ranges in north central Montana (Dooling and Eder 1981)—the Little Rocky Mountains (Phillips County), Bearpaw Mountains (Hill County) (Dooling 1973), and Sweetgrass (Whitlash) Mountains (Liberty County) (Thompson and Kuijt 1976). Dooling and Eder (1981) report that this dwarf mistletoe is known from all the isolated P. contorta populations in central Montana, except those in the Highwood and Snowy Mountains.

In central and northern Alberta, Arceuthobium americanum occurs on Pinus banksiana, and it is common on this host in Saskatchewan and Manitoba. The reports of A. americanum at White Otter Lake in western Ontario (Hord and Quirke 1956) and near Sprague in extreme southeastern Manitoba (Zalasky 1956) are misidentifications based on a rare infection of P. banksiana by A. pusillum (Laut 1967, Sippell and others 1968).

Arceuthobium americanum has been confirmed in Ontario (Larsen and Gross 1970) at the Sioux Lookout area (latitude 50°30' N, longitude 96°30' W), nearly 240 km east of the previously known eastern limits of the species near Belair at the south end of Lake Winnipeg, Manitoba. However, Sioux Lookout was severely burned by a wildfire in 1976, and the mistletoe was apparently eradicated (Myren and Gross 1977). The only other known populations in Manitoba south and east of Belair have also apparently been extirpated—Wallace Lake (latitude 49°50' N, longitude 95°30' W) by wildfire and Milner Ridge (latitude 50°N, longitude 96°10' W) by clearcutting (M. Slivitsky and T. Meyer, personal communication).

Arceuthobium americanum only occurs in the western part of the range of Pinus banksiana, so the spread of the parasite onto this tree has probably been relatively recent. Arceuthobium americanum likely evolved as a principal parasite of P. contorta, then spread to P. banksiana through central Alberta where these two species co-occur and frequently hybridize. Yeatman (1967) suggests that P. banksiana first became parasitized by A. americanum after the Wisconsin Glaciation, but available evidence indicates an earlier association (Zavarin and others 1969).

Arceuthobium americanum has been found on Pinus contorta x banksiana hybrids near Grand Prairie and Peace River, Alberta (Hawksworth and Wiens 1972). Also, A. americanum occurs on P. contorta that was planted in a naturally infested stand of P. banksiana at Prince Albert, Saskatchewan (Hawksworth and Wiens 1972).

In the central Rocky Mountains, Arceuthobium americanum occurs at the lower elevational limits of Pinus contorta, but its upper limits are usually about 200 m below the upper limit of stands dominated by P. contorta (Hawksworth 1956b and see fig. 7.2).

Arceuthobium americanum distribution maps have been published for western Canada (Kuijt 1963), Alberta (Baranyay 1970), British Columbia and adjacent southwestern Alberta (Baranyay 1975), British Columbia (Wood 1986), Manitoba and Saskatchewan (Zalasky 1956), Montana (Dooling and Eder 1981), Utah (Albee and others 1988), Colorado (Hawksworth 1987c), and California (Kuijt 1960a).

This dwarf mistletoe varies in elevation from 200 m near Lake Athabasca in northern Alberta and Saskatchewan to 3,350 m in central Colorado.

Discussion: Gill (1935) discusses the typification problem in Arceuthobium americanum. Engelmann believed that the specimen he was describing was collected by Nuttall, and it was designated as the type (Oregon, on Pinus). Gill states, however, that this specimen was actually collected by Douglas in the Blue Mountains of Oregon; we have designated this latter collection as the type specimen.

Despite the extensive geographic distribution of Arceuthobium americanum, we find no criteria for subspecific division. In general, the plants are larger in the Cascade Mountains of Oregon and in northern Idaho than elsewhere, but this is probably associated with the more vigorous host growth in these areas. Arceuthobium americanum induces characteristic systemic witches’ brooms (fig. 16.15) on Pinus contorta (Kuijt 1960b) and produces the same type of broom formations on P. ponderosa (Hawksworth 1956a, Weir 1916c). The witches’ brooms formed on Picea engelmannii, however, are non-systemic (Hawksworth and Graham 1963). Kuijt (1960a) noted that A. americanum cannot perpetuate itself for long periods of time on Pinus jeffreyi or P. ponderosa var. ponderosa in California. In northern Colorado and southern Wyoming, however, the parasite is aggressive in pure stands of P. ponderosa var. scopulorum, some of which are several kilometers from the nearest infection on P. contorta. Most areas where A. americanum occurs in pure stands of P. ponderosa are outside the range of A. vaginatum subsp. cryptopodum, which is the typical parasite on P. ponderosa in the Rocky Mountains (Hawksworth 1968b, 1969).

Arceuthobium apachecum
Apache Dwarf Mistletoe

4. A. apachecum Hawksworth & Wiens, Brittonia 22: 266, 1970. TYPE COLLECTION: ARIZONA: Pima County: Santa Catalina Mountains near the summit of Mt. Lemmon at 2,800 m, on Pinus strobiformis, Hawksworth, Lightle, & Gilbertson 1110, September 13, 1968. (Holotype US! Isotypes: ARIZ, COLO, DS, FPF, MO, RM, UC, UT.)

Description: Mean shoot height 3–4 (max. 9) cm. Shoots yellow, green, or reddish, flabellately branched and densely clustered (fig. 16.16). Basal diameter of dominant shoots 1–2 (mean 1.8) mm. Third internode 5–10 (mean 7.2 ± 2.0) mm long, 1–2 (mean 1.5) mm wide (12 collections), length/width ratio 4.8:1. Flowers axillary. Staminate flowers 2.7 mm across; perianth, 3- to 4-merous; same color as shoots; segments ca. 1.3 mm long, 0.9 mm wide. Mean anther diameter 0.5 mm, centered 0.7 mm from tip of segment. Pollen polar diameter 16–23 (mean 19) µm; equatorial diameter 18–23 (mean 21) µm; polar/equatorial diameter ratio 1:1.11; spine height (1.5 µm), slightly greater than the wall thickness (ca. 1.1 µm) (4 collections). Mature fruit 4 mm long, long 2.5 mm wide; proximal portion ca. 2.5 mm long. Seeds 2.8 x 1.2 mm. n = 14.

Phenology: Meiosis in July. Anthesis from late July to mid-September, with a peak in mid-August (Mathiasen 1982) (fig. 16.17). Fruits mature from mid-August to mid-October with a peak in September (Mathiasen 1982); maturation period averages about 13 months.

Host: Known only on Pinus strobiformis, the only member of subgenus Haploxylon (except pinyons) that occurs within its distribution.

Distribution: United States (Arizona and New Mexico) and Mexico (Coahuila) (fig. 16.18). This dwarf mistletoe has a limited distribution in southern Arizona and central New Mexico, with an outlier in the Sierra del Carmen in northern Coahuila, Mexico. In Arizona, it occurs in the White, Pinaleno, Santa Catalina, Santa Rita, and Chiricahua Mountains and in New Mexico in the Mangas, San Mateo, Magdalena, and Capitan Mountains. The report of a dwarf mistletoe on Pinus strobiformis on Mt. Livermore in the Davis Mountains of west Texas (Powell 1988) requires confirmation. The specimen on which the report was based (Coleman s. n. in 1936 at SRSC) is Phoradendron juniperinum, but there is no host material with the collection, and this mistletoe is not known to parasitize pines. A distribution map for Arceuthobium apachecum in New Mexico was published by Martin and Hutchins (1980). Elevational range is 2,000–3,000 m.

Discussion: We consider Gill’s Arceuthobium campylopodum f. blumeri to comprise 4 allopatric species—A. apachecum, A. blumeri, A. californicum, and A. monticola, all of which parasitize pines of subgenus Haploxylon. These species differ in a number of characteristics including morphology, hosts, phenology, and distribution. Our previous numerical analyses support the classification of these population systems as species (Hawksworth and Wiens 1972), although they may exhibit superficial similarities.

The exclusive occurrence of two dwarf mistletoes (Arceuthobium apachecum and A. blumeri) on a single host species (Pinus strobiformis) is unique in Arceuthobium. Originally, we suspected these taxa might represent a single, variable species. However, the occurrence of geographically consistent morphological differences (table 16.1) indicated that separate taxonomic status was warranted. This conclusion has been confirmed by Mathiasen (1982). The populations of A. apachecum in the Santa Rita Mountains tend to have taller shoots (up to 9 cm) than do those in other portions of its distribution.

When grown under common conditions in a greenhouse at Fort Collins, Colorado, the two species maintained morphological integrity. They are not sympatric, but they do approach to within about 60 km of each other in southern Arizona. Arceuthobium apachecum is abundant north and east of the Santa Rita and Chiricahua Mountains; A. blumeri occurs from the Huachuca Mountains south into Mexico. Unlike A. blumeri, A. apachecum frequently induces witches’ brooms (fig. 16.19).

Arceuthobium aureum
Golden Dwarf Mistletoe

5. A. aureum Hawksworth & Wiens, Brittonia 29: 414, 1977. TYPE COLLECTION: GUATEMALA: Alta Verapaz, 5 km west of San Cristóbal Verapaz, on Route 7W to Huehuetenango, on Pinus pseudostrobus, Hawksworth, Wiens, & Player 1596, in 1975 (Holotype US! Isotypes: ENCB, FPF, INIF, MO, UNAM, UT).

Description: Shoot height 12–40 cm yellow to golden brown, flabellately branched. Basal diameter of dominant shoots 3–24 mm. Third internode 16–35 mm long, 2–8 mm wide. Staminate flowers ca. 3 mm across, 3- or 4-merous. Mature fruits 4–5 mm long, 2.5 mm wide. Parasitic on pines of subgenus Diploxylon. n = 14.

Discussion: This taxon was originally considered to be part of the Arceuthobium globosum complex (Hawksworth and Wiens 1972), but further field studies in Mexico and Guatemala have shown that it is a distinct species (Hawksworth and Wiens 1977, see discussion under A. globosum). Arceuthobium aureum is comprised of two subspecies.

Key to the subspecies:

1. Shoots usually less than 20 cm tall, golden; dominant shoots usually less than 10 mm in diameter at base; mature fruits ca. 4 mm long; pedicels ca. 1.5 mm long; flowering and seed dispersal irregular throughout the year; witches’ brooms not formed; below 2,000 m in Guatemala.

5a. A. aureum subsp. aureum

1. Shoots usually over 20 cm tall, golden brown; dominant shoots usually more than 10 mm diameter at base; mature fruits ca. 5 mm long, pedicels ca. 4 mm long; flowering in September; seed dispersal June–July; witches’ brooms commonly produced; above 2,200 m in southern Mexico (Chiapas and Oaxaca).

5b. A. aureum subsp. petersonii

 

Arceuthobium aureum subsp. aureum
Golden Dwarf Mistletoe

5a. A. aureum Hawksworth & Wiens subsp. aureum.

Description: Shoots 12–30 (mean 16) cm tall, golden, flabellately branched (fig. 16.20). Basal diameter of dominant shoots 3–14 (mean 5) mm. Third internode 16–30 (mean 20) mm long, 2–4 (mean 3) mm wide. Mature fruits ca. 4 mm long, lightly glaucous; pedicel ca. 1.5 mm long. n = 14.

Phenology: Anthesis and fruit maturity apparently continuous throughout the year, or at least in the dry season.

Hosts: Pinus montezumae, P. oaxacana, and P. pseudostrobus.

Distribution: Guatemala (fig. 16.21). A common taxon of the low-elevation pine forests in Guatemala. Elevational range is 900–2,000 m.

Discussion: This dwarf mistletoe was formerly considered to be part of the Arceuthobium globosum complex (Hawksworth and Wiens 1977) and is discussed under that species in greater detail.

 

Arceuthobium aureum subsp. petersonii
Peterson’s Dwarf Mistletoe

5b. A. aureum Hawksworth & Wiens subsp. petersonii Hawksworth & Wiens, Brittonia 29: 415, 1977. TYPE COLLECTION: MEXICO: Chiapas, near km-136 (52 km southeast of San Cristóbal de las Casas) on Panamerican Highway, 2,250 m, on Pinus pseudostrobus, Hawksworth, Wiens, & Player 1598 in 1975 (Holotype US! Isotypes: ENCB, FPF, MO, UNAM, UT).

Description: Shoots 14–40 (mean 24) cm tall, golden to yellow-brown, flabellately branched. Basal diameter of dominant shoots 14–35 (mean 23) mm. Third internode 14–35 (mean 23) mm long, 2.5–8 (mean 5) mm wide. n = 14.

Phenology: Anthesis in September. Fruits mature June–July; maturation period of ca. 9–10 months. This is several months less than is common for many dwarf mistletoes.

Hosts: Pinus michoacana, P. montezumae, P. oaxacana, P. oocarpa, P. patula, and P. pseudostrobus are the principal hosts. Pinus michoacana is somewhat less susceptible and is infected only when it grows in association with the other principal hosts.

Distribution: Mexico (Oaxaca and Chiapas) (fig. 16.21). This dwarf mistletoe is common in Chiapas between San Cristóbal de las Casas and Teopisca. Its distribution in Oaxaca is poorly known, but there are a number of collections from the Miahuátlan–Suchixtepec area. Elevational range is 2,200–2,450 m.

Discussion: This subspecies is characterized primarily by its tall, slender, brown to golden shoots, slightly longer fruit (5 mm), long pedicels (4 mm), and general tendency to form witches’ brooms (fig. 16.22). The subspecies is named for R. S. Peterson, who first discovered these populations.

 

Arceuthobium bicarinatum
Hispaniolan Dwarf Mistletoe

6. A. bicarinatum Urban, Symbolae Antillanae 7: 204, 1912. TYPE COLLECTION: DOMINICAN REPUBLIC: Constanza, on Pinus occidentalis, von Turckheim 3241 in 1910 (Holotype Z! Isotypes: BM, F, ILL, K, MO, NY, S, US). Razoumofskya bicarinata (Urban) Tubeuf, Naturwissenschaftliche Zeitschrift für Forst -und Landwirtschaft 17: 195, 1919.

Description: Mean shoot height ca. 10 (max. 17) cm, dark brownish red, terminal branches of living plants conspicuously glaucous (character lost after drying), shoots frequently twisted when dry, flabellately branched (fig. 16.23). Basal diameter of dominant shoots 2–4 (mean 3) mm. Third internode 6–14 (mean 10.5 ± 2.1) mm long, 1.5–4.0 (mean 2.0) mm wide (6 collections), length/width ratio 5.2:1. Staminate flowers ca. 3 mm across; perianth 3- or 4-merous, reddish, segments ca. 1.5 mm long, 1.0 mm wide, apex acute. Mean anther diameter 0.5 mm, centered 1.0 mm from tip of segment. Pollen polar diameter 21–25 (mean 23) µm; equatorial diameter 25–28 (mean 26) µm; polar/equatorial diameter ratio 1:1.14; spine height (ca. 3 µm) 3 x wall thickness (ca. 1 µm) (3 collections). Mature fruits ca. 4 x 2 mm; proximal portion ca. 2.5 mm long. Seeds 2.5 x 1.2 mm. n = 14.

Phenology: Meiosis in August. Anthesis usually in September. Fruits mature in late August or September of the year following pollination; maturation period averages ca. 12 months.

Hosts: Known only on Pinus occidentalis, the only native pine within its distribution.

Distribution: Dominican Republic and Haiti (fig. 16.24). Arceuthobium bicarinatum is common on the island of Hispaniola. In Haiti, this dwarf mistletoe is common in the Morne la Selle and the Morne des Commissaires, in the southeastern part of the country. In the Dominican Republic, A. bicarinatum occurs on all three major mountain ranges where Pinus occidentalis is present—Cordillera Central, Sierra de Neiba, and Sierra de Bahoruco. More than half of the pine forests in the Dominican Republic are apparently infested by A. bicarinatum, and damage is particularly severe in Santiago and San Juan Provinces (Etheridge 1971). We commonly observed A. bicarinatum in La Vega Province between 1,000–2,600 m. Etheridge (1971) gives a distribution map for the species in the Dominican Republic. Thomas Zanoni of the Jardin Botánico Nacional, Santo Domingo, provided additional information on the distribution of this dwarf mistletoe in the Dominican Republic. Elevational range is 800–2,800 m (Darrow and Zanoni 1991, Etheridge 1971).

Discussion: This highly distinctive species is restricted to Hispaniola. Characteristic features are the dark brownish-red shoot color, sterile nodes near the tips of the pistillate shoots, and glaucous terminal internodes. Internodes on dried specimens often show a twist of about a quarter turn, but this trait is not discernible in living plants.

The witches’ brooms caused by Arceuthobium bicarinatum are mostly non-systemic (as illustrated by Marie-Victorin 1943). Near Valle Nuevo in the Cordillera Central, however, some witches’ brooms were anomalous. These brooms appeared to be systemic (fig. 16.25), but dwarf mistletoe shoots were not regularly produced on the infected branches, as is normally the case for systemic infections in other host–parasite combinations. Detailed studies are needed on the unusual witches’ brooms induced by A. bicarinatum.

This is a serious parasite of Pinus occidentalis in the Dominican Republic, where from 15–85% of the trees in the northern Cordillera Central are infected (Darrow and Zanoni 1991). At higher elevations in Haiti and the Dominican Republic, a leafy mistletoe (Dendropemon pycnophyllus, Loranthaceae) is also common on P. occidentalis and frequently occurs on trees that are also infected by Arceuthobium bicarinatum.

 

Arceuthobium blumeri
Blumer’s Dwarf Mistletoe

7. A. blumeri A. Nelson, Botanical Gazette 56: 65, 1913. TYPE COLLECTION: ARIZONA: Cochise County: Huachuca Mountains, on Pinus strobiformis, Blumer in October 1910 (Holotype RM No. 78604!). Razoumofskya blumeri Standley, Proceeding Biological Society Washington 29: 86, 1916. A. campylopodum Engelm. forma blumeri (Engelm.) Gill, Transactions Connecticut Academy Arts and Sciences 32: 207, 1935 (in part).

Description: Mean shoot height ca. 6–7 (max. 18) cm, gray to straw or light green, flabellately branched (fig. 16.26). Basal diameter of dominant shoots 1–3 (mean 2.1) mm. Third internode 5–14 (mean 9.1 ± 2.5) mm long, 1–2 (mean 1.6) mm wide (8 collections), length/width ratio 5.5:1. Staminate flowers ca. 2.5 mm long, 2.5–3.0 mm across; perianth 3- to 6-merous (mostly 3- or 4-merous), segments ca. 1.3 mm long, 1.0 mm wide, apex acute. Mean anther diameter 0.6 mm, centered 0.4 mm from tip of segment. Pollen polar diameter ca. 19 µm and an equatorial diameter ca. 20 µm; spine height and wall thickness each ca. 1 µm (1 collection, few grains available). Mature fruit ca. 4 x 2.5 mm, proximal portion ca. 2.5 mm long. Seeds 2.7 x 1.0 mm. n = 14.

Phenology: Meiosis in July. Anthesis from mid-July to late-August, with a peak in early August (Mathiasen 1982). Fruits mature from late August to early October, with a peak in mid-September (Mathiasen 1982); maturation period averages 13–14 months.

Hosts: The host affinities of Arceuthobium blumeri are not clear because of the taxonomic confusion surrounding Pinus strobiformis and the closely related P. ayacahuite var. brachyptera in the Sierra Madre Occidental, Mexico (Eguiluz 1991, Hawksworth 1991b, Perry 1991). However, most host populations of this dwarf mistletoe in the Sierra Madre Occidental are best referred to the latter taxon. On Cerro Potosí in Nuevo León, it parasitizes a different white pine that has been called "Pinus flexilis" (Martínez 1948, Perry 1991), but is perhaps best classified as P. strobiformis var. potosiensis (Silba 1990).

Distribution: United States (Arizona) and Mexico (Sonora, Chihuahua, Durango, Nuevo León, and Coahuila) (fig. 16.27). This dwarf mistletoe extends southward from the Huachuca Mountains in southern Arizona through the Sierra Madre Occidental in Chihuahua and Sonora to southern Durango. In the Sierra Madre Oriental it is known only from Cerro Potosí, Nuevo León, and San Antonio de las Alazanas, Coahuila (Cibrián and others 1980), but it probably occurs elsewhere over this extensive distribution. Elevational range is 2,150–3,250 m.

Discussion: The parasitism of Arceuthobium blumeri and A. apachecum on Pinus strobiformis is discussed under A. apachecum and the two species are compared in table 16.1.

Mathiasen (1982) studied 13 populations in 3 areas of Arizona, Chihuahua, and Durango and found that the perianth segments of the staminate flowers were distributed as follows: 3 segments, 42%; 4 segments, 52%; 5 segments, 6%, and 6 segments, trace. The northern populations (Arizona) had the highest number of segments per flower: 3 segments, 30%; 4 segments, 53%; and 5 segments, 16%. The Durango population had the lowest: 3 segments, 55%; 4 segments, 44%; and 5 segments, 1%.

Distinctive features of Arceuthobium blumeri include its gray-colored shoots, 4- to 6-merous staminate flowers, and rare formation of witches’ brooms. The populations in the southern portions of its distribution in Durango tend to have taller shoots (up to 18 cm).

 

Arceuthobium californicum
Sugar Pine Dwarf Mistletoe

8. A. californicum Hawksworth & Wiens, Brittonia 22: 266, 1970. TYPE COLLECTION: CALIFORNIA: Mariposa County, Fish Camp 0.4 km west of State Route 41 on Summit Camp Road, on Pinus lambertiana, Hawksworth & Hawksworth 1147, November 6, 1968 (Holotype US! Isotypes: ARIZ, COLO, DS, FPF, MO, RM, UC, UT). A. campylopodum Engelm. var. cryptopodum (Engelm.) Jepson, Manual of Flowering Plants of California 284. 1925.

Description: Mean shoot height ca. 8 cm (max. 12) cm, greenish to bright yellow, turning brown at base of older shoots, flabellately branched (fig. 16.28). Basal diameter of dominant shoots 1.5–4.0 (mean 2) mm. Third internode 6–16 (mean 10.5 ± 2.9) mm long, 1–2 (mean 1.5) mm wide (8 collections), length/width ratio 7.0:1. Flowers axillary. Staminate flowers 3.3 mm across; perianth 3- or 4-merous, segments ca. 1.5 mm long, 1.1 mm wide. Mean anther diameter 0.5 mm, centered 0.7 mm from tip of segment. Pollen polar diameter 18–23 (mean 20) µm; equatorial diameter 19–25 (mean 23) µm; polar/equatorial diameter ratio 1:1.12; spine height (ca. 1.6 µm), approximately 2 x wall thickness (ca. 0.8 µm) (4 collections). Mature fruit 4 x 2.5 mm; proximal portion ca. 2.0 mm long. Seeds 3.2 x 1.2 mm. n = 14.

Phenology: Meiosis in July. Anthesis usually in mid-July to mid-August, with extremes from early July to late August (fig. 16.29). Fruits mature from mid-September to mid-October, with extremes from late August to early November; maturation period averages ca. 13–14 months.

Hosts: The only principal host is sugar pine, Pinus lambertiana, but P. monticola is sometimes parasitized when it grows in association with infected P. lambertiana. This dwarf mistletoe induces large, compact witches’ brooms in P. lambertiana (fig. 16.30).

Distribution: United States (California) (fig. 16.31). This species is distributed from Mt. Shasta southward through the North Coast Range and through the Cascade Range south to Lake County and the west side of the Sierra Nevada, ultimately reaching the Cuayamaca Mountains in San Diego County. Elevational range is 600–2,000 m.

Discussion: Jepson’s (1925) variety cryptopodum, which was intended to include parasites of Pinus lambertiana and Abies grandis, is based on a misapplied name, Arceuthobium cryptopodum Engelm., a Rocky Mountain parasite of P. ponderosa. As discussed under Arceuthobium apachecum, Gill’s A. campylopodum f. blumeri includes four species—A. apachecum, A. blumeri, A. californicum, and A. monticola. A comparison of A. californicum and A. monticola is presented in table 16.2.

Concerning this dwarf mistletoe, Gill (1935) commented that "infected trees are infrequent and were found only in the presence of other infected species." Our studies do not support this observation. Arceuthobium californicum is common in many areas, rarely infects other species, and is a serious pathogen of Pinus lambertiana in California (Scharpf and Hawksworth 1968). Surveys found the parasite on 22% of 274 P. lambertiana plots in California (Anonymous 1968). In many instances A. californicum is the only dwarf mistletoe present in a stand, but it is sympatric with A. campylopodum or A. abietinum (this may have been the basis for Gill’s statement).

Arceuthobium cyanocarpum did not parasitize Pinus lambertiana in the San Jacinto Mountains of southern California, the only instance we know of where P. lambertiana has been exposed to this dwarf mistletoe. These two dwarf mistletoes are morphologically distinct, and each is restricted to its respective host.

 

Arceuthobium campylopodum
Western Dwarf Mistletoe

9. A. campylopodum Engelmann in Gray, Boston Journal Natural History 6: 214, 1850. TYPE COLLECTION: "Oregon," on Pinus ponderosa, Geyer 577 in 1843 (Lectotype GH! Isotype US). A. campylopodum Engelm. var. macrarthron Engelm., loc. cit. Razoumofskya campylopoda (Engelm.) Kuntze, Revision of Genera of Plants 2: 587, 1891. A. campylopodum Engelm. forma typicum Gill, Transactions Connecticut Academy Arts and Sciences 32: 185, 1935 (in part).

Description: Mean shoot height ca. 8 (max. 13) cm, olive green to yellow, flabellately branched (fig. 16.32). Staminate plants brownish, and pistillate plants greenish (particularly in northern populations). Basal diameter of dominant shoots 1.5–5.0 (mean 3) mm. Third internode 7–22 (mean 11.3 ± 3.8) mm long, 1.5–2.5 (mean 2.0) mm wide (27 collections), length/ width ratio 5.6:1. Staminate flowers 3.0 mm across; perianth 3-merous (occasionally 4-merous), segments ca. 1.4 mm long, 1.0 mm wide. Mean anther diameter 0.5 mm, centered 0.8 mm from tip of segment. Pollen polar diameter 18–25 (mean 20) µm; equatorial diameter 20–27 (mean 24) µm; polar/equatorial diameter ratio 1:1.18; spine height (ca. 2.2 µm) about 2.5 times wall thickness (ca. 0.8 µm) (5 collections). Mature fruit 5.0 x 3.0 mm. n = 14.

Phenology: Meiosis in July. Peak anthesis usually from mid-August to early October, with extremes from early August to late October (fig. 16.33). Fruits usually mature from early September to mid-November, with extremes from late August to late November; maturation period averages ca. 13 months.

Hosts: The principal and most commonly infected hosts are Pinus ponderosa var. ponderosa and P. jeffreyi. According to Wagener (1965) and R. F. Scharpf (personal communication), P. jeffreyi is somewhat more susceptible than P. ponderosa, but both species incur considerable damage and are classed here as principal hosts. Other trees frequently infected, particularly when associated with the above hosts, are P. coulteri and P. attenuata. The three races of variety ponderosa recognized by Conkle and Critchfield (1988)—North Plateau, Pacific, and Southern California—appear to be about equally susceptible. In the Spring (Charleston) Mountains of southern Nevada, P. ponderosa var. scopulorum is a common host, but this is the only known area where Arceuthobium campylopodum occurs naturally within the range of variety scopulorum. In Oregon, Roth (1967) inoculated varieties scopulorum and ponderosa and found them to be about equally susceptible to A. campylopodum. Occasional hosts for A. campylopodum are P. contorta vars. latifolia and murrayana. T. W. Childs (personal communication) informed us that P. lambertiana is very rarely infected by A. campylopodum in the vicinity of Hammer Butte in northern Klamath County, Oregon.

In certain areas in the southern Sierra Nevada where Pinus sabiniana occurs above the elevational range of Arceuthobium occidentale, A. campylopodum occasionally parasitizes P. sabiniana associated with infected P. ponderosa or P. jeffreyi. However, little or no cross-infection occurs in areas where the two dwarf mistletoes occur sympatrically (chapter 6 and 14).

Weir (1918a) successfully inoculated seedlings of Pinus sylvestris, P. mugo (as P. montana), P. resinosa, Picea abies, Abies concolor, A. grandis, and Larix occidentalis with Arceuthobium campylopodum. The latter three species are commonly associated with Pinus ponderosa when it is infected by A. campylopodum, but we have never observed the parasite on any of them. Kuijt (1960b) found the European Pinus pinaster infected naturally at the Institute of Forest Genetics Arboretum at Placerville, California, and J. R. Weir collected A. campylopodum on planted P. sylvestris near Hayden Lake, Idaho.

Pinus washoensis is probably susceptible to Arceuthobium campylopodum, but we know of no collections on this host. We found no infections on the extensive populations of this tree in the vicinity of Patterson Guard Station in the southern end of the Warner Mountains, Modoc County, California. Similarly, R. S. Peterson (personal communication) found no dwarf mistletoe in the type locality for P. washoensis on Mt. Rose, near Reno, Nevada.

Distribution: United States (Washington, Idaho, Oregon, California, and Nevada) and Mexico (Baja California Norte) (fig. 16.34). Arceuthobium campylopodum occurs from northern Washington and eastern Idaho, south through Oregon and California (but it is not known in the southern Coast Range) to the Sierra Juárez and Sierra de San Pedro Mártir in Baja California, Mexico. In Nevada, it occurs in the vicinity of Lake Tahoe and in the Spring (Charleston) Mountains of Clark County. Arceuthobium campylopodum is not known from British Columbia, but it occurs within about 30 km of the international boundary near Kettle Falls, Washington. Gill’s (1935) report of this species in western Montana is based on occasional parasitism of Pinus ponderosa by the larch dwarf mistletoe, A. laricis.

Arceuthobium campylopodum is distributed by elevation from 30 m along the Columbia River, near Hood River, Oregon, to 2,500 m in the Spring (Charleston) Mountains of southern Nevada.

Discussion: The type locality is listed as "Oregon," although Piper (1906) comments that the specimen must have come from northern Idaho or northeastern Washington, because Geyer did not collect in the area that is now Oregon.

G. G. Hedgcock (unpublished manuscript dated 1914) noted differences between Arceuthobium campylopodum and A. occidentale and suggested that they warranted separate taxonomic status. Gill’s (1935) A. campylopodum f. typicum is here considered to comprise four species—A. campylopodum (sensu stricto), A. occidentale, A. siskiyouense, and A. littorum (Hawksworth and others 1992b). These are separable on the basis of phenology, morphology, geographic distribution, and host preference.

Arceuthobium campylopodum is a serious pathogen of Pinus jeffreyi and P. ponderosa (fig. 16.35). Our observations suggest that host damage is more severe in the southern parts of the distribution; the most severely infested stands occur in the Laguna Mountains in San Diego County, California. In the northern Sierra Nevada, pathogenic effects are more severe on the drier east-side forests than along the wetter west-side forests. Roth (1966) reports that drooping-needled races of P. ponderosa in Oregon are less frequently infected by the dwarf mistletoe than are populations with typical needles, but grafts from these trees showed no evidence of resistance (Roth 1974a and chapter 13). Recently, Scharpf (1987) has shown that some populations of P. jeffreyi show high resistance to infection by A. campylopodum.

 

Arceuthobium cyanocarpum
Limber Pine Dwarf Mistletoe

10. A. cyanocarpum (A. Nelson ex Rydberg) Coulter & Nelson, New Manual of Botany of the Central Rocky Mountains, p. 146, 1909. TYPE COLLECTION: WYOMING: Carbon County: Ferris Mountains, on Pinus flexilis, Nelson 4959 in 1898 (Lectotype RM! Isotypes: NY, MO). Razoumofskya cyanocarpa A. Nelson in Rydberg, Colorado Agricultural Experiment Station Bulletin 100: 101, 1906. A. cyanocarpum (A. Nels.) Abrams, Illustrated Flora Pacific Coast States I: 531, 1923. A. campylopodum Engelm. forma cyanocarpum (A. Nels.) Gill, Transactions Connecticut Academy Arts and Sciences 32: 204, 1935.

Description: Mean shoot height ca. 3 (max. 7) cm, yellow green, flabellately branched, densely clustered (fig. 16.36). Basal diameter of dominant shoots 1–2 (mean 1.4) mm. Third internode 2–14 (mean 5.2 ± 2.0) mm long, 1.0–1.5 (mean 1.1) mm wide (15 collections); length/width ratio 4.7:1. Staminate flowers 3.0 mm across; perianth 3-merous (rarely 4-merous), same color as shoots; segments ca. 1.4 mm long, 1.0 mm wide, apex acute. Mean anther diameter 0.4 mm, centered 0.7 mm from tip of segment. Pollen polar diameter 15–21 (mean 19) µm; equatorial diameter 20–25 (mean 22) µm; polar/equatorial diameter ratio 1:1.19; spine height (ca. 1.5 µm) approximately 2 x wall thickness (ca. 0.8 µm) (6 collections). Mature fruit 3.5 x 2.0 mm; proximal portion ca. 2.0 mm long. Seeds 2.0 x 0.9 mm. n = 14.

Phenology: Meiosis in July. Peak anthesis from mid-July to early September, with extremes from early July to mid-September (fig. 16.37). Fruits mature from mid-August to late September; maturation averages ca. 12 months. Seed germination mostly in June.

Hosts: Pinus flexilis is the most common host of this dwarf mistletoe throughout its extensive geographical distribution. However, P. aristata, P. longaeva, and P. albicaulis are also classed as principal hosts because, even though they are not as common within the range of Arceuthobium cyanocarpum, they appear to be about as susceptible as P. flexilis. Infection of P. aristata is known from only an area near La Veta Pass in southern Colorado where this mistletoe is also common on associated P. flexilis. Pinus longaeva is parasitized in many areas in Utah and Nevada, and P. albicaulis is infected in western Wyoming, northern Nevada, central Oregon, and northern California.

Pinus monticola in northern California and Tsuga mertensiana in central Oregon are secondary hosts of this dwarf mistletoe (Mathiasen and Hawksworth 1988). Pinus balfouriana subsp. balfouriana is an infrequently but severely infected host in northern California (Miller and Bynum 1965, Mathiasen and Hawksworth 1988). No dwarf mistletoe has been found on the southern subspecies of foxtail pine, P. balfouriana subsp. austrina, in the southern Sierra Nevada in California. Pinus ponderosa var. scopulorum and P. contorta var. latifolia are rare hosts in the Rocky Mountains (Hawksworth and Peterson 1959). Von Tubeuf (1919) mentions a single collection of dwarf mistletoe on Picea engelmannii from Pikes Peak, Colorado, which Hawksworth and Graham (1963) surmised was Arceuthobium cyanocarpum.

Greenhouse inoculations of the seedlings of both Pinus strobus and P. strobiformis show that they are also susceptible to infection by Arceuthobium cyanocarpum (Hawksworth and Wiens 1972).

Distribution: United States (Idaho, Montana, Oregon, California, Nevada, Utah, Wyoming, and Colorado) (fig. 16.38). This dwarf mistletoe occurs from southern Montana (Brown 1970) and northern Wyoming south to southern Colorado and west to Oregon and California where it occurs on the east side of the Sierra Nevada, in the Panamint Mountains in Death Valley National Monument (Johnson 1976) and in the San Bernardino and San Jacinto Mountains of southern California.

Arceuthobium cyanocarpum occurs on many mountain ranges in Wyoming and Utah, including an isolated locality in the La Sal Mountains, near the Utah–Colorado border. Arceuthobium cyanocarpum is common in northern Colorado from near Idaho Springs to the Wyoming border, and it occurs in three other isolated localities in Colorado—Pikes Peak, Sangre de Cristo Mountains near Coaldale, and Silver Mountain near La Veta Pass. Arceuthobium cyanocarpum is presently known from only four localities in Idaho. The species is widely distributed in the mountain ranges of Nevada on Pinus flexilis, P. longaeva, and P. albicaulis (Mathiasen and Hawksworth 1990). The specimens listed by Gill (1935) as A. campylopodum f. cyanocarpum on P. albicaulis from the Crater Lake region of Oregon are referable to A. tsugense subsp. mertensianae. The only verified collection of A. cyanocarpum in Oregon is at Three Creek Lake, southwest of Sisters, on P. albicaulis and Tsuga mertensiana (Knutson and Tinnin 1981). We have tentatively listed a collection on P. albicaulis from "Obsidian Cliff," Oregon, as this species, but we have not been able to locate the site. We previously identified the dwarf mistletoe on P. aristata on the San Francisco Peaks, Arizona, as A. cyanocarpum (Hawksworth and Wiens 1972). That population, however, has since been transferred to A. microcarpum (Crawford and Hawksworth 1979, Mathiasen and Hawksworth 1980). Arceuthobium cyanocarpum apparently does not occur in Arizona.

Weir collected Arceuthobium cyanocarpum in 1913 and 1917 on Pinus flexilis in the vicinity of Anaconda, Montana. Several attempts to locate these populations were unsuccessful. Possibly sulfur dioxide fumes have resulted in its extirpation in this area. Scheffer and Hedgcock (1955), who studied the area in the late 1920’s, noted that P. flexilis and A. cyanocarpum were relatively rare in the fume-damaged area.

Distribution maps for Arceuthobium cyanocarpum are available for Colorado (Hawksworth 1987c) and Nevada (Mathiasen and Hawksworth 1990). Elevational range is 1,600 m in southern Montana to nearly 3,050 m in central Colorado.

Discussion: Although Rydberg (1906) stated that he described no new species in that publication, Tiehm (1989) concluded that he did, in fact, validly describe several new species, including Razoumofskya cyanocarpa. We concur with Tiehm and accept Rydberg (1906) as having provided the first valid description of the species.

This dwarf mistletoe, which characteristically infects Pinus flexilis and associated high-altitude white pines, is easily recognized by its small, densely clustered shoots, especially the staminate plants (fig. 16.36). In many cases shoot mortality is very high (the cause is unknown but is probably associated with a secondary fungal pathogen).

Arceuthobium cyanocarpum causes heavy mortality in Pinus flexilis in many areas in the Rocky Mountains and in P. albicaulis on the north slopes of Mt. Shasta, California. In the latter locality, the high mortality caused by the dwarf mistletoe has resulted in large areas of "ghost forests" (Cooke 1955, Mathiasen and Hawksworth 1988).

Witches’ brooms induced by Arceuthobium cyanocarpum are typically small and compact (fig. 16.39). In many instances, however, infection is general throughout the crown of the tree.

 

Arceuthobium divaricatum
Pinyon Dwarf Mistletoe

11. A. divaricatum Engelmann in U.S. Geographical Survey West of 100th Meridian (Wheeler Report) 6: 253, 1878. TYPE COLLECTION: ARIZONA: County unknown, Salt River Valley, on Pinus edulis, Gilbert 116 in 1873 (Lectotype MO! Isotype US). A. gracile Engelmann in Gray, Memoires American Academy N.S. 4: 59, 1849, nomen nudum. Razoumofskya divaricata Coville, Contributions U.S. National Herbarium 4: 192, 1892. A. campylopodum Engelmann forma divaricatum (Engelm.) Gill, Transactions Connecticut Academy Arts and Sciences 32: 193, 1935.

Description: Mean shoot height ca. 8 (max. 13) cm, olive green to brown, flabellately branched (fig. 16.40). Basal diameter of dominant shoots 1.5–4.0 (mean 2) mm. Third internode 6–15 (mean 9.8 ± 2.4) mm long, 1–2 (mean 1.6) mm wide (19 collections), length/width ratio 6.1:1. Staminate flowers 2.5 mm across; perianth 3-merous; segments ca. 1.1 mm long, 0.9 mm wide. Mean anther diameter 0.5 mm, centered 0.5 mm from tip of segment. Pollen polar diameter 18–26 (mean 20) µm; equatorial diameter 21–30 mean 24 µm; polar/equatorial diameter ratio 1:1.17; spine height (1.6 µm) ca. 1.5 x the wall thickness (ca. 1.0 µm) (8 collections). Mature fruit 3.5 x 2.0 mm; proximal portion ca. 2.0 mm long. Seeds 2.0 x 0.9 mm. n = 14.

Phenology: Meiosis in July. Peak anthesis usually from early August to late September (fig. 16.41). Fruits usually mature from early September to late October in the year following pollination; maturation period averages ca. 13 months.

Hosts: Arceuthobium divaricatum is restricted to pinyons. It is most common on Pinus edulis and P. monophylla. Other pinyons are also classed as principal hosts because they appear to be about equally susceptible to infection when the dwarf mistletoe is present—P. quadrifolia, P. cembroides, P. discolor, and P. californiarum subspecies californiarum and fallax (Bailey and Hawksworth 1988). Arceuthobium divaricatum, however, does not commonly occur within the distribution of the latter group of pinyons.

The principal host in California and Nevada is Pinus monophylla, but in Arizona, Colorado, New Mexico, Texas, and Utah the primary host is P. edulis. This dwarf mistletoe has been collected on P. quadrifolia only in the Sierra Juárez, Baja California (Hawksworth and others 1968), but it has been reported on this tree on the east slope of the Laguna Mountains in San Diego County, California (Beauchamp 1986), and in the Sierra San Pedro Mártir, Baja California (Hawksworth and others 1968). Pinus cembroides is parasitized only at the northern limits of its distribution in the Davis Mountains, Jeff Davis County, in west Texas. Pinus discolor is also parasitized only at its northern limits near Fort Bayard, and in the Mule Mountains, New Mexico, where this tree is associated with infected P. edulis. Infection is local on the two subspecies of P. californiarum it parasitizes—subsp. californiarum in several Mojave Desert ranges (New York Mountains, Providence Mountains, and in Joshua Tree National Monument) and subsp. fallax in southwestern Utah (Zion National Park), northwestern Arizona (Black Hole Mountains), and central Arizona near Prescott, Sedona, and Strawberry. Infected pinyons are not often associated with other pines, but where they are sympatric with P. ponderosa and P. jeffreyi, these trees were not parasitized by Arceuthobium divaricatum. No other dwarf mistletoe infects pinyon in the United States. Arceuthobium pendens, however, also parasitizes pinyons in Mexico.

Distribution: United States (California, Nevada, Utah, Colorado, Arizona, New Mexico, and Texas) and Mexico (Baja California) (fig. 16.42). Arceuthobium divaricatum occurs in eastern and southern California (the White and Inyo Mountains, the Mount Pinos area, the San Bernardino Mountains and the Mojave Desert Ranges), the southern three-fourths of Nevada and Utah, western Colorado, Arizona (except southwest), New Mexico (except northeast), and south to the Davis Mountains of western Texas. In Mexico it is known only in northern Baja California. J. R. Weir (unpublished manuscript, 1920) reported A. divaricatum on Pinus monophylla in Cassia County in southern Idaho, but we have not confirmed its presence there, nor do we know of any collections of this dwarf mistletoe from Idaho. The northernmost population of which we are aware is in the Pilot Range, western Box Elder County in northwestern Utah (latitude 44°30'N) (Albee and others 1988). Arceuthobium divaricatum has been reported on P. discolor (as P. cembroides) in the Santa Catalina Mountains in southern Arizona (G. G. Hedgcock, unpublished manuscript, 1914), but this has not been confirmed. We are aware of no collections from Arizona south of the Gila River.

Distribution maps have been published for Arceuthobium divaricatum in Colorado (Hawksworth 1987c), Utah (Albee and others 1988), and New Mexico (Martin and Hutchins 1980). Elevational range is from 1,200 m near Sedona, Arizona, to 3,000 m in the San Mateo Mountains of New Mexico.

Discussion: The type locality for Arceuthobium divaricatum is given by Gill (1935) as Salt "Run" Valley. The original label, however, is handwritten and scarcely legible, but it appears that Salt "River" Valley was the intended locality. Kuijt (1960a) points out the scattered distributions of this parasite in California. We suggest that A. divaricatum is probably more common than indicated by collections to date. The witches’ brooms induced by this dwarf mistletoe are often poorly developed and not conspicuous because of the stunted habit of even healthy trees (fig. 16.43). Our observations suggest that witches’ brooms are more consistent in Pinus edulis than in P. monophylla. Shoots of the mistletoe are often long, slender, and spreading, especially the staminate plants which also tend to have relatively few flowers per shoot.

Arceuthobium divaricatum is often associated with another mistletoe, Phoradendron juniperinum, a parasite of junipers, in the juniper–pinyon communities where both mistletoes may co-occur. The extent of their co-occurrence and the factors underlying their distribution could be a subject of considerable interest.

 

Arceuthobium douglasii
Douglas-fir Dwarf Mistletoe

12. A. douglasii Engelmann in U.S. Geographical Survey West of 100th Meridian (Wheeler Report) 6: 253, 1878. TYPE COLLECTION: NEW MEXICO: Santa Fe County: Santa Fe River, on Pseudotsuga menziesii , Rothrock 69 in 1874 (Lectotype MO! Isotypes: F, US). Razoumofskya douglasii (Engelm.) Kuntze, Revision of Genera of Plants 2: 587, 1891.

Description: Mean shoot height ca. 2 (max. 8) cm, olive green, flabellately branched (fig. 16.44 and 16.45). Basal diameter of dominant shoots 1.0–1.5 (mean 1) mm. Third internode 2–6 (mean 3.6 ± 1.2) mm long, ca. 1.0 mm wide (29 collections), length/width ratio 3.6:1. Flowers usually axillary in pairs, occasionally borne on pedicel-like segments as in Arceuthobium americanum. Staminate flowers 2.0 mm long, 2.3 mm across; perianth mostly 3-merous (occasionally 4- or 2-merous); segments rounded at the apex, without a keel, inner surface reddish to purple, lower surface same color as shoots, ca. 1.0 mm long, 1.0 mm wide. Mean anther diameter 0.4 mm, centered 0.6 mm from tip of segment. Pollen polar diameter 16–25 (mean 21) µm; equatorial diameter 20–28 (mean 24) µm; polar/equatorial diameter ratio 1:1.16; spine height approximately equal to wall thickness (1.0–1.2 µm) (8 collections). Pistillate flowers ca. 1.5 mm long, 1.5 mm across. Mature fruit olive green 3.5–4.5 (mean 4) mm long, 1.5–2.0 mm wide, obovate; proximal portion ca. 2.5 mm long. Seeds 2.4 x 1.1 mm. n = 14.

Phenology: Staminate meiosis in September, pistillate meiosis in April (Jones and Gordon 1965). Peak anthesis is usually in April or May (fig. 16.46). However, this species exhibits a marked latitudinal variation in flowering—March in Mexico, late April to early May in Arizona and New Mexico, late May in Colorado, Utah, Oregon, and early to mid-June in Washington, north Idaho, and Montana. In contrast, fruit maturity is more uniform and usually occurs from late August to late September throughout the distribution; maturation period averages 17–18 months. The seeds germinate in March (Wicker 1967a).

Hosts: The principal and only commonly infected host is Pseudotsuga menziesii. Both var. menziesii (Washington, Oregon, and California) and var. glauca (from British Columbia through the Rocky Mountains to central Mexico) are parasitized, although it is much more common on the inland variety. The following trees are occasional hosts when they are associated with infected Pseudotsuga menziesii: Abies amabilis in Oregon (Mathiasen and Loftis 1987) and A. lasiocarpa var. arizonica in Arizona and New Mexico (Mathiasen 1984). Rare hosts are A. grandis (Kuijt 1954a), A. concolor (Hawksworth 1952), A. lasiocarpa var. lasiocarpa, Picea pungens, and P. engelmannii. We concur with Kuijt (1960a) that reports of this dwarf mistletoe parasitizing Pseudotsuga macrocarpa (Jepson 1925, Munz 1935) are erroneous.

Distribution: Canada (British Columbia), United States (Washington, Idaho, Montana, Oregon, Wyoming, California, Nevada, Utah, Colorado, Arizona, New Mexico, and Texas) and Mexico (Chihuahua, Durango, Coahuila, and Nuevo León) (figs. 16.47, 16.48, and 16.49). Arceuthobium douglasii has the greatest latitudinal range (3,000 km) of any species in the genus (fig. 16.47). Its northernmost distribution is near Lytton and Sicamous in southern British Columbia, about 210 km north of the international boundary (Smith 1974, Wood 1986). This is somewhat north of the distribution given by Kuijt (1963). In Washington and Oregon, it occurs east of the Cascade Crest but is common on the Pacific slope from the Umpqua River drainage south to about latitude 40°N in northern California (California Forest Pest Council 1968). The report by Palmer and others (1983) of A. douglasii from the headwaters of the North Fork of the American River in Placer County, California, has not been confirmed and is probably in error. This dwarf mistletoe is common in eastern Washington, eastern Oregon, Idaho, western Montana, Idaho, Utah, Colorado, and New Mexico. It is rare in Nevada (Wheeler Peak, Hawksworth 1965c), Wyoming (Teton County, Hawksworth 1965c), and Texas (Guadalupe Mountains, Warnock 1974). The distribution of the dwarf mistletoe in Mexico is poorly known, and it is probably more widespread than the few available records in Chihuahua, Coahuila, Durango, and Nuevo León suggest (fig. 16.49).

The distribution of this species exhibits some interesting peculiarities. For further details refer to chapter 5.

Distribution maps of Arceuthobium douglasii have been published for British Columbia (Wood 1986), Montana (Dooling and Eder 1981), Utah (Albee and others 1988), Colorado (Hawksworth 1987c), New Mexico (Martin and Hutchins 1980), and California (Kuijt 1960a).

The altitudinal range of this dwarf mistletoe is correlated with latitude, and it occurs as low as 300 m near Lytton in southern British Columbia and as high as 3,250 m on Cerro Potosí in Nuevo León, Mexico.

Discussion: Arceuthobium douglasii is one of the most distinctive and stable species in the genus. In spite of its extensive latitudinal distribution, we find no characteristics that suggest subspecific separation. Much of the confusion in the literature on this species is due to its supposed affinities with various members of the A. campylopodum complex (Gill 1935). Weir (1918a) discussed the occurrence of "a small purple-flowered form on Picea" in the Inland Empire that he (and we) consider to be merely A. douglasii on an unusual host.

This dwarf mistletoe is the smallest in western North America, but its typically systemic mode of infection produces large witches’ brooms (fig. 16.50) and causes severe growth loss and mortality in Pseudotsuga menziesii.

 

Arceuthobium durangense
Durangan Dwarf Mistletoe

13. A. durangense (Hawksworth & Wiens) Hawksworth & Wiens, Phytologia 66: 7, 1989. TYPE COLLECTION: MEXICO: Durango: 58 km west of El Salto on Route 40, on Pinus durangensis, Hawksworth & Wiens 3507 in 1963 (Holotype COLO! Isotypes: DS, F, FPF, INIF, MEXU, MO, US). A. vaginatum subsp. durangense Hawksworth & Wiens, Brittonia 17: 230, 1965.

Description: Mean shoot height ca. 20–30 (max. 50) cm, bright orange; older shoots becoming pendulous (fig. 16.51). Basal diameter of dominant shoots 4–8 (mean 6) mm. Third internode 9–22 (mean 17.9 ± 4.1) mm long, 3.5–6.0 (mean 4.5) mm wide (3 collections), length/width ratio 3.3:1. Internodes often slightly swollen at base. Staminate flowers ca. 2.5 mm long, 2.5 mm across, segments ca. 1 mm long, 1 mm wide. Pollen polar diameter 18–19 (mean 18) µm; equatorial diameter 20–21 (mean 21) µm; polar/equatorial diameter ratio 1:1.15; spine height (1.5 µm) about 1.5 x wall thickness (1.0 µm) (1 collection). Mature fruit 7 x 3.5 mm; bluish; proximal portion ca. 4 mm long. Seeds 4 x 1.5 mm. n = ?

Phenology: Time of meiosis unknown (probably in February). Anthesis usually in April. Fruits mature from mid-July to September of the year following pollination; maturation period averages 15–18 months.

Hosts: Pinus douglasiana, P. durangensis, P. montezumae, P. michoacana, and P. pseudostrobus are the principal hosts. Pinus herrerai is secondarily parasitized when it occurs near infected principal hosts. Pinus oocarpa is reputed to be a host in Jalisco and southeastern Durango, but this requires confirmation. We have seen no collections from this tree. In the Sierra de Quilla, Jalisco, P. oocarpa growing with infected P. montezumae and P. pseudostrobus was not infected by Arceuthobium durangensis. The following pines have been observed growing near infected trees but were not themselves parasitized: P. leiophylla var. leiophylla, P. lumholtzii, and P. teocote.

Distribution: Mexico (Durango, Sinaloa, and Jalisco) (fig. 16.52). This rather local dwarf mistletoe occurs in extreme western Durango and adjacent Sinaloa on the western edge of the escarpment near the crest of the Sierra Madre Occidental and in the Sierra de Quilla in Jalisco. It probably occurs in other areas along the western slopes of the Sierra Madre Occidental, e.g., in Nayarit, but this region is difficult of access and little collected. Elevational range is 1,450–2,750 m.

Discussion: We originally classified this taxon as a subspecies in the Arceuthobium vaginatum complex, but our subsequent field studies have convinced us that it is a distinct species (Hawksworth and Wiens 1989). This dwarf mistletoe is not sympatric with either subspecies of A. vaginatum, and it differs from A. vaginatum subsp. vaginatum by its bright orange shoots and from A. vaginatum subsp. cryptopodum by its larger size, brighter color, branching pattern of the staminate spikes, larger fruit, and geographical distribution.

 

Arceuthobium gillii
Chihuahua Pine Dwarf Mistletoe

14. A. gillii Hawksworth & Wiens, Brittonia 16: 55, 1964. TYPE COLLECTION: ARIZONA: Cochise County: Huachuca Mountains, 0.8 km east of Reef, on Pinus leiophylla var. chihuahuana, Hawksworth & Lightle 236, in 1962 (Holotype COLO!, Isotypes: FPF, MO, ARIZ, DS).

Description: Mean shoot height ca. 8–15 (max. 25) cm, greenish brown, flabellately branched (fig. 16.53). Basal diameter of dominant shoots 2.5–8.0 (mean 4) mm. Third internode 5–18 (mean 10.7 ± 3.4) mm long, 2.0–4.5 (mean 2.8) mm wide (9 collections), length/width ratio 3.8:1. Staminate flowers 3.5 mm long, 2.5–4.0 (mean 3.2) mm across. Pollen polar diameter 19–25 (mean 22) µm; equatorial diameter 24–28 (mean 26) µm; polar/equatorial diameter ratio 1:1.19; spine height (1.7 µm) slightly greater than wall thickness (1.3 µm) (4 collections). Pistillate flowers ca. 1.5 mm long, 1 mm across. Mature fruit 4–5 mm long, 2–3 mm wide, the proximal portion of fruit conspicuously glaucous. Seeds 3.1 x 1.4 mm. n = 14.

Phenology: Meiosis in September. Anthesis usually in March and April. Fruits mature in October of the year following pollination; maturation period averages ca. 19 months, the longest in the genus. Seed germination begins in April (Chihuahua).

Hosts: The principal and only commonly infected hosts are Pinus leiophylla var. chihuahuana, P. lumholtzii, and P. herrerai. However, P. leiophylla var. leiophylla is about equally susceptible to the parasite but not common within the range of Arceuthobium gillii. In western Chihuahua, this dwarf mistletoe rarely parasitizes P. cooperi and P. arizonica var. arizonica. Pinus ponderosa var. scopulorum and P. engelmannii frequently occur in association with infected Pinus leiophylla var. chihuahuana in Arizona, but these two species are not parasitized.

Distribution: United States (Arizona and New Mexico) and Mexico (Chihuahua, Durango, Sinaloa, and Sonora) (fig. 16.54). This dwarf mistletoe occurs in southeastern Arizona (Santa Catalina, Rincon, Santa Rita, Huachuca, and Chiricahua Mountains) (Hawksworth and Wiens 1964) and the Animas Mountains in southwestern New Mexico (Hawksworth and Weiss 1975). It is most common in western Chihuahua but is also distributed in adjacent northern and eastern Sonora, northern Durango, and northeastern Sinaloa.

We have examined several additional populations of Pinus leiophylla var. chihuahuana in New Mexico (Bear Mountain near Silver City and in the Mule and Peloncillo Mountains) but Arceuthobium gillii was not found. Several localities in Arizona where P. leiophylla var. chihuahuana occurs have also been examined, but no A. gillii were observed in the Pinaleno (Mount Graham) and Pinal Mountains nor below the Mogollon Rim near Colcord Mountain, Gila County. Elevational range is from 1,700 m in southern Arizona to 2,650 m in southern Chihuahua.

Discussion: This dwarf mistletoe is named for Lake S. Gill. It had long been confused with Arceuthobium vaginatum subsp. cryptopodum, but it differs in host preference, phenology, and its conspicuously glaucous fruits (Hawksworth and Wiens 1964). These two dwarf mistletoes are usually separated by at least 300 m of elevation in Arizona and New Mexico. They co-occur, however, in several areas in central Chihuahua, but there was no evidence of hybridization.

A characteristic feature of Arceuthobium gillii is its strong sexual dimorphism—staminate plants are tall and openly branched and the pistillate plants are small and densely branched.

This dwarf mistletoe causes open, non-systemic witches’ brooms similar to those induced by Arceuthobium nigrum. It causes serious mortality in Pinus leiophylla var. chihuahuana and P. lumholtzii in Chihuahua, particularly on poor, rocky sites.

 

Arceuthobium globosum

15. A. globosum Hawksworth & Wiens, Brittonia 17: 223, 1965. TYPE COLLECTION: MEXICO: Durango: 2.4 km east of El Salto on Route 40, on Pinus cooperi, Hawksworth & Wiens 3414 in 1963 (Holotype COLO! Isotypes: DS, F, FPF, INIF, MEXU, MO, US).

Description: Shoot height ca. 20–50 (max. 70) cm, yellow to greenish, flabellately branched. Basal diameter of dominant shoots 3–48 mm. Third internode 4–37 mm long, 2–24 mm wide. Staminate flowers ca. 3.5–5.0 mm long, 3.0–3.5 mm across; perianth 3- or 4-merous; same color as shoots; segments ca. 1.3 mm long, 1.0 mm wide. Mean anther diameter 0.6 mm, centered 0.9 mm from tip of segment. Pollen polar diameter 19–23 (mean 22) µm; equatorial diameter 24–28 (mean 26) µm; polar/equatorial diameter ratio 1:1.18; spine height (ca. 1.9 µm), ca. 2 x wall thickness (ca. 1.0 µm) (4 collections). Pistillate flowers ca. 1.5 mm long, 1.5 mm across. Mature fruit ca. 5–7 mm long, ca. 3–4 mm wide; proximal portion ca. 3.5 mm long, with pedicels 4.0–5.0 mm long. Seeds 5 x 2 mm. n = 14.

Hosts: Common on pines of subgenus Diploxylon.

Discussion: Previously, we (Hawksworth and Wiens 1972) noted that there was considerable variation within this dwarf mistletoe and that further field research was needed to determine whether the variation was sufficiently consistent to warrant the segregation of additional taxa. Subsequent studies in Mexico, Guatemala, and Belize have shown that the former Arceuthobium globosum (sensu lato) consists of five taxa (Hawksworth and Wiens 1977, Wiens and Shaw 1994):

Key to the subspecies:

1. Plants yellowish; shoots usually 15–20 (max. 50) cm tall, less than 1 cm in diameter at the base; witches’ brooms not formed; shoots usually only on the host branches (not on trunk); northern Mexico

15a. A. globosum subsp. globosum

1. Plants greenish to yellow-green, typically dark at the base of older shoots; shoots usually 25–40 (max. 70) cm tall, greater than 1 cm in diameter at the base; witches’ brooms usually induced; shoots on branches, but also frequently on the lower main trunks; central and southern Mexico to the highlands of western Guatemala

15b. A. globosum subsp. grandicaule.

 

Arceuthobium globosum subsp. globosum
Rounded Dwarf Mistletoe

15a. A. globosum Hawksworth & Wiens subsp. globosum

Description: Shoots 15–20 (max. 50) cm high, bright yellow, flabellately branched (fig. 16.55). Basal diameter of dominant shoots 3–10 (mean 7) mm. Third internode 19 mm long, 4 mm wide. Staminate flowers ca. 4 mm wide. Mature fruit 5 x 2.5 mm. Seeds 4 x 2 mm. n = ?

Phenology: Anthesis usually March–April. Fruits mature June–July; maturation period averages 15–16 months.

Hosts: The principal hosts are Pinus cooperi, P. engelmannii, and P. durangensis. Pinus arizonica is occasionally parasitized, but it appears to be less susceptible to infection than the three principal hosts. Pinus teocote is a rare host. We have classified the host near the summit of Pico Candelaria (2,650 m) south of Chuchuichupa, Chihuahua, as P. cooperi, but the trees exhibit some characteristics of P. rudis.

Distribution: Mexico (Sonora, Chihuahua, Durango, and Jalisco) (fig. 16.56). This subspecies is widely distributed in the pine forests of the Sierra Madre Occidental from northwestern Chihuahua and adjacent Sonora, through Durango to northern Jalisco. Elevational range is 2,300–2,800 m.

Discussion: This dwarf mistletoe is characterized by its bright yellow, globose clusters and absence of witches’ broom formation.

 

Arceuthobium globosum subsp. grandicaule
Large-Stemmed Dwarf Mistletoe

15b. A. globosum Hawksworth & Wiens subsp. grandicaule Hawksworth & Wiens, Brittonia 29: 413, 1977. TYPE COLLECTION: MEXICO: Mexico: km-59 on Highway 59, 35 km east of Zitácuaro, on Pinus montezumae, Hawksworth & Player 1607 in 1975 (Holotype US! Isotypes: ENCB, FPF, INIF, MO, UNAM, UT).

Description: Shoots 18–50 (max. 70, mean 25) cm tall, yellow green, typically dark at the base of older shoots, flabellately branched. Basal diameter of dominant shoots 10–48 (mean 17) mm. Third internode 14–37 (mean 27) mm long, 3–20 (mean 7) mm wide. Staminate flowers ca. 5 mm wide, 4-merous. Mature fruits 6–7 mm long, 3.5 mm wide. Seeds 5 x 3 mm. n = 14.

Phenology: Meiosis in December. Anthesis from January–May, with peak March–April. Fruits maturing July–October. Detailed studies by Escudero and Cibrián (1985) in the Sierra Nevada between the states of Mexico and Puebla showed that seed dispersal lasted for 95 days from early July to early November, with a peak from mid-August to mid-September; maturation period averages 16–18 months.

Hosts: This subspecies has one of the broadest host ranges of any dwarf mistletoe. It infects at least 12 species of Mexican pines, all of which appear to be about equally susceptible: Pinus douglasiana, P. durangensis, P. hartwegii, P. lawsonii, P. maximinoi, P. michoacana, P. montezumae, P. patula, P. pringlei, P. pseudostrobus, P. teocote, and P. rudis. Valdivia (1964) studied the hosts of Arceuthobium globosum subsp. grandicaule in northeastern Michoacán, listing them in decreasing order of susceptibility: P. maximinoi (as P. tenuifolia), P. pseudostrobus, P. pseudostrobus f. protuberans, P. montezumae, P. michoacana, P. durangensis, P. douglasiana, P. lawsonii, P. pringlei, P. teocote, and P. rudis. Pinus leiophylla and Abies religiosa were recorded as immune.

The hosts of Arceuthobium globosum subsp. grandicaule in Guatemala are poorly known. However, Pinus pseudostrobus, P. maximinoi, P. rudis, and Cupressus sp. are reported to be infected there (Standley and Steyermark 1946). This report on Cupressus sp. is the only report of Arceuthobium on a host other than the Pinaceae in the Western Hemisphere and requires confirmation. The putative collection on Cupressus (Steyermark 4904 in 1942 from the Sierra Cuchumatanes) has no host material. If the report were correct, the occurrence of A. globosum subsp. grandicaule on Cupressus would be rare. We did not observe any infections on Cupressus during our field studies in this region, even though the tree was common in many stands where P. rudis was heavily parasitized by A. globosum subsp. grandicaule.

Distribution: Mexico (Jalisco, Michoacán, Mexico, Hidalgo, Distrito Federal, Guerrero, Puebla, Tlaxcala, Veracruz, and Oaxaca) and Guatemala (Huehuetenango) (fig. 16.56). Arceuthobium globosum subsp. grandicaule is the most abundant dwarf mistletoe in central Mexico, and it is also locally common in the highlands of western Guatemala. Elevational range is 2,450–4,000 m (Hernandez and others 1992).

Discussion: This subspecies has the largest shoots in the genus, reaching a height of 70 cm and with a basal diameter of 5 cm.

Seed dispersal was studied by Escudero and Cibrián (1985). They showed that the maximum distance of seed dispersal was 15 m but that 91% of the seeds traveled less than 8.7 m. More than 7.3 million seeds per hectare landed on seed traps on the ground in stands of Pinus hartwegii. This number does not include the large proportion of seeds that were undoubtedly intercepted by the trees.

Valdivia (1964), who surveyed 400,000 ha of pine forest in northeastern Michoacán, reported that Arceuthobium globosum was present on nearly 40% of the forest area.

 

Arceuthobium guatemalense
Guatemalan Dwarf Mistletoe

16. A. guatemalense Hawksworth & Wiens, Brittonia 22: 267, 1970. TYPE COLLECTION: GUATEMALA: Department Huehuetenango, Sierra Cuchumatanes, on road from Huehuetenango to Santa Eulalia; 54 km north of Huehuetenango (10 km south of San Juan Ixcoy), on Pinus ayacahuite, Hawksworth & Wiens 1221, September 11, 1969. (Holotype US! Isotypes: COLO, DS, EAP, FPF, MO, UT).

Description: Mean shoot height ca. 1–3 cm on systemic witches’ brooms, but shoots on non-systemic infections up to ca. 7 cm high; living shoots greenish to purple, yellow to brown when dried, flabellately branched (fig. 16.57). Basal diameter of dominant shoots 2.0–2.5 mm. Third internode 8–15 (mean 11.4 ± 2.8) mm long, 1.5–2.0 (mean 1.7) mm wide; length/ width ratio 6.7:1 (3 collections). Staminate flowers ca. 2 mm across; perianth 2- or 3-merous, segments 0.9 mm long, 0.7 mm wide. Mean anther diameter 0.5 mm. Pollen polar diameter 21 µm and mean equatorial diameter 24 µm; polar/equatorial diameter ratio 1:1.14; spine height (ca. 1.5 µm), wall thickness (ca. 1.0 µm) (1 collection, few grains available). Mature fruit ca. 3.5–4.0 mm long, 1.5–2.0 mm wide; distal portion ca. 1.2 mm long; dark green, glabrous, with a slightly swollen ring at the base of the fruit where it joins the pedicel. Seeds 2.0 x 0.8 mm. n = ?

Phenology: Time of meiosis unknown. Anthesis apparently in August and early September. Fruits mature in September; maturation period ca. 12–13 months. Seed germination in September.

Hosts: Known only on Pinus ayacahuite var. ayacahuite. The only other pine in subgenus Haploxylon that occurs within the range of Arceuthobium guatemalense is P. chiapensis. This tree, however, usually grows at lower elevations than P. ayacahuite (Martínez 1948). These two species, however, co-occur from about 2,700–2,800 m just north of the Continental Divide on Highway 175 north of Ixtlán, Oaxaca (Hawksworth and Wiens 1977), but Arceuthobium guatemalense was not present in these mixed stands.

Distribution: Mexico (Oaxaca and Chiapas) and Guatemala (Huehuetenango and Totonicapán) (fig. 16.58). This distinctive species is known only from the high mountains of western Guatemala and southern Mexico. It is common at the type locality in the vicinity of Santa Eulalia, in the Sierra Cuchumatanes in Guatemala. Elevational range is poorly known. Our collections were made between 2,450–3,100 m.

Discussion: The consistent formation of systemic witches’ brooms is a distinctive characteristic of this species, and brooms sometimes measure 3–5 m across (fig. 16.59). An unusual feature of these witches’ brooms is that the shoots of the dwarf mistletoe are consistently formed on 1-year-old host shoots and, in some cases, we found buds on the current year’s growth (September). This species causes extensive damage and considerable mortality to Pinus ayacahuite.

 

Arceuthobium hawksworthii
Hawksworth’s Dwarf Mistletoe

17. A. hawksworthii Wiens & C. G. Shaw III, Journal of Idaho Academy of Science 30 (1): 25-32. 1994. TYPE COLLECTION: BELIZE: Cayo District, Mountain Pine Ridge Region, 2.4 km south of Cooma Cairn Lookout, elevation 940 m, on Pinus caribaea var. hondurensis, Wiens and Shaw 7768, February 16, 1994 (Holotype US! Isotypes FPF, MO, UC).

Description: Shoot height 10–20 (max. 30) cm, pale yellow to light green, flabellately branched (fig. 16.60 and 16.61). Basal diameter of dominant shoots 3–5 mm. Third internode 10–19 (mean 14) mm long, 2–4 mm wide. Staminate flowers ca. 2.5 mm long, 3 mm across; perianth mostly 3-merous or occasionally 4-merous, adaxial surface dark red or purple, 1.2–1.5 mm long, 1.0–1.5 mm wide; anther diameter ca. 0.75 mm, sepals, stigma, and subtending bracts often dark brown to gray when flowering, giving the pistillate flowers a dark appearance. Mature (?) fruit 4.0 mm long, 1.5 mm wide, equatorial line approximately medial. n = 14.

Phenology: Meiosis probably in December and/or early January. Anthesis probably from mid-late January to early March, with a peak from mid-January to mid-February. Fruit dispersal was complete by late February, but a few fruits were present on previous collections (Carty 3) made sometime in June (exact date not specified). Fruit maturation therefore presumably requires about 3–4 months, the shortest period known for any dwarf mistletoe. If this is correct, then Arceuthobium hawksworthii likely has at least two flower and fruit crops annually and possibly a third (depending on whether it flowers during the wet season). This type of multiple, periodic pattern of flowering also occurs in other tropical species, e.g., A. juniperi-procera, A. nigrum, and A. abietis-religiosae (see discussion under those species). Arceuthobium aureum subsp. aureum produces new flowers continuously, at least during the dry season.

Hosts: The only known hosts are Pinus caribaea var. hondurensis and what appears to be P. oocarpa at higher elevations. Previous collections of Arceuthobium hawksworthii (Carty 3A) indicate these two pines may hybridize at higher elevations.

Distribution: Belize, known only from the Mountain Pine Ridge Region of Cayo District (fig. 16.62). Arceuthobium hawksworthii is not present in the coastal stands of P. caribaea south of Belize City. Pinus caribaea var. hondurensis also occurs at higher elevations in eastern Guatemala in the Poptún area, which is only about 75 km from the population in western Belize, and this area should be examined for the presence of A. hawksworthii. Elevational range is about 700–900 m. Arceuthobium hawksworthii does not occur in the lower 300 m of the elevational distribution of P. caribaea var. hondurensis in the Mountain Pine Ridge Region.

Discussion: This dwarf mistletoe commemorates Frank G. Hawksworth. The presence of a dwarf mistletoe in Belize has been known since 1957, but it was previously confused with Arceuthobium aureum subsp. aureum (table 16.3). The presence of red staminate flowers and its geographic distribution suggest affinities with the section Campylopoda series Rubra; but the lack of red pigment in the shoots, general stature, and presumed multiple flowering periods indicate a relationship with section Vaginata where it is placed provisionally (see tables 14.1 and 15.2).

 

Arceuthobium hondurense
Honduran Dwarf Mistletoe

18. A. hondurense Hawksworth & Wiens, Brittonia 22: 267, 1970. TYPE COLLECTION: HONDURAS: Department Francisco Morazán, Piedra Herrada, 22 km southeast of Tegucigalpa on road to Escuela Agricola Panamericana, on Pinus oocarpa, Hawksworth, Wiens, & Molina 1203, September 4, 1969 (Holotype US! Isotypes: EAP, COLO, DS, F, FPF, MO, UT).

Description: Mean shoot height ca. 14 (max. 21) cm, olive brown to grayish green, markedly glaucous; flabellately branched (fig. 16.63). Basal diameter of dominant shoots 3–9 (mean 5) mm; nodes of older shoots swollen; lateral branches of staminate plants at nearly right angles to the axis of the main shoot; third internode 7–12 (mean 9.1 ± 1.5) mm long, 2.5–4.0 (mean 3.2) mm wide; length/width ratio 6.1:1 (2 collections). Staminate flowers ca. 2.5 mm across; inner surface reddish, lower surface the same color as the shoots; perianth usually 3-merous (sometimes 2- or 4-merous), segments ca. 1.2 mm long, 0.8 mm wide; nectary with 2 large and 1 small lobe. Pollen polar diameter 18–23 (mean 20) µm, equatorial diameter 21–25 (mean 24) µm; polar/equatorial diameter ratio 1:1.21; spine height (ca. 2.5 µm) ca. 2 x wall thickness (1.1 µm) (2 collections). Pistillate flowers with stigmas exserted ca. 0.5 mm, with copious stigmatic exudate at anthesis. Mature fruit 5.5 x 3.0 mm, greenish glaucous; proximal portion ca. 4.0 mm long; stigma exserted. Seeds ca. 3.1 x 1.5 mm. n = 14.

Phenology: Meiosis in August or early September. Anthesis and fruit maturity in September; maturation period averages ca. 12 months.

Hosts: The only known hosts are Pinus oocarpa and P. maximinoi.

Distribution: Honduras and possibly El Salvador (fig. 16.62). The distribution of this species is poorly known, but it is presently one of the rarest dwarf mistletoes in the New World with the possible exception of Arceuthobium hawksworthii . Only four collections are known, all from Honduras. Three are from the same general area near Piedra Herrada, southeast of Tegucigalpa on the road to Escuela Agricola Panamericana. The fourth population occurs 200 km to the northwest near Choloma. A roadside reconnaissance of 225 km through forests of Pinus oocarpa in the Departments of Francisco Morazán, El Paraiso, and Comayagua failed to uncover additional populations of A. hondurense. Recently, Jerry Beatty (personal communication) traveled extensively in the pine forests of Honduras and noted that this dwarf mistletoe was exceedingly rare and limited to the localities noted above. The indiscriminate cutting of the original forests has apparently reduced the populations of A. hondurense in these areas. These lands now have been either converted to agriculture or consist of second-growth forests. Unless A. hondurense has a broader distribution and occurs in areas less subject to deforestation, the species is in danger of extinction.

James Davis of the New Mexico State Forest Service (personal communication.) observed a dwarf mistletoe on pines in the Monte Cristo area (Santa Ana Province) of northeastern El Salvador near the Honduran border. This could be Arceuthobium hondurense, but unfortunately no collections of this dwarf mistletoe were obtained. Elevational range is poorly known. Our collections were made between 1,200 and 1650 m.

Discussion: Originally, we believed Arceuthobium hondurense might be conspecific with A. bicarinatum, a species endemic to Hispaniola. Observations of living plants of both dwarf mistletoes, however, demonstrated discontinuities in many characters. The species nonetheless appear closely related. Arceuthobium hondurense also exhibits some similarities with A. rubrum and A. oaxacanum. Arceuthobium bicarinatum and A. hondurense occupy the southern distribution limits of Arceuthobium in the New World.

 

Arceuthobium laricis
Larch Dwarf Mistletoe

19. A. laricis (Piper) St. John, Flora of Southeast Washington and Adjacent Idaho: 115, 1937. TYPE COLLECTION: WASHINGTON: Kittitas County: North of Ellensburg, on Larix occidentalis, Brandegee 1071 in 1883 (Lectotype US! Isotypes: GH, PH, UC). Razoumofskya douglasii (Engelm.) Kuntze subsp. laricis Piper, Contributions U.S. National Herbarium 11: 223, 1906, nomen nudum. A. douglasii Engelm. var. laricis M. E. Jones, University of Montana Bulletin 61 (Biological Series 15): 25, 1910, nomen nudum. Razoumofskya laricis Piper, in Piper and Beattie, Flora Southeast Washington and Adjacent Idaho: 80, 1917. A. campylopodum Engelm. forma laricis (Piper) Gill, Transactions Connecticut Academy Arts and Sciences. 32: 202, 1935.

Description: Mean shoot height ca. 4 (max. 6) cm, mostly dark purple, flabellately branched (fig. 16.64). Basal diameter of dominant shoots 1.5–3.0 (mean 2) mm. Third internode 5–14 (mean 8.0 ± 2.0) mm long, 1.0–2.5 (mean 1.3) mm wide (12 collections), length/ width ratio 6.1:1. Staminate flowers 2.7 mm across; perianth mostly 3-merous (sometimes 4-merous); segments ca. 1.4 mm long, 1.1 mm wide. Mean anther diameter 0.5 mm, centered 0.5 mm from tip of segment. Pollen polar diameter 18–25 (mean 20) µm; equatorial diameter 21–27 (mean 23) µm; polar/equatorial diameter ratio 1:1.16; spine height (ca. 2 µm) about 2 x wall thickness (ca. 1.0 µm) (3 collections). Pistillate flowers ca. 1 mm long, 1 mm across. Mature fruit 4.5 x 2.5 mm; proximal portion ca. 2.5 mm long. n = 14.

Phenology: Meiosis in June. Peak anthesis from mid-July to late August, with extremes from early July to early September (fig. 16.65). Fruits usually mature in September, with extremes from early August to early October; maturation period averages 13–14 months.

Hosts: The principal and most commonly infected host is Larix occidentalis, but Tsuga mertensiana is also a principal host. Pinus contorta var. latifolia is a secondary host, and occasional hosts include Abies lasiocarpa and Pinus ponderosa var. ponderosa. Rare hosts are Abies grandis (Smith and others 1972), Picea engelmannii (Kuijt 1945b), Pinus albicaulis and P. monticola (Kuijt 1953, 1954b). Larix lyallii is no longer considered to be a natural host of Arceuthobium laricis. J. R. Weir’s two fragmented collections from the Bitterroot and Cabinet Mountains of Montana are clearly A. laricis, but the host was misidentified as L. lyallii. Recent field studies in these areas have discovered numerous infected L. occidentalis at lower elevations but no infected L. lyallii at higher elevations (R. M. Mathiasen, personal communication).

Exotic trees infected naturally by Arceuthobium laricis include Pinus banksiana (Graham 1959a), P. resinosa (Hawksworth and Wiens 1972), P. sylvestris (Graham and Leaphart 1961), and Picea abies (Hawksworth and Wiens 1972). Weir (1918a) has shown that, on the basis of seedling inoculations, Larix decidua (as L. europaea) and L. kaempferi (as L. leptolepis), are susceptible to infection by this dwarf mistletoe.

The occurrence of Arceuthobium laricis on Tsuga mertensiana in Idaho and Montana is of interest because this dwarf mistletoe will not infect T. heterophylla. Our observations in many mixed Larix–Tsuga stands in Oregon, Washington, Idaho, and Montana confirm that T. heterophylla is not infected by A. laricis. In mixed T. heterophylla–T. mertensiana stands in the Coeur d’Alene National Forest, Idaho, only the latter was infected by A. laricis. It is unknown if A. laricis is associated with T. mertensiana in the Cascade Mountains of Oregon and Washington.

Distribution: Canada (British Columbia) and the United States (Washington, Oregon, Idaho, and Montana) (fig. 16.66). Arceuthobium laricis occurs generally throughout the range of its principal host, Larix occidentalis, in southern British Columbia, east of the Cascade Mountains in Washington and northern Oregon, north and central Idaho, and western Montana.

Distribution maps have been published for Arceuthobium laricis from British Columbia (Wood 1986) and Montana (Dooling and Eder 1981). Elevational range is 650–2,250 m.

Discussion: Arceuthobium laricis is a serious pathogen of Larix occidentalis (Weir 1916a). Infection usually results in the formation of heavy but compact brooms; because larch branches are brittle, larger brooms are readily broken off (fig. 16.67). Surveys show that about two-thirds of L. occidentalis stands are infested on the Coeur d’Alene National Forest, Idaho, and the Kootenai National Forest, Montana (Graham 1959b, 1959c). On the Colville National Forest and adjacent private lands in northeastern Washington, infestation was 86% (Graham and Frazier 1962).

 

Arceuthobium littorum
Coastal Dwarf Mistletoe

20. A. littorum Hawksworth, Wiens & Nickrent, Novon 2: 206, 1992. TYPE COLLECTION: CALIFORNIA: Mendocino County: 4.8 km miles east of Novo on State Highway 20, on Pinus muricata, elevation 30 m, R. L. Mathiasen 8940, December 12, 1989. Holotype US! Isotypes: CAS, FPF, HSC, MO, UC).

Description: Shoots 8–20 (mean 12) cm, brown to yellow-brown, flabellately branched (fig. 16.68). Basal diameter of dominant shoots 2–5 (mean 3.5) mm. Third internode 10–20 (mean 15) mm long, 2–2.5 (mean 2.2) mm wide, mature fruits 4–5 mm long; staminate flowers mostly 4-merous. n = 14.

Phenology: The phenology is poorly known. Meiosis occurs in July, flowering begins in August, with peak anthesis probably occurring in September. Seed dispersal probably peaks in September or October.

Hosts: Pinus radiata and P. muricata are the primary hosts. It occasionally infects P. contorta var. bolanderi where this tree is associated with infected P. muricata.

Distribution: United States: (California: Mendocino, Sonoma, Marin, Monterey, and San Luis Obispo Counties) (fig. 16.69). Arceuthobium littorum is restricted to within about 10 km of the Pacific Ocean from Fort Bragg south to the Point Reyes area on Pinus muricata and along the central coast at Monterey and Cambria on P. radiata. It also parasitizes the small population of P. muricata surrounded by infected P. radiata on Huckleberry Hill near Monterey.

This dwarf mistletoe has been established in at least three locations in the Bay Area, presumably by transplantation of infected Pinus radiata: Monterey-Stanford Arboretum (Peirce 1905), North Berkeley (Offord 1964a), and Hillsborough (Butler 1992). Elevational range is from sea level to ca. 250 m.

Discussion: Previously, we included Arceuthobium littorum in A. occidentale (Hawksworth and Wiens 1972, 1984). Arceuthobium occidentale is primarily a parasite of Pinus sabiniana in the foothills surrounding the Central Valley of California and is not sympatric with A. littorum near the coast. Electrophoretic studies support the segregation of A. littorum as a species distinct from other members of the A. campylopodum–A. occidentale complex (Nickrent and Butler 1989, 1990). A primary feature for distinguishing A. littorum from A. occidentale is the production of large, non-systemic witches’ brooms by A. littorum (fig. 16.70); other differences between A. littorum and A. occidentale are presented in table 16.4.

 

Arceuthobium microcarpum
Western Spruce Dwarf Mistletoe

21. A. microcarpum (Engelmann) Hawksworth & Wiens, Brittonia 22: 268, 1970. TYPE COLLECTION: ARIZONA: Apache County: Sierra Blanca, on Picea engelmannii, Gilbert 112 in 1873 (Lectotype MO! Isotypes: ILL, US). A. douglasii Engelm. var.? microcarpum Engelm. in U.S. Geographical Survey West of 100th Meridian (Wheeler Report) 6: 253, 1878. Razoumofskya microcarpa (Engelm.) Wooton & Standley, Contributions U.S. National Herbarium 19: 179, 1915. Razoumofskya douglasii (Engelm.) Kuntze var. microcarpa (Engelm.) Tubeuf, Naturwissen-schaftliche Zeitschrift für Forstund Landwirtschaft 17: 216, 1919. A. campylopodum Engelm. forma microcarpum (Engelm.) Gill, Transactions Connecticut Academy Arts and Sciences. 32: 209, 1935.

Description: Mean shoot height ca. 5 (max. 11) cm, green to purple, flabellately branched (fig. 16.71). Basal diameter of dominant shoots 1.5–3.0 (mean 2) mm. Third internode 5–16 (mean 9.3 ± 2.2) mm long, 1–2 (mean 1.5) mm wide (9 collections), length/width ratio 6.2:1. Staminate flowers 2.3 mm across; perianth mostly 3-merous (rarely 4-merous); segments ca. 1.2 mm long, 1.0 mm wide. Mean anther diameter 0.4 mm, centered 0.6 mm from tip of segment. Pollen polar diameter 18–21 (mean 19) µm; equatorial diameter 21–23 (mean 22) µm; polar/equatorial diameter ratio 1:1.6; spine height (1.2 µm) approximately equal to the wall thickness (1.4 µm) (1 collection). Pistillate flowers ca. 1 mm long, 1 mm across. Mature fruit 3.5 x 2.0 mm; proximal portion ca. 2.5 mm long. Seeds 2.4 x 1.0 mm. n = 14.

Phenology: Meiosis in July. Anthesis in mid-August to early September, with extremes from late July to late September (fig. 16.72). Fruits mature in September, with extremes from late August to early October; maturation period averages 12–13 months.

Hosts: This dwarf mistletoe is a common and serious pathogen only on Picea engelmannii and P. pungens, but on the San Francisco Peaks (northern Arizona) it also parasitizes Pinus aristata (Mathiasen and Hawksworth 1980). Pinus strobiformis and Abies lasiocarpa var. arizonica are rarely infected, even where they are associated with infected Picea or Pinus.

Distribution: United States (Arizona and New Mexico) (fig. 16.73). Arceuthobium microcarpum has one of the more restricted distributions in the genus. In Arizona, the parasite occurs on the North Rim of the Grand Canyon, the San Francisco Peaks and nearby Kendrick Peak, White Mountains, and in the Pinaleno (Graham) Mountains, but not in the Chiricahua Mountains. In New Mexico, this dwarf mistletoe is present at several locations in the Mogollon Mountains, and it was discovered in the Sacramento Mountains, an outlier some 275 km from the closest known populations in the Mogollon Mountains (Mathiasen and Jones 1983).

A distribution map of Arceuthobium microcarpum in New Mexico was published by Martin and Hutchins (1980). Elevational range is 2,400–3,150 m (Acciavatti and Weiss 1974).

Discussion: The original specimens cited by Engelmann are Gilbert 100 and 102. The only specimen now at the Missouri Botanic Garden is Gilbert 112, which is labeled in Engelmann’s handwriting as the type. The original label for specimen 100 is in the same packet. Presumably, Gilbert 102 in the original citation should have been 112. The collections are now combined, so Gilbert 112 is our designate for a lectotype.

This localized species in Arizona and New Mexico is characterized by its near exclusive occurrence on Picea. Gill (1935) gave a much larger distribution for Arceuthobium microcarpum (to California, Idaho, and Montana). Hawksworth and Graham (1963) have shown, however, that Gill’s records are based on rare parasitism of spruce by other dwarf mistletoes (A. laricis, A. douglasii, A. americanum, A. tsugense, and, possibly, A. cyanocarpum).

This species induces small, dense witches’ brooms. Heavily infected trees may bear hundreds of such witches’ brooms. This dwarf mistletoe causes heavy mortality in stands of Picea pungens and, to a lesser extent, of P. engelmannii. This dwarf mistletoe is not found in the principal range of P. engelmannii in the central or northern Rocky Mountains.

 

Arceuthobium monticola
Western White Pine Dwarf Mistletoe

22. A. monticola Hawksworth, Wiens & Nickrent, Novon 2: 205, 1992. TYPE COLLECTION: OREGON: Josephine County, 11 km south of O’Brien on Old Gasquet Toll Road (Oregon Mountain Road); parasite of Pinus monticola, T. 13 S., R. 9 W., Section 9; latitude 42°1' N, longitude 123°46' W, elevation 650 m, D. Wiens 6575, 20 July 1987 (Holotype US! Isotypes: FPF, MO, ORE, OSC, UC).

Description: Shoots 5–10 (mean 7) cm tall, dark brown, flabellately branched (fig. 16.74). Basal diameter of dominant shoots 2–4 (mean 3) mm; third internode 8–15 (mean 12) mm long, 1.5–2.0 mm wide. Staminate flowers mostly 3-merous. Mature fruits 4.0–4.5 mm long, 2.0–2.5 mm wide. n = 14.

Phenology: The period of anthesis is poorly known but apparently occurs in late July–August. Fruits mature October–November; maturation period averages ca. 15 months.

Hosts: The principal and only commonly infected host is Pinus monticola. Pinus lambertiana is a secondary host, Picea breweriana an occasional host, and Pinus jeffreyi a rare host (only one case known).

Distribution: United States (Oregon and California) (fig. 16.75). Arceuthobium monticola is a local endemic in the Klamath Mountains of southwestern Oregon (Coos, Curry, and Josephine Counties) and the Siskiyou Mountains of adjacent northwestern California (Del Norte County and possibly in western Siskiyou County). Elevational range is 700–1,900 m.

Discussion: Previously, we included this taxon under Arceuthobium californicum (Hawksworth and Wiens 1972). Subsequent field and laboratory studies have demonstrated that it is a distinct species and not apparently sympatric with A. californicum, a species restricted to California. Arceuthobium monticola differs from A. californicum in its much darker shoot color, later flowering and seed dispersal periods, and occurrence on Pinus monticola, as opposed to P. lambertiana (table 16.2).

 

Arceuthobium nigrum
Black Dwarf Mistletoe

23. A. nigrum (Hawksworth & Wiens) Hawksworth & Wiens, Phytologia 66: 9, 1989. TYPE COLLECTION: MEXICO: Durango, 51 km east of El Salto on Route 40, on Pinus teocote, Hawksworth & Wiens 304 in 1963 (Holotype COLO! Isotypes: FPF, MEXU, MO, US). A. gillii subsp. nigrum Hawksworth & Wiens, Brittonia 17: 233, 1965.

Description: Mean shoot height ca. 15–35 (max. 45) cm, dark brown to black (fig. 16.76). Basal diameter of dominant shoots 3–8 (mean 5) mm. Third internode 5–19 (mean 10.8 ± 3.8) mm long, 2.5–6.0 (mean 3.7) mm wide (6 collections), length/width ratio 2.9:1. Staminate flowers ca. 3 mm long, 3.5 mm across. Pollen polar diameter ca. 20 µm, equatorial diameter ca. 23 µm; spine height (ca. 2.0 µm), ca. 2 x wall thickness (1.0 µm). Mature fruit 6–9 (mean 7) mm long, ca. 3.5 mm wide, proximal portion ca. 2–3 mm. Seeds 3.5 x 1.3 mm. n = 14.

Phenology: This dwarf mistletoe is unusual in Arceuthobium because it has distinct flowering periods in March–April and September–October. We observed seed dispersal in September; presumably these fruits originated from the crop of flowers pollinated the previous September–October; however, when the seeds from the March–April pollinations are mature is unknown.

Hosts: This dwarf mistletoe is most common on pines of subsection Leiophyllae (P. leiophylla vars. leiophylla and chihuahuana and P. lumholtzii). However, P. oaxacana, P. patula, P. teocote, and P. lawsonii are also highly susceptible and are classed as principal hosts. Pinus montezumae and P. pseudostrobus are occasional hosts and P. cooperi and P. arizonica var. arizonica are rare hosts.

Distribution: Mexico (Durango, Zacatecas, Guanajuato, Querétaro, Hidalgo, Michoacán, Mexico, Tlaxcala, Puebla, Veracruz, Oaxaca, Chiapas) and possibly in western Guatemala (Hawksworth and Wiens 1977) (fig. 16.77). This mistletoe has been recently reported from the northeastern slope of Volcán la Malintzi (Malinche), Tlaxcala (Hernandez and others 1992). This species is common on pines in central and eastern Mexico. Elevational range is 1,800–2,800 m.

Discussion: Arceuthobium nigrum resembles A. gillii, a species occurring in Chihuahua, northern Durango, southeastern Arizona, and southwestern New Mexico. Both species possess markedly glaucous fruits, strong sexual dimorphism (open, divaricate branching in staminate plants versus densely branched in pistillate plants), and parasitize members of subsection Leiophylla. For these reasons, we originally classified A. nigrum as a subspecies of A. gillii (Hawksworth and Wiens 1965, 1972). Subsequent studies, however, have shown that they differ in a sufficient number of characters that specific status is warranted (Hawksworth and Wiens 1989).

Arceuthobium nigrum is a larger plant than A. gillii and has dark green to black shoots 15–35 (max. 45) cm high, whereas those of A. gillii are only 8–15 (max. 25) cm tall, and colored greenish brown. Arceuthobium nigrum also has the two flowering periods (March–April and September–October) compared to only one for A. gillii (March–April). To our knowledge, these species are not sympatric, although they occur in the same mountain range near Tepehuanes in northern Durango. Even there, however, they are separated both by elevation and by hosts. Arceuthobium gillii occurs at lower elevations (below 2,200 m) on Pinus leiophylla var. chihuahuana and P. lumholtzii, and A. nigrum occurs at higher elevations (over 2,600 m) on P. teocote and P. leiophylla var. leiophylla.

Arceuthobium oaxacanum Oaxacan Dwarf Mistletoe

24. A. oaxacanum Hawksworth & Wiens, Phytologia 66: 7, 1989. TYPE COLLECTION: MEXICO: Oaxaca: Mpio. Tamazulapán, 13 km south of Mihuatlán (and 0.5 km south on side road) on Puerto Angel Road (Route 175), on Pinus lawsonii, Wiens & Calvin 6003 in 1985 (Holotype US! Isotypes: ENCB, FPF, INIF, MO, UNAM).

Description: Shoots 8–20 (mean 12) cm tall, pale brown to reddish, flabellately branched (fig. 16.78). Basal diameter of dominant shoots 2–4 (mean 3) mm. Third internode 10–17 (mean 12) mm long and 2–3 mm wide (5 collections). n = ?

Phenology: Anthesis in July. Fruits mature in August of the following year; maturation period averages 13 months.

Hosts: Pinus lawsonii, P. michoacana, and P. pseudostrobus are principal hosts and are about equally susceptible; P. oaxacana is an occasional host.

Distribution: Mexico (Oaxaca) (fig. 16.79). This species is known from only three localities (two in southern Oaxaca south of Miahuatlán and another in northern Oaxaca near Ixtlán). Elevational range is 2,000–2,200 m.

Discussion: Arceuthobium oaxacanum was first discovered by R. S. Peterson in 1972 (Hawksworth and Wiens 1977). We originally classified it as an extreme disjunct (ca. 1200 km) of A. rubrum, but we noted that plants from Oaxaca were larger than those of A. rubrum from the Sierra Madre Occidental in Durango (Hawksworth and Wiens 1977). Subsequent studies of both taxa, however, indicate that they are distinct species (Hawksworth and Wiens 1989). Some characters of A. oaxacanum and A. rubrum are compared in table 16.5. In general, A. oaxacanum (fig. 16.78) is a larger, lighter colored, more openly branched plant and causes larger witches’ brooms (fig. 16.80) than A. rubrum. Arceuthobium oaxacanum parasitizes principally Pinus lawsonii, P. michoacana, and P. pseudostrobus, and occasionally P. oaxacana; none of these pines occur within the range of A. rubrum. Arceuthobium oaxacanum, A. rubrum, and A. bicarinatum are the only species in the genus that have reddish shoots.

 

Arceuthobium occidentale
Digger Pine Dwarf Mistletoe

25. A. occidentale Engelmann, U.S. Geographical Survey West of 100th Meridian (Wheeler Report) 6: 375, 1878 (see also Botany of California 2: 107, 1880). TYPE COLLECTION: CALIFORNIA: Kern County; Walker’s Basin, on Pinus sabiniana, Rothrock 429 in 1875 (Lectotype MO! Isotype US). Razoumofskya occidentale (Engelm.) Kuntze, Revision of Genera of Plants 2: 587, 1891.

Description: Mean shoot height ca. 8 (max. 17) cm, yellowish, glaucous, flabellately branched (fig. 16.81). Basal diameter of dominant shoots 1.5–5.0 (mean 2) mm. Third internode 7–18 (mean 12.7 ± 2.0) mm long, 1.5–3.5 (mean 1.8) mm wide (11 collections), length/width ratio 7.1:1. Staminate flowers 3.0 mm across; perianth 3- or 4-merous; segments ca. 1.5 mm long, 1.0 mm wide. Mean anther diameter 0.5 mm, centered 0.6 mm from tip of segment. Pollen polar diameter 18–23 (mean 20) µm; equatorial diameter 21–25 (mean 23) µm; polar/equatorial diameter ratio 1:1.16; spine height (ca. 2 µm) ca. 2 x wall thickness (1.1 µm) (3 collections). Mature fruit 4.5 x 3.0 mm; proximal portion ca. 3.0 mm long. Seeds 2.6 x 1.0 mm. n = 14.

Phenology: Meiosis in August. Peak anthesis from late September to late November, extremes from early September to early December (fig. 16.82). Most fruits mature from mid-October to mid-January, with extremes from late September to early February; maturation period averages ca. 13 months.

Hosts: Pinus sabiniana is the most common host. Pinus coulteri and P. attenuata are often parasitized where they are associated with infected P. sabiniana. Pinus ponderosa and P. jeffreyi may also be infected where they occur in areas outside the range of their typical parasite, Arceuthobium campylopodum (e.g., in the Coast Range of central California) and they are associated with infected P. sabiniana. Arceuthobium occidentale, however, infrequently infects P. ponderosa in the Sierra Nevada, even though this tree is frequently associated with P. sabiniana infected by A. occidentale.

Hedgcock and Hunt (1917) successfully inoculated seedlings of Pinus banksiana, P. bungeana, P. caribaea, P. pinea, and P. virginiana with Arceuthobium occidentale. Kuijt (1960a) inoculated P. torreyana with this dwarf mistletoe, and Scharpf (1969a) also successfully inoculated P. radiata with A. occidentale. We have found this latter host–parasite combination occurring spontaneously at Mt. Hamilton, California, where planted P. radiata was parasitized by A. occidentale when it grew near infected P. sabiniana. Some exotic pines recently found to be infected by this dwarf mistletoe in southern California are P. halepensis, P. pinea, and P. thunbergii (Scharpf and Hawksworth 1986).

Distribution: United States (California) (fig. 16.83). This dwarf mistletoe is a California endemic and commonly occurs on Pinus sabiniana throughout the foothills and low mountains surrounding the Central Valley of California and along the Coast Ranges from the Mt. Pinos area (Ventura County) north to Mendocino County. Elevational range is ca. 30–1,200 m (the higher elevations occurring in the southern Sierra Nevada).

Discussion: Engelmann’s 1878 report specifically lists only the specimen Rothrock 429 on Pinus sabiniana, but he also mentions that it occurs on P. ponderosa. No specimens are cited by Engelmann (1880) in his additional comments on the species. Engelmann thus apparently considered Arceuthobium occidentale to include both the parasite on P. ponderosa (our A. campylopodum) and P. sabiniana (our A. occidentale sensu stricto). Engelmann’s writings are not clear as to what he considered to be the relationship of A. occidentale to his previously described A. campylopodum (in Gray 1850). Traditionally, the name A. campylopodum has been applied to the dwarf mistletoe associated with Pinus ponderosa. We maintain this usage.

Since the publication of our previous monograph (Hawksworth and Wiens 1972), we have continued our studies of the Arceuthobium campylopodum–A. occidentale complex (Hawksworth and others 1989). We now recognize four taxa in this complex (see also discussions under A. campylopodum, A. littorum, and A. siskiyouense).

Even though Pinus sabiniana typically occurs in open, savanna-like forests, Arceuthobium occidentale is widely distributed in these stands. Isolated, infected trees more than 100 m away from the closest infected trees are sometimes found, which suggests that bird vectors (possibly phainopepla) are involved in long-distance seed transport of this dwarf mistletoe. Studies of dispersal in A. occidentale would be a useful contribution to our knowledge of this species.

 

Arceuthobium pendens
Pendent Dwarf Mistletoe

26. A. pendens Hawksworth & Wiens, Brittonia 32: 348, 1980. TYPE COLLECTION: MEXICO: San Luis Potosí, Mpio. Villa de Reyes, Sierra San Miguelito, 5 km west of the village of Cerritos, on Pinus discolor, Hawksworth, Wiens & Bailey 1877 in 1979 (Holotype US! Isotypes: ENCB, FPF, INIF, MO, UNAM, UT).

Description: Mean shoot height ca. 15 (max. 22) cm, light green, flabellately branched (fig. 16.84). Basal diameter of dominant shoots 1.5–3.5 (mean 2) mm. Third internode 12–20 (mean 16) mm long, 1–2 (mean 1.5) mm wide. Staminate flowers ca. 2.5 mm across, 3-merous. Pistillate flowers unknown. n = ?

Phenology: Time of meiosis unknown. Peak anthesis in September? Fruit maturity from June to September (Cházaro and Oliva 1987).

Hosts: Known only on pinyons—Pinus discolor in San Luis Potosí and P. orizabensis in Veracruz and Puebla. At the type locality in the Sierra San Miguelito (San Luis Potosí), this dwarf mistletoe parasitized P. discolor but did not infect associated P. cembroides.

Distribution: Mexico (San Luis Potosí, Veracruz, and Puebla) (fig. 16.85). The distribution of this dwarf mistletoe is poorly known. It has been collected in the areas mentioned above, i.e., the Sierra San Miguelito (San Luis Potosí) and the Frijol Colorado (Veracruz), and also on the nearby Cerro Pizzaro, Puebla (Cházaro and Oliva 1987). Elevational range is 2,250-2,700 m.

Discussion: This dwarf mistletoe likely occurs in other areas in Mexico and on other pinyons. The species exhibits striking sexual dimorphism—pistillate plants are densely branched and typically less than 8 cm tall and mature staminate plants are openly branched, and pendant, 15–20 cm long (fig. 16.84). A unique feature of this species, at least in the population at Sierra Miguelito, is that only the staminate plants appear to induce systemic witches’ brooms.

 

Arceuthobium pusillum
Eastern Dwarf Mistletoe
*

27. Arceuthobium pusillum Peck, Transactions Albany Institute 7: 191, 1872. TYPE COLLECTION: NEW YORK: Rensselaer County: Sandlake, on Picea mariana, Peck in 1871 (Isotype MO!). Arceuthobium minutum Engelmann, Bulletin of the Torrey Botany Club 2: 43, 1871, nomen nudum. Arceuthobium oxycedri (DC.) M. Bieb. var. abigenium Wood, American Botanist and Florist, p. 446, 1871. Arceuthobium abigenium Wood, Class-Book of Botany. p. 832, 1881. Razoumofskya pusilla (Peck) Kuntze, Revision of Genera of Plants. 2: 587, 1891. Razoumofskya minuta (Engelm.) Kuntze, Revision of Genera of Plants. 2: 587, 1891.

Description: Mean shoot height ca. 1 (max. 3) cm, green to brown, usually without secondary branching, but the rare branching that does occur is flabellate (fig. 16.86). Basal diameter of dominant shoots ca. 1.0 mm. Third internode 1–4 (mean 1.9 ± 0.8) mm long, 0.5–1.5 (mean 1.0) mm wide (17 collections), length/width ratio 1.9:1, often markedly wider at top than at base. Pistillate shoots often longer than the staminate. Staminate flowers 1.7–2.2 (mean 1.8 mm) across; perianth mostly 3-merous (sometimes 2- or 4-merous); segments ca. 0.8 mm long, 0.7 mm wide. Mean anther diameter 0.4 mm, centered 0.5 mm from tip of segment. Pollen polar diameter 19–25 (mean 22) µm; equatorial diameter 23–28 (mean 26) µm; polar/ equatorial diameter ratio 1:1.16; spine height (2.2 µm) ca. 1.5 x wall thickness (1.4 µm) (4 collections). Mature fruit ca. 3.0 mm long, 1.25–1.75 mm wide (mean 1.5 mm); proximal portion ca. 2.0 mm long. Seeds 2.0 x 0.9 mm. n = 14.

Phenology: Staminate meiosis in September, pistillate meiosis in May (Tainter 1968). Anthesis usually in April or May, with extremes from late March to June (Fernald 1950, Tainter 1968). Fruits mature in September or early October of the same year as pollination (the only temperate North American species with this characteristic); maturation period averages ca. 5 months, one of the shortest in the genus, depending on the periods of fruit maturation ultimately determined in some of the tropical species, e.g., Arceuthobium hawksworthii. Seed germination mostly in May and June (Tainter 1968).

Hosts: Arceuthobium pusillum is most common and widely distributed on Picea mariana. Picea glauca and P. rubens appear to be about as susceptible as P. mariana and are also classed here as principal hosts, although the dwarf mistletoe is not so common on them. Picea glauca is commonly parasitized along the Maine Coast (Nash 1955), on the north shore of Lake Michigan, and near Glenboro, Manitoba (French and others 1981). Its occurrence on P. rubens was previously thought to be rare, but it is now known to be common on the old-growth stands in New York, Vermont, New Hampshire, and Maine (Hawksworth and Shigo 1980, Livingston and others 1991, Souto and Hawksworth 1988). Larix laricina is an occasional host in Québec (Pomerleau 1942), Wisconsin (Anderson and MacDonald 1976, Kliejunas 1969), and Minnesota (Tainter and French 1967, 1968). Pinus banksiana is a rare host in Québec (Pomerleau), Manitoba (Laut 1967), Ontario (Sippell and others 1968), and Minnesota (Baker and others 1978). Pinus strobus is also a rare host in New York (House 1935). Abies balsamea and P. resinosa are also rare hosts. Picea pungens, a western species, is infected when planted in Maine (U.S. Department of Agriculture 1960) and in Ontario (Davis and Myren 1990). We found a witches’ broom on Picea abies in a plantation around naturally infected P. rubens near Hancock, Vermont. The tree was dead, but it was not possible to confirm that death was induced by infection with A. pusillum.

Distribution: Canada (Saskatchewan, Manitoba, Ontario, Québec, New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland) and United States (Minnesota, Wisconsin, Michigan, New York, Pennsylvania, New Jersey, Connecticut, Massachusetts, Vermont, New Hampshire, Rhode Island, and Maine) (figs. 16.87, 16.88, 16.89). Arceuthobium pusillum occurs in Canada from Hudson Bay, the Cumberland areas in eastern Saskatchewan to southern Manitoba, southern Ontario, Québec, and the Maritime Provinces including Newfoundland. Its northern limits in Ontario and Québec are poorly known. In the United States it occurs in the northern parts of Minnesota, Wisconsin, and Michigan, northeastern Pennsylvania, extreme northwestern New Jersey, and all of New England. It is rare near the limits of its southern distribution and is listed as an endangered plant in Rhode Island (Enser and Caljouw 1989), Connecticut (Dowhan and Craig 1976, Mehrhoff 1989), and New Jersey (Snyder and Vivian 1981) and a plant of "special concern" in Massachusetts (Anonymous 1989, Sorrie 1987).

Several distribution maps have been published for Arceuthobium pusillum:

Elevational range is from sea level in Maine and the Maritime Provinces to ca. 800 m on Mt. Katahdin, Maine (Brower 1960).

Discussion: Wood’s (1871) variety abigenium was based on Peck’s specimen from Sandlake, New York. Because it was proposed as a variety, it cannot replace Peck’s (1872) validly published specific name pusillum.

Thoreau (1858) provided an accurate description of the witches’ brooms caused by Arceuthobium pusillum in spruce at Walden Pond, some 13 years before the species itself was described. The dwarf mistletoe is now extinct at Walden Pond (Eaton 1974). Smith (1992) presents an interesting account of the discovery of A. pusillum by Lucy B. Millington at Warrensburg, New York in 1871 and her correspondence with C. H. Peck about the new species (Millington 1871a, 1871b, 1872). This discovery triggered a flurry of activity that eventually showed the parasite to be widely distributed in the spruce forests of the Great Lakes region, New England, and southern Canada. The plant is now recognized as a damaging pathogen of spruce in many parts of its distribution. Mortality is severe in Picea glauca along the Maine Coast (Nash 1955), and the parasite is considered to be the most serious disease agent of P. mariana in the Great Lakes region (Beckwith and Anderson 1956).

Arceuthobium pusillum has the smallest shoots of any New World dwarf mistletoe (averaging about 1 cm high), but A. minutissimum in the Himalayas has even smaller shoots (averaging about 0.5 cm).

The witches’ brooms induced by Arceuthobium pusillum have been the subject of several investigations: von Schrenk (1900), Gill (1935), Pomerleau (1942), Anderson and Kaufert (1959), Kuijt (1960b), and Singh (1982). Witches’ brooms appear to be mostly of the systemic type; thus shoot formation is regular and progressive. Shoots usually first appear in late summer or autumn as small eruptions in the bark of host branches 2 years old; the shoots mature during the third season; they flower the following spring, and the fruits mature by autumn. Thus, shoots with mature fruits usually occur only on 4-year-old host growth. Shoots usually fall after flowering or fruiting. Only rarely are the shoots perennial and produce a second crop of flowers (Baker and French 1979). This pattern of reproduction is unique among north temperate species of the genus.

Thomas (1954) stated that the witches’ brooms formed on Picea mariana are dense and dark, whereas those on P. glauca are light and loose (fig. 16.90). The witches’ brooms he observed possibly were in different stages of development, because Anderson and Kaufert (1959) demonstrated that in P. mariana the witches’ brooms grew rapidly for the first 5–10 years and then abruptly declined in vigor. Large swellings on the main trunk are commonly induced by Arceuthobium pusillum in old-growth P. rubens in New England and New York (Hawksworth and Shigo 1980, Livingston and others 1991), but such swellings have not been reported on other spruces.

Brower (1960) and Nash (1955) suggest that the dwarf mistletoe on Picea glauca along the Maine Coast is taxonomically distinct from the inland populations characteristically found in spruce bogs. However, our studies of both populations showed little evidence of morphological differences. Although P. glauca appears to be more severely infected along the Maine coast, the occurrence of Arceuthobium pusillum on this host is not unique to this area. It occurs commonly on P. glauca in the Mackinac Straits area of Michigan (Voss 1985) and in the Sprucewoods area of southwestern Manitoba (French and others 1981), and it is found sporadically on P. glauca throughout the range of the parasite.

Arceuthobium pusillum is common in spruce bogs, where it most frequently infects Picea mariana. In Québec, A. pusillum is apparently restricted to within 2 km of lakes or rivers (Bernard 1957). In Maine, it occurs on P. glauca only within ca. 300–400 m of the coast (Nash 1955). Bonga (1969a) suggests that A. pusillum in New Brunswick requires an uninterrupted period of high atmospheric humidity in the spring for normal growth. This might explain its presence in bogs and its absence from drier, upland sites farther inland.

Eaton (1931) reported a 130–160 km break in the distribution of Arceuthobium pusillum in southern New England. However, A. pusillum has since been reported from many locations within this supposed gap (fig. 16.89) (Eaton and Dow 1940).

Arceuthobium pusillum occurs on several islands that are isolated 30 km or more from the nearest known populations of the dwarf mistletoe—Mantinicus and Monhegan Islands off the Maine Coast and Beaver Island in Lake Michigan. Arceuthobium pusillum may occur on Isle Royale in Lake Superior, although a recent flora of the island does not list it (Slavick and Janke 1987). Also, the plant is not included in a flora of Anacosti Island in the Gulf of St. Lawrence, where it might be expected to occur (Marie-Victorin and Rolland-Germain 1969).

* "In spite of Peck's efforts to see that [Lucy Bishop] Millington was remembered as the discoverer of this new species, her name has long been forgotten by present day investigators of A. pusillum, if indeed they ever knew it" (Smith 1992).

 

Arceuthobium rubrum
Ruby Dwarf Mistletoe

28. A. rubrum Hawksworth & Wiens, Brittonia 17: 233, 1965. TYPE COLLECTION: MEXICO: Durango: 47 km east of El Salto on Route 40, on Pinus teocote, Hawksworth & Wiens 3490 in 1963 (Holotype COLO! Isotypes: FPF, INIF, MEXU, MO, US).

Description: Mean shoot height ca. 10 (max. 18) cm, dark red, brown to blackish, flabellately branched (fig. 16.91). Staminate plants usually taller than pistillate plants. Basal diameter of dominant shoots 2–3 (mean 2.4) mm. Third internode 4–12 (mean 6.9 ± 2.7) mm long, 2–3 (mean 2.3) mm wide (17 collections), length/width ratio 3.4:1. Staminate flowers 1.0–1.5 mm across; mostly 3-merous; segments ca. 0.6 mm long, 0.6 mm wide. Pollen polar diameter 23–26 (mean 24) µm; equatorial diameter ratio 1:1.05; spine height (ca. 1.0 µm) slightly greater than the wall thickness (0.8 µm) (1 collection). Mature fruit ca. 3.5 x 2.0 mm. Seeds 2.0 x 1.0 mm. n = ?

Phenology: Meiosis probably in June. Anthesis usually in July. Fruits mature from mid-July to August of the year following pollination; maturation period averages 12–13 months.

Hosts: Principal hosts are Pinus teocote, P. cooperi, P. engelmannii, P. herrerai, and P. durangensis, all of which appear to be highly susceptible.

Distribution: Mexico (Durango and Sinaloa) (fig. 16.92). This species has a localized distribution in the Sierra Madre Occidental of western Mexico. Elevational range is 2,250–2,900 m.

Discussion: This distinctive, slender, reddish dwarf mistletoe is apparently widespread in the mountains of Durango. As the plants dry, the red color turns to dull brown, and the apical portion of each internode becomes golden yellow. This gives dried specimens a characteristic banded appearance. The shiny fruits, a character shared only with Arceuthobium oaxacanum, readily distinguish this species. The fruits remain shiny when dried, but the bright red turns to dark brown. Another distinctive characteristic of A. rubrum is the exceptionally small, scarcely opened staminate flowers. In all July collections examined, the staminate flowers were still not expanded, although they were past maturity and nearly all the pollen had been shed. The perianth segments formed a nearly closed cup with only a slight separation between the segments, and the flowers had to be examined closely to determine whether pollen was being dispersed. This species may possess an interesting mode of pollen dispersal that should be investigated further.

The populations in the Pueblo Altares area in northern Durango, about 150 km north of the populations around El Salto, have taller, darker, and stouter shoots. Superficially, they resemble Arceuthobium vaginatum subsp. vaginatum. These populations are almost black and have shoots up to 23 cm tall and up to 1 cm diameter at the base. However, they have small, shiny fruits typical of other populations of A. rubrum. Further studies may show that these populations warrant separate taxonomic status.

Arceuthobium rubrum exhibits a number of features similar to A. oaxacanum which occurs about 1,200 km to the south in Oaxaca. See discussion under A. oaxacanum and table 16.5.

 

Arceuthobium siskiyouense
Knobcone Pine Dwarf Mistletoe

29. A. siskiyouense Hawksworth, Wiens & Nickrent, Novon 2: 204, 1992. TYPE COLLECTION: OREGON: Josephine County, 9.6 km south of O’Brien on Old Gasquet Toll Road (Oregon Mountain Road), parasitic on Pinus attenuata, T. 13 S., R. 9 W., Section 9, latitude 42°1'N, longitude 123°46'W; elevation 650 m, Wiens 6756, 20 July, 1987 (Holotype US! Isotypes: FPF, MO, ORE, OSC, UC).

Description: Mean shoot height ca. 8 (max. 10) cm, dark brown, flabellately branched (fig. 16.93). Basal diameter of dominant shoots 2.0–2.5 mm across. Third internode 8–15 (mean 9) mm long, ca. 2 mm wide. Mature fruits 3.6 x 2.1 mm. n = ?

Phenology: Peak anthesis in August. Fruits at maturation not observed.

Hosts: Pinus attenuata is the principal and only common host of Arceuthobium siskiyouense. This dwarf mistletoe rarely parasitizes P. ponderosa, P. jeffreyi, and P. contorta var. contorta in areas where these trees grow in close association with infected P. attenuata.

Distribution: United States (California and Oregon) (fig. 16.94). The distribution of Arceuthobium siskiyouense is restricted to the Klamath Mountains of southwestern Oregon (Curry and Josephine Counties) and the Siskiyou Mountains in adjacent northwestern California (Del Norte and Siskiyou Counties). Elevational range is 400–1,200 m.

Discussion: In our previous taxonomic treatments, this dwarf mistletoe was included in Arceuthobium campylopodum (Hawksworth and Wiens 1972, 1984). Subsequent studies, however, have demonstrated that it is a distinct species, but certainly closely related to A. campylopodum. Previously, we noted an anomalous situation near Gasquet, Del Norte County, California, where a dwarf mistletoe (then presumed to be A. campylopodum) was common on Pinus attenuata but rare on associated P. ponderosa (Hawksworth and Wiens 1972). This situation is now clarified—the population on P. attenuata is A. siskiyouense. The two species are sympatric in this and several other areas, yet both maintain their distinctive morphologies and host preferences, even though their flowering periods partially overlap (table 16.6). In marked contrast to the restricted distribution of A. siskiyouense, A. campylopodum has an extensive range and occurs from northern Idaho and northern Washington to Baja California. Furthermore, A. siskiyouense does not induce witches’ brooms.

 

Arceuthobium strictum
Unbranched Dwarf Mistletoe

30. A. strictum Hawksworth & Wiens, Brittonia 17: 234, 1965. TYPE COLLECTION: MEXICO: Durango: 26 km south of the city of Durango on road to La Flor, on Pinus leiophylla var. chihuahuana, Hawksworth & Wiens 3465 in 1963 (Holotype COLO! Isotypes: DS, FPF,INIF, MEXU, MO, US).

Description: Mean shoot height ca. 7 (max. 13) cm, pistillate shoots generally greenish yellow brown, flabellately branched. Staminate plants brownish, rarely branching (fig. 16.95). Staminate plants usually taller than pistillate plants. Basal diameter of dominant shoots 2.5–4.0 (mean 3.1) mm. Third internode 1–8 (mean 3.6 ± 2.0) mm long, 1.5–3.5 (mean 2.3) mm wide (5 collections); length/width ratio 1.6:1. Staminate flowers ca. 3 mm across, perianth 3-, 4-, or 5-merous (rarely 6- or 7-merous), segments ca. 1.5 mm long, 1 mm wide. Pollen polar diameter 18–19 (mean 18) µm; equatorial diameter 20–25 (mean 22) µm; polar/equatorial diameter ratio 1:1.19; spine height (ca. l µm) approximately equal to the wall thickness (1 collection). Mature fruit ca. 4 x 2.5 mm. Seeds 2.5 mm x 1.0 mm. n = 14.

Phenology: Meiosis in July. Anthesis late July–October, with peak in September. Fruits mature from mid-September to October of the year following pollination; maturation period averages ca. 13 months.

Hosts: Pinus leiophylla var. chihuahuana is the principal host; P. teocote is an occasional host; and P. engelmannii is a rare host. Pinus cembroides frequently grows in close association with heavily infected P. leiophylla var. chihuahuana, but no infections have been observed on this tree.

Distribution: Mexico (Durango) (fig. 16.96). This species is known only in the Sierra Madre Occidental south and west of the city of Durango. Elevational range is 2,200–2,500 m.

Discussion: The most distinctive feature of this dwarf mistletoe is the lack of branching in the staminate plants (Hawksworth and Wiens 1965, Kuijt 1970). The staminate shoots at anthesis become single spikes 6–13 cm long (fig. 16.95). The staminate flowers also possess a greater number of perianth segments (up to 7) than any other dwarf mistletoe (Kuijt 1970). The pistillate plants, however, have abundant secondary branching with dense clusters of small, yellow-green fruits. This dwarf mistletoe causes heavy mortality in its principal host, Pinus leiophylla var. chihuahuana.

 

Arceuthobium tsugense
Hemlock Dwarf Mistletoe

31. A. tsugense (Rosendahl) G. N. Jones, University of Washington Publications in Biology 5: 139, 1936 (as A. tsugensis). TYPE COLLECTION: CANADA: British Columbia: Vancouver Island, Port Renfrew on Tsuga heterophylla, Rosendahl 826 in 1902, (Lectotype MIN, Isotypes: COLO!, DS, FPF!, K!, ILL!, MO!, RM!). Razoumofskya tsugensis Rosendahl, Minnesota Botanical Studies 3: 272, 1903. Razoumofskya douglasii tsugensis (Rosendahl) Piper, Contributions U.S. National Herbarium 11:222, 1906. A. douglasii Engelm. var. tsugensis (Rosendahl) M. E. Jones, University of Montana Bulletin 61 (Biology Series 15): 25, 1910. A. campylopodum Engelm. forma tsugensis (Rosendahl) Gill, Transactions Connecticut Academy Arts and Sciences. 32: 200, 1935.

Description: Shoot height ca. 5–7 (max. 13) cm, greenish to reddish, darker in winter, flabellately branched (fig. 16.97). Basal diameter of dominant shoots 1.5–4.0 (mean 2.0) mm. Third internode 4–16 (mean 9.2 ± 2.5) mm long, 1–2 (mean 1.5) mm wide (15 collections), length/width ratio 6.1:1. Staminate flowers 2.8 mm across; perianth 3- or 4-merous, segments ca. 1.2 mm long, 1.0 mm wide. Mean anther diameter 0.5 mm, centered 0.6 mm from tip of segment. Pollen polar diameter 19–21 (mean 20) µm; equatorial diameter 23–25 (mean 24) µm; polar/equatorial diameter ratio 1:1.19; spine height (2.4 µm) 3 x wall thickness (0.8 µm) (4 collections). Pistillate flowers ca. 1 mm long, 1 mm across. Mature fruit 3 x 2 mm; proximal portion ca. 2.0 mm long. n = 14.

Discussion: This dwarf mistletoe has been variously treated as a distinct species or as allied to Arceuthobium douglasii or A. campylopodum (Gill 1935). Its phenology and general morphology, however, clearly align it with the A. campylopodum complex.

We commented previously on the unusually broad host range of Arceuthobium tsugense, which encompasses not only both western species of hemlock but also several species of fir, spruce, and pine (Hawksworth and Wiens 1972). Its occurrence on Pinus contorta var. contorta in British Columbia and the San Juan Islands of Washington also has long been known (Hawksworth and Wiens 1972; Smith 1971; Smith and Wass 1976, 1979; Wass 1976). The taxonomic status of these populations has now been re-evaluated. We separate A. tsugense into two subspecies (tsugense and mertensianae) and two physiologically differentiated host races (western hemlock and shore pine) that are not accorded formal taxonomic recognition (Hawksworth 1987a, Hawksworth and others 1992, Mathiasen and Hawksworth 1988).

Key to the subspecies:

1. Parasitic primarily on Tsuga heterophylla or Pinus contorta var. contorta; shoots 3-13 (mean 7) cm high

31a. tsugense subsp. tsugense

1. Parasitic primarily on Tsuga mertensiana; shoots 3-9 (mean 5) cm high

31b. tsugense subsp. mertensianae

The subspecies are similar morphologically, but the shoots are about 30% taller in subsp. tsugense than in subsp. mertensianae. The differences are statistically highly significant (P < 0.01).

The phenology of flowering of the subspecies also differ somewhat. Flowering averages about 1–2 weeks earlier in subsp. tsugense (peak anthesis in August, with extremes from late July to late September, 56 observations; fig. 16.98) than for subsp. mertensianae (peak anthesis from mid-August to mid-September, 33 observations; fig. 16.99). In contrast to flowering, the seed dispersal period for subsp. tsugense (late September to early November, 57 observations) averaged about 2–4 weeks later than for subsp. mertensianae (mid-August, rarely to late October, 31 observations). Fruit maturation thus requires about 13–14 months for subsp. tsugense and approximately 12–13 months for subsp. mertensianae.

Extensive field studies of these subspecies in the Cascade Mountains and Coast Range in Oregon and Washington during the last 2 weeks of September 1993 showed that by mid-September all the populations of subsp. mertensianae had completed dispersal, whereas subsp. tsugense did not initiate dispersal until about the last week of September. Peak dispersal in subsp. tsugense likely took place in early October. No overlap in dispersal was noted, even where the subspecies occurred in relatively close proximity, e.g., the populations of subsp. tsugense at Union Creek, Klamath County, Oregon, and those of subsp. mertensianae about 25 km farther north.

 

Arceuthobium tsugense subsp. tsugense
Western Hemlock Dwarf Mistletoe

31a. A. tsugense (Rosendahl) G. N. Jones subsp. tsugense.

Description: As the species, but shoots vary from 3–13 cm high, mean ca. 7 cm. n = 14.

Phenology: Meiosis in July. Peak anthesis usually in August, with extremes from late July to late September (fig. 16.98) or even December in British Columbia (Baranyay 1962). Fruits mature from late September to early November (Smith 1966a); maturation period averages ca. 13–14 months. Seed germination is usually from February to May in British Columbia (Smith 1966b).

Hosts: Tsuga heterophylla is the common principal host. Abies amabilis, A. lasiocarpa var. lasiocarpa, and A. procera are reported as principal hosts (Mathiasen 1994), but classifications of these hosts are based on few observations and considered tentative. Abies grandis and Pinus contorta var. latifolia are occasional hosts (Mathiasen 1994). Rare hosts are Picea engelmannii, P. sitchensis (in Alaska, Laurent 1966; in British Columbia, Molnar and others 1968; and in Oregon), Pinus monticola (Gill 1935), Pseudotsuga menziesii (Hunt and Smith 1978), and Tsuga mertensiana (in Alaska, Shaw 1982a; and in Oregon, Mathiasen 1994). The status of Pinus contorta var. contorta as a host is discussed separately.

Seedlings of Tsuga canadensis were susceptible to inoculation with Arceuthobium tsugense (Weir 1918a). Larix decidua (cited as L. europaea) became infected when planted under T. heterophylla parasitized by A. tsugense on Vancouver Island (Kuijt 1964). Although swelling occurred, no shoots developed, and L. decidua was apparently not a compatible host. Smith (1965) successfully inoculated potted seedlings of the following trees originating beyond the natural distribution of Tsuga heterophylla: Picea glauca from interior British Columbia, Pinus contorta var. latifolia from Montana, and Picea abies from Europe. Smith (1970a,b) also successfully infected Picea engelmannii and, with difficulty, Larix occidentalis. In the latter host–parasite combination, most infections resulted only in swellings, but in one instance a few short-lived shoots were produced. We have examined several mixed L. occidentalis–T. heterophylla stands in the Oregon Cascade Mountains and A. laricis on Larix never infected associated Tsuga. Based on greenhouse and plantation inoculations in British Columbia, Pinus ponderosa, P. radiata, and P. sylvestris are also susceptible to infection by A. tsugense subsp. tsugense (Smith and Craig 1968).

Distribution: Canada (British Columbia) and the United States (Alaska, Washington, Oregon, and California) (figs. 16.100, 16.101, 16.102, 16.103). Arceuthobium tsugense subsp. tsugense is distributed from near Haines, Alaska, to the vicinity of Mendocino in northwestern California. A general distribution map is given in fig. 16.100. The distribution in Alaska and British Columbia is presented in fig. 16.101, and that in Washington, Oregon, and California in fig. 16.102. The distribution of the populations infecting shore pine (Pinus contorta var. contorta) is illustrated in fig. 16.103. Arceuthobium tsugense subsp. tsugense occurs throughout the coastal Tsuga heterophylla forests of Alaska, British Columbia (Kuijt 1963), Oregon, and Washington. It has been collected rarely in northwestern California (Humboldt and Mendocino Counties). Hultén (1968) mentions that this dwarf mistletoe is "much overlooked" in Alaska, and this is emphasized by his distribution map for southeast Alaska, which shows only two localities.

Our field investigations have not confirmed the reports of Arceuthobium tsugense in Idaho (Weir 1916b). Weir reports and illustrates an infected hemlock on Marble Creek on the St. Joe National Forest, labeled as Tsuga heterophylla, but we suspect it is T. mertensiana. An intensive survey of dwarf mistletoe throughout northern Idaho, western Montana, and eastern Washington by D. P. Graham (1959b) found no infections on T. heterophylla in Marble Creek or elsewhere in this region. Graham (1959b) reports dwarf mistletoe on T. mertensiana on the Coeur d’Alene National Forest. Our studies there in 1966, however, identified the parasite as A. laricis, which typically infects Larix occidentalis and T. mertensiana, with which Larix is often associated. Tsuga heterophylla was not infected even though several individuals grew in close association with infected T. mertensiana, L. occidentalis, and Abies lasiocarpa. The specimens that Weir collected in Idaho, reportedly on T. heterophylla (see appendix), are so fragmentary that specific identification of the dwarf mistletoe is impossible. Thus, whether A. tsugense occurs in Idaho remains moot, but we consider it unlikely. Marble Creek and nearby areas have been extensively logged during the 75 years since Weir’s report, and it is remotely possible that the parasite has been eradicated there. Although logging might be expected to reduce the amount of dwarf mistletoe in a stand (Stewart 1976), we know of no instances where it has eliminated Arceuthobium from an area. Elevational range is from sea level in Alaska, British Columbia, and Washington to about 1,250 m in Oregon.

Discussion: Many of the lower infections in Tsuga heterophylla produce few shoots of the dwarf mistletoe. Because dwarf mistletoes are sensitive to light, the absence of dwarf mistletoe shoots from the lower infections may be explained by the dense shade in the lower portions of coastal hemlock forests (Smith 1969). In such situations vigorous shoots are often found only along margins of stands, on young trees in openings, or in higher branches of older trees.

Arceuthobium tsugense subsp. tsugense has long been known to parasitize Pinus contorta var. contorta in southwestern British Columbia and the San Juan Islands, Washington (fig. 16.103). The populations of this dwarf mistletoe are distributed along the east coast of Vancouver Island, on the Channel Islands, and the mainland of British Columbia north of Vancouver. Two outlying populations have been reported about 250 km farther north at Port Clements on the Queen Charlotte Islands and at Terrace on the British Columbia mainland. Field studies are needed in these localities to determine whether they are populations of A. tsugense restricted to P. contorta var. contorta or rare crossovers from populations that also infect Tsuga heterophylla. In the United States, the only known populations occur on Orcas Island, San Juan Islands, Washington, where the mistletoe is common near the summit of Mt. Constitution and on Turtleback Mountain, about 7 km southwest of Mt. Constitution (K. Russell, personal communication). The elevational range is about 100–800 m.

The taxonomic status of the dwarf mistletoe populations on Pinus contorta var. contorta has been debated for several decades (Hawksworth and Wiens 1972; Hunt and Smith 1978; Kuijt 1956, 1963; Smith 1971, 1974; Smith and Wass 1976, 1979; Wass 1976). The western hemlock and shore pine races of subsp. tsugense are morphologically similar and appear to differ primarily in their host relationships (Smith 1971, 1974; Smith and Wass 1976, 1979; Wass 1976). Inoculation of P. contorta var. contorta with seeds from the western hemlock race produced low levels of infection. The few successful infections, however, developed abundant aerial shoots. In contrast, seeds of the shore pine race produced moderate levels of infection on Tsuga heterophylla, but only a few of these infections produced shoots (Smith and Wass 1979). Maximum shoot height of the plants on Tsuga heterophylla was about 30% taller than shoots of the shore pine race (Smith 1971). Results of inoculation tests in areas where these two races are sympatric in British Columbia confirm their morphological similarity (E. Wass, personal communication). The only statistically significant difference that Wass obtained between the two populations was the occurrence of slightly larger fruits in the shore pine race. Anthesis and seed dispersal peaked about 10 days earlier in the shore pine race than in the western hemlock race on the same sites.

These populations show clear host affinities, but there is little else to warrant taxonomic recognition. In addition to their general morphological similarities, they are also similar in terms of their flavonoid composition (Crawford and Hawksworth 1979 and table 14.2) and their isozyme patterns (Nickrent and Stell 1990). Nickrent and Stell (1990) indicate the races are sympatric in some areas, but many of the populations on Pinus contorta var. contorta are isolated because this host is often confined to rocky outcrops, especially in the southern portions of Vancouver Island. Although the shore pine race of A. tsugense does not show sufficient differentiation to justify taxonomic recognition on the basis of the present data, further study of its systematic status is warranted.

The status of the dwarf mistletoe infecting Abies procera on Mary’s Peak, west of Corvallis, Oregon, has long been uncertain. Arceuthobium tsugense subsp. tsugense is common in the area on Tsuga heterophylla at lower elevations. In some stands, however, it is common on both T. heterophylla and associated Abies procera; but near the summit of the mountain (1,250 m), it parasitizes Abies procera extensively and is rare on associated T. heterophylla. The populations on Abies are morphologically similar to populations infecting Tsuga, and isozyme studies indicate that the dwarf mistletoe population on Mary’s Peak is subsp. tsugense (Nickrent and Stell 1990).

 

Arceuthobium tsugense subsp. mertensianae
Mountain Hemlock Dwarf Mistletoe

31b. A. tsugense (Rosendahl) G. N. Jones subsp. mertensianae Hawksworth & Nickrent, Novon 2: 209, 1992. TYPE COLLECTION: OREGON: Douglas County: 26 km north of Union Creek on State Highway 230, on Tsuga mertensiana; elevation 1,500 m, Mathiasen 9002, January 2, 1990 (Holotype US! Isotypes: FPF, NO, ORE, OSC, UC, UWT).

Description: As the species, but shoots typically shorter (ca. 5 cm) than in subsp. tsugense (ca. 7 cm). n = 14.

Phenology: Peak anthesis from mid-August to mid-September (fig. 16.99). Seed dispersal from mid-August to early September (or rarely to October); maturation period averages ca. 12–13 months.

Hosts: The principal hosts of Arceuthobium tsugense subsp. mertensiana are Tsuga mertensiana, Abies amabilis, A. lasiocarpa var. lasiocarpa, and A. procera. Pinus albicaulis is a secondary host and Pinus monticola is an occasional host in areas where trees of these species are closely associated with infected principal hosts. Picea breweriana and recently Abies grandis, Pinus contorta var. latifolia, and Tsuga heterophylla are recognized as rare hosts (Mathiasen1994 and personal communication).

Distribution: Western Canada (southern British Columbia) and western United States (Washington, Oregon, and California) (fig. 16.104). The distribution of Arceuthobium tsugense subsp. mertensianae is still poorly known, but it extends from near Vancouver, British Columbia, through the Cascade Mountains of Washington and Oregon to the central Sierra Nevada in Alpine County, California.

A number of collections of dwarf mistletoe on Tsuga mertensiana from the east side of the Cascade Mountains (Chelan County, Washington) have not been studied in the field, but they are presumably this subspecies. Arceuthobium tsugense subsp. mertensianae was recently reported for the first time in the southern Olympic Mountains, Washington, (Henderson and others 1989) and confirmed by us. This is the first population of A. tsugense subsp. mertensianae to be discovered west of the Cascades in Washington or Oregon. This dwarf mistletoe is most common in the Cascade Mountains from central Oregon to central California. Elevational range is 1,200–2,500 m.

Discussion: Some populations of Tsuga mertensiana are exceptionally heavily infected by this dwarf mistletoe. Notable among such populations are those at Mt. Baker in the northern Cascade Mountains, and in the southwestern Olympic Mountains of Washington.

 

Arceuthobium vaginatum

32. A. vaginatum (Willdenow) Presl in Berchtold, O P´rirozenosti Rostlin aneb Rostlinár 2: 28, 1825.

Description: Shoot height mostly from 20–30 (max. 5) cm, orange to dark brown, reddish, or black, usually densely branched and erect, but large older plants sometimes become pendulous; flabellately branched; basal diameter of dominant shoots ca.12–20 mm long, 0.2–0.4 cm wide. Staminate flower up to 3.5 mm long and up to 3.5 mm across, mostly 3-merous (sometimes 4-merous), segments up to 2.0 mm long and up to 1.5 mm wide, apex acute to obtuse. Pistillate flower up to 2.5 mm long, up to 1.5 mm across. Fruit 4–6 mm long, 2–3 mm wide, elliptical to obovate.

Phenology: Anthesis from approximately late March through May.

Hosts: Parasitic on pines of subgenus Diploxylon.

Discussion: Our studies in Mexico and the United States show that Arceuthobium vaginatum is comprised of two subspecies (vaginatum and cryptopodum) distinguishable by shoot color, shoot size, staminate flowers, fruit, time of anthesis, and geographic distribution (fig. 16.105). The taxon durangense that we originally included in A. vaginatum (Hawksworth and Wiens 1972) has since been raised to specific status (Hawksworth and Wiens 1989).

The distributions of the two subspecies overlap in the mountains of central Chihuahua (between latitudes 28°00' and 28°30' N), where intermediate characteristics are shown in some populations. Even here, however, there is a tendency to segregate by elevation, with subsp. vaginatum occurring at lower elevations and subsp. cryptopodum at higher elevations. The characteristics distinguishing these subspecies are perhaps greater than those distinguishing others, e.g., those in Arceuthobium tsugense. A case could be made for specific status, but we retain them as subspecies because they represent the only examples where we find intermediate populations between taxa of dwarf mistletoes.

Key to the subspecies:

1. Plants dark brown to black, usually over 20 cm tall; staminate flowers usually greater than 3 mm long and wide; anthesis March-April; Sierra Madre Occidental from central Chihuahua southward to the Central Cordillera, and in the Sierra Madre Oriental

31a. A. vaginatum subsp. vaginatum

1. Plants orange, usually less than 20 cm tall; staminate flowers usually less than 3 mm long and wide; anthesis May–June; Sierra Madre Occidental of central Chihuahua and Sonora and mountains of central Coahuila northward to central Utah and northern Colorado

31b. A. vaginatum subsp. cryptopodum

 

Arceuthobium vaginatum subsp. vaginatum
Mexican Dwarf Mistletoe

32a. A. vaginatum (Willdenow) Presl subsp. vaginatum, Berchtold, O P´rirozenosti Rostlin aneb Rostlinár 2: 28, 1825. TYPE COLLECTION: MEXICO: Veracruz: Cofre de Perote, on Pinus sp., Humboldt & Bonpland in 1804 (Lectotype B, Isotype MO!). Viscum vaginatum Willdenow, Sp. Pl. ed. 4, 4: 740, 1806. Razoumofskya mexicana Hoffman, Hortus Mosquensis (unpaged), 1808. Viscum vaginatum (Willd.) Kunth, Nov. Gen. et Spec. 3: 445, 1820. A. vaginatum (H.B.K.) Eichler, Mart. Fl. Bras. 5(2); 105, 1868. Razoumofskya vaginata (Willd.) Kuntze, Revision of Genera of Plants 2: 587, 1891.

Description: Mean shoot height ca. 20 (max. 55) cm, dark brown to black, rarely reddish (fig. 16.106). Basal diameter of dominant shoots 4–20 (mean 7) mm. Third internode 5–30 (mean 17.4 ± 6.0) mm long, 2.5–8.5 (mean 5.0) mm wide (11 collections), length/ width ratio 2.9: 1. Staminate flower segments ca. 1.6 mm long, 1.1 mm wide. Mean anther diameter 0.6 mm centered 0.8 mm from tip of segment. Pollen polar diameter 19–23 (mean 21) µm; equatorial diameter 22–26 (mean 25) µm; polar/equatorial diameter ratio 1:1.17; spine height (1.7 µm) ca. 1.5 x wall thickness (1.2 µm) (3 collections). Mature fruit ca. 5.5 x 3.5 mm. n = ?

Phenology: Meiosis in February. Anthesis usually March–April. Fruits mature in August of the year following pollination; maturation period averages 16–17 months.

Hosts: Arceuthobium vaginatum subsp. vaginatum has the broadest known host range of any taxon in the genus. It has been collected on 13 species of Mexican pines and undoubtedly occurs on others. It is common on Pinus montezumae, P. hartwegii, P. herrerai, P. lawsonii, P. rudis, P. cooperi, P. arizonica vars. arizonica and stormiae, P. durangensis, P. engelmannii, and P. patula, all of which appear to be highly susceptible and are classed as principal hosts. Pinus teocote is a secondary host because it was parasitized only when it was associated with one of the infected principal hosts listed above. It rarely infects Pinus culminicola on Cerro Potosí, Nuevo León, where P. culminicola occurs in the understory of infested P. rudis forests (Hawksworth and Wiens 1965).

Distribution: Mexico (Chihuahua, Coahuila, Distrito Federal, Durango, Hidalgo, Jalisco, Mexico, Nayarit, Nuevo León, Oaxaca, Puebla, Querétaro, Sinaloa, Tamaulipas, Tlaxcala, Veracruz, and Zacatecas) (fig. 16.107). This is the most widely distributed dwarf mistletoe in Mexico, extending from the Sierra Madre Occidental in western Chihuahua south through Durango, Jalisco, and into the Central Cordillera of Mexico and Puebla and occurring in the Sierra Madre Oriental from Coahuila and Nuevo León to Oaxaca. Elevational range is from 2,100 m in Nuevo León to 3,900 m on Nevado de Toluca near Mexico City.

Discussion: The shoots of A. vaginatum subsp. vaginatum may exceed 55 cm in height in central Mexico. Only the shoots of A. globosum subsp. grandicaule are taller.

The plants exhibit considerable sexual dimorphism. The staminate plants tend to be taller than the pistillate plants, but the character is variable. Vasquez (1981) studied this dwarf mistletoe near Texcoco, Mexico, and noted that the pistillate plants were short, erect, and dark, whereas staminate plants were long, pendulous, and reddish. Plants in some areas of the northern Sierra Madre Oriental (e.g., the Sierra de la Marta, Coahuila) are often reddish, but plants just 40 km to the south (Cerro Potosí, Nuevo León) are again typically black (Hawksworth and Cibrián 1985).

The hosts and ecological requirements of Arceuthobium vaginatum subsp. vaginatum and the two subspecies of A. globosum are similar, and they frequently co-occur, sometimes even on the same tree (Hawksworth and Wiens 1965) (fig. 5.4).

 

Arceuthobium vaginatum subsp. cryptopodum
Southwestern Dwarf Mistletoe

32b. A. vaginatum subsp. cryptopodum (Engelmann) Hawksworth & Wiens, Brittonia 17: 230, 1965. TYPE COLLECTION: NEW MEXICO: Santa Fe County, Santa Fe, on Pinus ponderosa, Fendler 283 in 1847 (Lectotype GH! Isotypes: MO, K, NY). A. robustum Engelmann in Gray, Memoirs American Academy n. s. 4: 59, 1849, nomen nudum. A. cryptopodum Engelmann in Gray, Boston Journal Natural History 6: 214, 1850. Razoumofskya robusta (Engelm.) Kuntze, Revision of Genera of Plants 2: 587, 1891. Razoumofskya cryptopoda (Engelm.) Coville, Contributions U.S. National Herbarium 4:192, 1893. A. vaginatum (Willd.) Presl forma cryptopodum (Engelm.) Gill, Transactions Connecticut Academy Arts and Sciences. 32: 178, 1935.

Description: Mean shoot height ca. 10 cm (max. 27) cm, usually orange to reddish brown, sometimes dark to near black (fig. 16.108). Basal diameter of dominant shoots 2&150;10 (mean 4) mm. Third internode 4&150;16 (mean 7.8 ± 3.2) mm long, 2.0&150;4.5 (mean 3.1) mm wide (25 collections), length/width ratio 2.5:1. Staminate flowers 2.5&150;3.0 (mean 2.7) mm across; perianth segments ca. 1.3 mm long, 1.0 mm wide. Mean anther diameter 0.5 mm, centered 0.5 mm from tip of segment. Pollen polar diameter 18&150;28 (mean 22) µm; equatorial diameter 23&150;30 (mean 26) µm; polar/equatorial diameter ratio 1:1.15; spine height (ca. 1.5 µm) 1.5 x the wall thickness (1.0 µm) (5 collections). Mature fruit 4.5&150;5.5 (mean 5.0) mm long, 2.0&150;3.0 (mean 2.5) mm wide.; proximal portion ca. 3.5 mm long. Seeds 2.7 x 1.1 mm. n = 14.

Phenology: Meiosis in March or April. Anthesis usually in May&150;June, with extremes from late April to early July (fig. 16.109). Fruits mature in late July or early August, with extremes from early July to early September. Both anthesis and seed dispersal in Colorado occur 1&150;2 weeks later than in Arizona and New Mexico; maturation period averages 14&150;15 months. Seed germination from August to September, immediately following dispersal.

Hosts: Pinus ponderosa var. scopulorum is the most common host in Arizona, New Mexico, Colorado, Utah, and Texas. The two races of var. scopulorum recognized by Conkle and Critchfield (1988) (Rocky Mountain and southwestern) appear to be about equally susceptible, but most of the distribution of the Rocky Mountain race is primarily north of that of Arceuthobium vaginatum subsp. cryptopodum. Other principal hosts include P. arizonica var. arizonica (Arizona, New Mexico, Chihuahua, and Sonora) and var. stormiae (Coahuila), P. engelmannii (Arizona, Chihuahua, and Sonora), and P. durangensis (Chihuahua and Jalisco). Pinus cooperi is a secondary host. Occasional hosts are P. contorta var. latifolia in Colorado and Utah (Hawksworth 1956a) and P. aristata in Colorado and New Mexico (Hawksworth 1965a). Rare hosts are P. flexilis in Colorado (Hawksworth and Peterson 1959) and P. strobiformis in New Mexico.

Distribution: Northern Mexico (Sonora, Chihuahua, and Coahuila) (fig. 16.107) and United States (Utah, Arizona, Colorado, New Mexico, and Texas) (fig. 16.110). Arceuthobium vaginatum subsp. cryptopodum is widely distributed on Pinus ponderosa var scopulorum from central Utah (Sevier and Emery Counties) and northern Colorado (to latitude 40°52' N in Larimer County) to Arizona, New Mexico, west Texas (Guadalupe and Davis Mountains, but not the Chisos Mountains), at least as far south as the Sierra de la Madera in central Coahuila and the Sierra Madre Occidental in Sonora and Chihuahua. Confirmation is needed for the reports of A. vaginatum subsp. cryptopodum in southern Duchesne County and northwestern Emery County, Utah (Albee and others 1988). The Duchesne record is probably based on the infection of P. ponderosa by A. cyanocarpum.

Arceuthobium vaginatum subsp. cryptopodum occurs in nearly every mountain range where P. ponderosa var scopulorum grows, including some very isolated ranges such as the Virgin, Trumbull, and Hualapai Mountains in northwestern Arizona; the Ladron, Organ, Guadalupe, and San Andreas Mountains (Salinas Peak) in New Mexico; Navajo Mountain in southern Utah, and Mesa de Maya in southeastern Colorado. Andrews and Daniels (1960) recorded the parasite on 36% of about 3,000 plots scattered throughout the range of P. ponderosa var. scopulorum in Arizona and New Mexico.

Arceuthobium vaginatum subsp. cryptopodum distribution maps have been published for Colorado (Hawksworth 1987c), Utah (Albee and others 1988), and New Mexico (Martin and Hutchins 1980). Elevational range is 1,700&150;3,000 m, although it is found primarily between 2,000 and 2,600 m in Arizona and New Mexico.

Discussion: Arceuthobium vaginatum subsp. cryptopodum is characterized by thick, orange-colored shoots, but certain populations show various gradations from yellow to red and rarely may be purple to nearly black. Dark plants are particularly common in the Black Forest northeast of Colorado Springs, Colorado. Plants growing in deep shade tend to be greenish, as do those on Pinus contorta var. latifolia.

This dwarf mistletoe is particularly damaging to Pinus ponderosa in the Sacramento Mountains in south central New Mexico (Lincoln National Forest and adjacent Mescalero Apache Indian Reservation), central Arizona, and along the Front Range in Colorado. For reasons yet to be explained, the parasite, although common, is less damaging to P. ponderosa var. scopulorum in southwestern Colorado and southeastern Utah than in other areas.

The witches’ brooms induced by Arceuthobium vaginatum subsp. cryptopodum are similar on all hosts (fig. 16.111), except for Pinus contorta var. latifolia (Hawksworth 1961a). On this host the witches’ brooms are small, but the swellings at the point of infection are much more pronounced than on P. ponderosa var. scopulorum (often 2&150;3 times the diameter of uninfected branches).

 

Arceuthobium verticilliflorum
Big-Fruited Dwarf Mistletoe

33. A. verticilliflorum Engelmann, Botany of California 2:107, 1880. TYPE COLLECTION: MEXICO: Durango: Sierra Madre, (on Pinus sp.?), Seemann 2138 in 1852 (Lectotype MO! Isotypes: K!, ILL!). Razoumofskya verticillata (Engelm.) Kuntze, Revision of Genera of Plants 2:587, 1891.

Description: Mean shoot height ca. 7 (max. 11) cm, mostly yellow to yellow-green to purplish, without secondary branching, lightly glaucous when young (fig. 16.112). Basal diameter of dominant shoots 2.5&150;5.0 (mean 3.6) mm. Third internode 2&150;7 (mean 3.0 ± 1.2) mm long, 2.5&150;4.5 (mean 3.2) mm wide (4 collections), length/width ratio 0.9:1. Staminate flowers 3.5&150;4.5 (mean 4.0) mm across; perianth mostly 4-merous (sometimes 3-merous); verticillate, with 5&150;10 flowers per whorl; segments 1.8 mm long, 1.2 mm wide. Mean anther diameter 1.0 mm, centered 0.8 mm from tip of segment. Pollen polar diameter 21&150;25 (mean 24) µm; equatorial diameter 26&150;30 (mean 28) µm; polar/ equatorial diameter ratio 1:1.18; spine height ca. 1.0 µm or two-thirds wall thickness (1.5 µm) (2 collections). Mature fruit ca. 15 x 10 mm (figs. 16.113 and 16.114). Seeds ca. 11 x 6 mm; embryos 4 x 1 mm. n = 14.

Phenology: Meiosis September&150;October. Anthesis usually March&150;April. Fruits mature in September and October of the year following pollination; maturation period averages 18&150;19 months.

Hosts: This dwarf mistletoe parasitizes Pinus engelmannii, P. cooperi, P. arizonica, and P. durangensis. Pinus leiophylla vars. leiophylla and chihuahuana, and P. teocote are not parasitized even when they are associated with infected pines.

Distribution: Mexico (Durango) (fig. 16.115). This dwarf mistletoe is not as rare as we originally believed (Hawksworth and Wiens 1975). Several populations occur 11&150;54 km east of El Salto on the Durango&150;Mazatlán Highway. Recently populations have been discovered in the Sierra Candella, Sierra Huacol (Cibrián Tovar and others 1980), Sierra Guanacevi, and along the road from Santiago Papasquiaro to Altares. Elevational range is 2,000&150;2,750 m.

Discussion: This unusual species, although first described by Engelmann in 1880, was essentially ignored until we rediscovered the populations near El Salto (Hawksworth and Wiens 1965). The species is perhaps the most distinctive and primitive in the genus. The staminate shoots are characterized by thick spikes (4&150;6 mm) with verticillate 4-merous flowers, mostly 6 in a whorl; the entire spikes are deciduous after flowering. The species has fruits that are more than twice as large as any other dwarf mistletoe; they averaged 6 x 3 mm in March, 14 x 7 mm in July, and 15 x 10 mm in September when they were near maturity. A few fruits are probably dispersed in early September, but the peak period of seed dispersal is likely in October.

This is the only dwarf mistletoe in which the pedicels do not elongate and curve downward when the fruits mature (fig. 16.114), and the seeds are not explosively dispersed upward. In Arceuthobium verticilliflorum the pericarp appears to "ooze off" the top of the seed, which remains attached to the receptacle. There is no evidence of explosive discharge. This feature, plus apparent evidence of birds feeding among the fruits, suggests dispersal by birds. The primitive morphological features associated with this species indicate that birds are likely the original mode of dispersal and not a derived system. Its occurrence in very open stands and the seemingly random distribution of the parasites within stands and trees suggests that dispersal by birds is common in this species (other species tend to occur in closed stands as pockets of trees more severely infected in lower crowns than in upper crowns).

Arceuthobium verticilliflorum is also chemically unique in the genus because it has no flavonoids (Crawford and Hawksworth 1979, see table 14.2). Another unique feature of A. verticilliflorum is the heaviness of the seeds (200&150;270 mg). This is about 100 x the weight of dwarf mistletoe seeds that are explosively dispersed (Hinds and others 1963, Knutson 1984). It is extremely unlikely that seeds weighing 200 mg could be effectively dispersed explosively.

Kuijt (1970) concluded that the branching pattern of Arceuthobium verticilliflorum is basically verticillate. This dwarf mistletoe causes massive witches’ brooms, and the diameter of infected branches is sometimes greater than that of the trunk where the infected branch emerges. Infections on the main trunks of pines sometimes extend up to 3 m in length.

 

Arceuthobium yecorense
Yecoran Dwarf Mistletoe

34. A. yecorense Hawksworth & Wiens, Phytologia 66: 6, 1989. TYPE COLLECTION: MEXICO: Sonora: Mpio. Yecora: 2 km east-southeast of Yecora, on Pinus herrerai, latitude 28°22' N, longitude 108°54' W, elevation 1600 m, Hawksworth 2168, 7 May 1987 (Holotype US! Isotypes: ENCB, FPF, INIF, MO, UNAM, UC).

Description: Mean shoot height 12 (max. 17) cm, yellow-green to brown, flabellately branched (fig. 16.116). Basal diameter of dominant shoots 2&150;5 (mean 3) mm. Third internode 10&150;21 (mean 15) mm long, 2&150;4 (mean 2.4) mm wide. No flowers or mature fruits available for study. n = ?

Phenology: Anthesis has not been observed, but judging by the stage of development of the floral buds in early May, we suspect that it flowers in June. Time of fruit maturity is unknown, but it is presumably September&150;October.

Hosts: The principal hosts in the Yecora area of Sonora are Pinus leiophylla var. chihuahuana and P. herrerai. It occurs secondarily on P. engelmannii, and does not parasitize associated P. arizonica. In the Sierra Madre Occidental in western Durango, it occurs on P. herrerai, P. lumholtzii, P. leiophylla var. chihuahuana, P. engelmannii, and P. durangensis.

Distribution: Mexico (Sonora, Chihuahua, and Durango) (fig. 16.117). The distribution of this dwarf mistletoe is poorly known; it has been collected only in the Yecora area of southeastern Sonora and adjacent Chihuahua and about 100 km west of Santiago Papasquiaro in Durango. It is abundant at both locations. These populations are more than 400 km apart, so the dwarf mistletoe is to be expected in the intervening forest areas of western Chihuahua and northwestern Durango. Elevational range is 1,600&150;2,500 m.

Discussion: This distinctive dwarf mistletoe was first collected in 1986 near Yecora, Sonora, by Ignacio Carbajal V. of Forestal Sanidad, SARH, Mexico City (Hawksworth and Wiens 1989). Arceuthobium yecorense is characterized by its slender, greenish yellow to brownish shoots and early summer flowering period. The plants are morphologically most similar to A. aureum subsp. aureum of the lowlands of Guatemala (Hawksworth and Wiens 1977). The populations in western Durango have more yellowish and slightly taller shoots than the Sonora populations, but otherwise they are similar. The Pueblo of Yecora is the primary pine-producing area of Sonora where this dwarf mistletoe is both common and damaging.

 

Old World Taxa

Arceuthobium azoricum
Azores Dwarf Mistletoe

35. A. azoricum Hawksworth & Wiens, Kew Bulletin 31:73, 1976. TYPE COLLECTION: AZORES: Faial, southeast slopes of the Caldiera, 900 m, on Juniperus brevifolia, Wiens 4953, October 1973 (Holotype US! Isotypes: FPF, K, LISU, UT).

Description: Plants 7&150;14 cm high, greenish-yellow; secondary branching is verticillate, but rare (figs. 16.118 and 16.119). Basal diameter of dominant shoots 5&150;9 mm. Third internode 8&150;15 (mean 12) mm long, 3&150;6 (mean 4) mm wide; length/width ratio 3:1; staminate flowers ca. 2.5 mm across, mostly 4-merous (97%), rarely 3-merous (3%). Pistillate flowers verticillate. Mature fruit is unknown. Pollen is unknown. n = ?

Phenology: Anthesis September&150;October, fruits mature in November.

Host: Exclusively on Juniperus brevifolia (the only juniper on the Azores).

Distribution: The species is endemic to the Azores and occurs on the islands of Terceira, San Jorge, Pico, and Faial. Because the endemic host, Juniperus brevifolia, occurs on several additional islands (Palhinha 1966, Sjogren 1973), the dwarf mistletoe may also have a wider distribution. Most of the islands are severely deforested, however. Elevational range is 600&150;1200 m.

Discussion: This dwarf mistletoe was not discovered until 1913 (Guppy 1914, 1917). An account of the early collections and distribution of Arceuthobium azoricum (as A. oxycedri) is given by Palhinha (1942). Recent descriptions of distribution are given by Palhinha (1966) and Sjogren (1973). The parasite had previously been considered to be an extreme outlier (ca. 1,600 km) of A. oxycedri, but A. azoricum is one of the most distinctive of all dwarf mistletoes (Hawksworth and Wiens 1977). The high frequency of 4-merous staminate flowers is unusual. The consistently whorled pistillate flowers subtended by minute bracts are unique in the genus, although A. verticilliflorum has consistently whorled staminate flowers. A comparison of A. azoricum with the other two Old World parasites of Juniperus (A. oxycedri and A. juniperi-procerae) is given in table 16.7.

The basal internodes are unusually wide and approach 1 cm across. The shoots then taper gradually upward. This dwarf mistletoe typically occupies the distal ends of the host branch, where a gall up to about four times the size of the host branch is formed at the point of infection (fig. 16.119). The portions of the host branch distal to the original point of the infection are killed and ultimately fall away. This host reaction is typical of many other genera of mistletoes, but is otherwise unknown in Arceuthobium.

Arceuthobium azoricum and A. bicarinatum (a Caribbean species) are the only dwarf mistletoes restricted to oceanic islands. Arceuthobium azoricum might be of exceptional interest from phylogenetic and biogeographical points of view. The distinctiveness of the species, as well as the host, suggests a long and isolated evolutionary history. Ridley (1930) specifically mentioned A. azoricum (as A. oxycedri) as an example of long-distance dispersal, and hence probably a recent immigrant to the islands. The necessity for seeds of both sexes to be distributed almost simultaneously and in close proximity, the ecological differences between the mainland and island habitats, and the general continental distribution of the genus argue strongly against long-distance dispersal. Also, the juniper host would not appear to be well adapted for long-distance dispersal. Hawksworth and Wiens (1976) suggested that A. azoricum and its host may be survivors of early Tertiary continental genera that became restricted to recurrent volcanic islands along the mid-Atlantic ridge (McKenna 1972).

 

Arceuthobium chinense
Keteleeria Dwarf Mistletoe

36. A. chinense Lecomte, Notulae Systematicae 3: 170, 1915. TYPE COLLECTION: CHINA: Yunnan: Bois audessus de Ta-pin-ze; Delavay s. n. in 1887 (Lectotype P, Isotype K!).

Description: Plants 2&150;12 (mean ca. 5) cm high, yellowish green, secondary branching verticillate. Basal diameter of dominant shoots 1&150;2 (mean 1.5) mm. Third internode 4&150;6 (mean 5) mm long, 1.0&150;1.5 (mean 1.1) mm wide (4 collections); length/width ratio 4.4:1. Staminate flowers ca. 2 mm across; 4-merous (rarely 3-merous); segments ca. 1 mm long, 1 mm wide, tapered to tip, widest at base. Pollen polar diameter 14&150;18 (mean 16) µm; equatorial diameter 16&150;19 (mean 17) µm; polar/equatorial diameter ratio 1:1.09; spine height and wall thickness ca. 1.0 µm (2 collections). Mature fruit ca. 4&150;6 mm long, 3&150;4 mm wide, distal portion ca. 2 mm long, proximal portion yellowish green or glaucous. n = ?

Phenology: Time of meiosis unknown. Anthesis July&150;November, with a peak from mid-September to mid-October (Kiu 1984, Tong and Ren 1980), fruits mature in late-August to mid-October (Tong and Ren 1980); maturation period is 11&150;12 months.

Hosts: The only known host is Keteleeria evelyniana. The original description by Lecomte (1915), lists the host as "Abies," but it was undoubtedly Keteleeria. Kiu (1984) also believes that the host was misidentified because Abies does not occur at the comparatively low elevation (2,500 m) reported for this species.

Distribution: China (Yunnan and Sichuan Provinces). Tong and Ren (1980) report that it occurs throughout the range of Keteleeria evelyniana in southwestern China. A distribution map is given by Kiu and Ren (1982). Elevational range is 1,500&150;2,700 m (Kiu 1984).

Discussion: The only information on this species is from the reports of Anonymous 1939, Danser 1936, Handel-Mazzetti 1929, Kiu 1984, Kiu and Ren 1982, Lecomte 1915, and Ton and Ren 1980. Some of these papers, however, do not distinguish between Arceuthobium chinense and A. pini. Arceuthobium chinense causes extensive damage in both pure and mixed stands of Keteleeria evelyniana (Tong and Ren 1980). Handel-Mazzetti (1929) reports a young stand of Keteleeria in Yunnan Province that was completely killed by this dwarf mistletoe, which apparently forms systemic witches’ brooms (Kiu 1984b).

 

Arceuthobium juniperi-procerae
East African Dwarf Mistletoe

37. A. juniperi-procerae Chiovenda, Annali di Botanica, Roma 9:134, 1911. TYPE COLLECTION: Eritrea: Acchele Guzai, Ambra Debra, on Juniperus procera, Pappi 355, August 1902, and Mt. Mamahot, on Juniperus procera, Pappi 1161, August 1902 (Lectotype, Pappi 1161 FT).

Description: Plants 4&150;14 (mean 7) cm high; greenish yellow; secondary branching verticillate, but rare (fig. 16.120). Basal diameter of dominant shoots 1.0&150;3.5 (mean 2) mm. Third internode averages 7 mm long, 1 mm wide. Staminate flowers 1.8 mm across, all 3-merous in the 1 collection studied. n = 14.

Phenology: Anthesis from November to March, probably having 2 distinct, or less likely, 1 extended flowering period(s) (Hawksworth and Wiens 1976). Fruit maturation period poorly known, mature fruits were present March&150;April (Hawksworth and Wiens 1976). Developing fruits observed in mid-December were approximately two-thirds of their mature size.

Host: Parasitic only on Juniperus procera (fig. 16.121).

Distribution: Eastern Africa (Kenya, Eritrea, and Ethiopia) (fig. 16.122). Elevation range is 2,000&150;3,000 m.

Discussion: The earliest mention of a dwarf mistletoe from sub-Saharan Africa was by Chiovenda (1911) who described Arceuthobium juniperi-procerae from the northern highlands of Eritrea. With the exception of Cufodontis (1953), who cited A. juniperi-procerae in his checklist of Ethiopian plants, no one else has previously accepted this species as valid.

Arceuthobium juniperi-procerae is closely related to A. oxycedri. Consequently, it is useful to compare these species (table 16.7). Whorled branching is relatively common in A. oxycedri (45%), but uncommon to rare (3%) in A. juniperi-procerae. Occasionally, the staminate flowers occur in whorls in A. juniperi-procerae, a condition unknown in the other two species of dwarf mistletoes that parasitize junipers. The fruit of A. juniperi-procerae is also characterized by a distinctive shoulder below the perianth segments that is absent in A. oxycedri.

The living plants of Arceuthobium oxycedri are often conspicuously glaucous, especially the apical portions of the shoots and the fruit. The glaucous character, however, tends to be lost on drying. Arceuthobium juniperi-procerae is only slightly glaucous. There is also a marked difference in coloration between A. oxycedri and A. juniperi-procerae. The former is a deep green and rather resembles the color of most Viscum and Phoradendron species, whereas A. juniperi-procerae is a yellowish green that is more typical of dwarf mistletoes.

The distribution of Arceuthobium juniperi-procerae is poorly known. It parasitizes only Juniperus procera, which is the only juniper occurring in sub-Saharan Africa. The only extensive highland areas in Kenya from which the parasite is not known are the Cherangani Hills and Mt. Elgon. Preliminary searches for the parasite in the southern Cherangani Hills failed to reveal its presence, but deforestation has reduced the populations of J. procera there. Only the eastern slopes of Mt. Elgon extend into Kenya, and juniper was not observed there. The western slopes of all the principal highland regions of Kenya are typically much drier than the eastern slopes, and this dwarf mistletoe seems to favor relatively dry areas. In addition to the western highlands of Kenya, J. procera is also known from several mountain ranges in the northern arid regions of Kenya that exceed 1,800 m (such as in the Maralal area); the dwarf mistletoe might also be expected to occur there. Arceuthobium juniperi-procerae may possibly be scattered from the principal highland regions of Kenya to Eritrea and northern Ethiopia wherever suitable host populations might occur. In southern Kenya, the Loita Hills should be favorable habitat for the dwarf mistletoe, as well as the interesting stand of comparatively low elevation junipers at the northern end of the Chyulu Hills.

Presently, Arceuthobium juniperi-procerae is unknown in East Africa outside Kenya. There might be favorable habitat, however, in Uganda and Tanzania, especially on the western slopes of Mt. Elgon and in some of the higher isolated northeastern mountain ranges in the Karamoja region. In Tanzania, the juniper stands at the northeastern base of Mt. Meru and the northwestern side of Mt. Kilimanjaro were examined for dwarf mistletoe, but none was observed. The juniper stand at Mt. Meru was not extensive, and the stands on the northwestern slopes of Mt. Kilimanjaro have been significantly reduced through deforestation. Several limited surveys of the northern portion of the western Usambara Mountains in northeastern Tanzania, where substantial stands of Juniperus procera occur, failed to reveal the presence of dwarf mistletoe. The highland areas around Ngorongoro Crater should also be explored for dwarf mistletoe, as well as regions around Loliondo, which is essentially an extension of the Loita Hills in southern Kenya.

The Kipengere Range in the southern Tanzanian highlands has a juniper stand in the Ndumbe forest. These trees are likely the largest individuals of Juniperus procera in Africa and are probably more than 30 m tall. A brief visit to this stand showed no evidence of dwarf mistletoe infection. This area, however, may be too moist for Arceuthobium.

Juniperus procera occurs in scattered localities as far south as Zimbabwe, but the stands have apparently not been examined for dwarf mistletoe, with the exception of those on the Nyika Plateau in northern Malawi. The Nyika stand, however, is also much reduced through deforestation, and the junipers consisted of only a few hectares of scattered trees; no evidence of dwarf mistletoe infection was observed.

In Ethiopia and Eritrea, Arceuthobium juniperi-procerae is known from only three localities, all in the north; two original collections of this species were from the Eritrean Highlands south of Asmara. We collected the dwarf mistletoe in the same general region, but south of the Tigre Provincial border northwest of Makale. Ethiopia and Eritrea are so badly deforested that the original distribution of A. juniperi-procerae will likely never be known. Considering the degree of infection of Juniperus procera in the forest reserve northwest of Makale, where perhaps more than 90% of the trees were infected, one might suspect that A. juniperi-procerae originally had a rather widespread distribution in these countries, at least in the drier regions.

We suspect that Arceuthobium juniperi-procerae may also occur across the Red Sea in Yemen and adjoining areas.

 

Arceuthobium minutissimum
Himalayan Dwarf Mistletoe

38. A. minutissimum J. D. Hooker, Flora of British India 5: 227, 1886. TYPE COLLECTION: NEPAL: Kumaon Himalaya, 3,250 m, on Pinus wallichiana (as Abies excelsa), Duthie s. n. in 1884 (Lectotype K!). Razoumofskya minutissima (Hook.) Tubeuf, Naturwissenschaftliche Zeitschrift für Forst- und Landwirstschaft 17: 195, 1919.

Description: Plants ca. 5 (max. 10) mm high; pistillate plants greenish (fig. 16.123), usually about twice as tall as the staminate plants; staminate plants yellow-green (fig. 16.124); primary branches multiple and arising from basal cups, but without secondary branching. Basal diameter of dominant shoots ca. 1 mm. Third internode 0.5&150;1.4 (mean 0.8 ± 0.3) mm long, 0.3&150;1.0 (mean 0.4) mm wide (3 collections); length/width ratio 1.5:1. Staminate flowers 2.0&150;2.5 mm across; perianth mostly 4-merous (sometimes 3-merous), segments ca. 0.8 mm long, 0.8 mm wide. Anther diameter ca. 0.5 mm, centered ca. 0.4 mm from tip of segment. Pollen polar diameter 16&150;19 (mean 18) µm; equatorial diameter 19&150;21 (mean 21) µm; polar/equatorial diameter ratio 1:1.16; spine height about the same as wall thickness (0.8 µm) (3 collections). Mature fruit 2.0&150;2.5 mm long, 1.0&150;1.5 mm wide. n = ?

Phenology: Time of meiosis unknown. Anthesis is reported August&150;October (Datta 1956), but all flowering specimens we have collected or examined in herbaria were obtained in September. Seed dispersal is also in September, so the fruit maturation period averages ca. 12 months.

Hosts: The principal and only commonly infected host is Pinus wallichiana. However, Zakaullah and Badshah (1984) found that P. gerardiana is sometimes infected in northern Pakistan in areas where this tree is associated with infected P. wallichiana. Arceuthobium minutissimum may occur on Abies pindrow in the Simla Hills of East Punjab, India (Rajagopaliengar 1955), but this requires confirmation. Similarly, Rechinger’s (1976) report of it on Cedrus deodara in the Swat area of northwest Pakistan has not been verified. We, and several Pakistani forest pathologists, have found A. minutissimum to be common on Pinus wallichiana in this area, but it did not infect closely associated Cedrus. Gaur’s (1981) report of A. minutissimum on Debregeasia hypoleuca (an angiosperm) at low elevations on the Gangetic Plain of northern in India is in error. The observation was based on a fungus that superficially resembled the fruit of a dwarf mistletoe.

Distribution: Arceuthobium minutissimum is known from the Himalayas in Pakistan, India, Nepal, and Bhutan. Bagchee (1952) reports that its distribution in India is associated with the dry zone of the Himalayas in Upper and Lower Bashahr in Himachal Pradesh, Kulu in East Punjab, and the four divisions of Kashmir. Several workers note that this dwarf mistletoe occurs in the dry zone of Pinus wallichiana forests in India and Pakistan, but not in the more mesic zones of the host’s distribution (Bakshi and Puri 1971, Gorrie 1929, Hawksworth and Zakaullah (1985). Elevational range is 2,400&150;3,500 m (Brandis 1907).

Discussion: The type folder for Arceuthobium minutissimum at Kew contains 3 collections by Duthie from Nepal: Duthie s. n. in 1884, Duthie 3359 in 1884 (2 sheets), Duthie 5947 in 1886. Apparently Duthie s. n. in 1884 was intended as the type because the elevation of this collection agrees with the published description.

Hooker (1886) mentions that Arceuthobium minutissimum was perhaps the smallest dicotyledonous plant. The shoots are clearly the smallest of any dwarf mistletoe. Although the shoots (inflorescences) are usually less than 5 mm high, the "plant" is much more extensive if the endophytic system within the host tissues were included.

Several workers have questioned whether Arceuthobium minutissimum is dioecious, as are all the other members of the genus (Danser 1931, Datta 1951, and Hooker 1886). Our field observations indicate that the species is clearly dioecious. The suggestion of monoecism was likely based on the frequent occurrence of mixed pistillate and staminate infections. Datta (1951) described a single hermaphroditic flower in A. minutissimum, but this was undoubtedly a rare developmental aberration. Bhandari and Nanda (1968) discuss the embryology and morphology of A. minutissimum. The species is unique in the genus because the shoots emerge not only from the host cortex, but also from the needles, about 1&150;2 mm above the base of the fascicles.

Kuijt (1960b) suggested that the fruits of Arceuthobium minutissimum may mature in a single growing season (ca. 5 months) as in the North American species A. pusillum. However, our field studies in Kashmir and Pakistan show that peak periods for both anthesis and seed dispersal occur in September. Thus, a seed maturation period of about 12 months is indicated or the species has two flowering periods, which is unlikely.

This dwarf mistletoe is a serious pathogen of Pinus wallichiana in India and Pakistan, particularly on the drier sites where it causes severe brooming (fig. 16.125) and mortality (Ciesla 1993, Gorrie 1929, Zakaullah 1988, Zakaullah and Badshah 1982). Severe host damage and mortality are often associated with dwarf mistletoes that induce systemic infections. Our field studies indicate this is certainly the case for Arceuthobium minutissimum, and so have others (Gorrie 1929, Kuijt 1960b).

 

Arceuthobium oxycedri
Juniper Dwarf Mistletoe

39. A. oxycedri (DC.) M. Bieb., Flora Taurico-Caucasica III: 629, 1819. TYPE COLLECTION: unknown but Komarov (1936) states it is at P. Viscum oxycedri DC., Fl. Fr. 4: 274, 1805. Razoumofskya caucasica Hoffman, Hortus Mosquensis (unpaged), 1808. Razoumofskya oxycedri (DC.) Schultz, ex Nym. Consp.: 320, 1853. Viscum caucasicum Steud. No. ed 1: 888, 1891. A. juniperi Bubani, Flora Pyrenacea: 131, 1897. A. juniperorum Reynier, Bull. Soc. Bot. Fr. 66: 97, 1919. A. oxycedri (DC.) M. Bieb. var. cupressii Zefirov, Akademi Nauk SSR. Bot. Inst. 17:110, 1955.

Description: Shoots mostly 5&150;10 (max. 20) cm high, green; verticillate branching common (fig. 16.126). Basal shoots 1&150;4 (mean 2) mm across. Third internode 5&150;9 (mean 7) mm long, ca. 1 mm wide; shoots terete; internodes often markedly wider at the top than at the base. Staminate flowers 1.5&150;2.5 (mean 2.0) mm across, perianth mostly 3-merous (ca. 95%), occasionally 4-merous (ca. 5%), rarely 2-merous. Pollen polar diameter 17&150;19 (mean 18) µm, equatorial diameter 19&150;22 (mean 21) µm; polar/equatorial diameter ratio 1:1.17; spine height (1.5 µm) slightly greater than the wall thickness (1.0 µm) (2 collections). Mature fruit ca. 3 mm long, 1.5&150;2.0 mm wide. n = 13&150;17.

Phenology: Anthesis mostly September&150;October (August&150;September in China according to Kiu 1984b). Seed dispersal in October and November; maturation period about 13 months.

Hosts: Parasitic primarily on Juniperus oxycedrus, but also on several other native species of junipers and introduced Juniperus, Cupressus, Chamaecyparis, Thuja, and Platycladus (table 16.8). The most intensive studies of its hosts have been in Crimea, where Isikov and Zakharenko (1988) reported it on 18 species of trees including the native Juniperus oxycedrus, J. excelsa, J. macropoda, and J. sabina; but also on introduced J. virginiana (from the United States), J. thurifera (from the western Mediterranean region), Cupressus macrocarpa (3 varieties from California), C. lusitanica (2 varieties from Mexico), C. arizonica (3 varieties from Arizona), C. macnabiana (from California), C. funebris (from China), and Platycladus orientalis (2 varieties from China).

Distribution: Although described as "very local" in Europe (Tutin and others 1964), Arceuthobium oxycedri has the most extensive geographical distribution in the genus (fig. 16.122). Its range (table 16.9) extends over 100° of longitude or about 10,000 km from Spain and Morocco to the Himalayas of China (Hawksworth and Wiens 1976; Turrill 1920, 1926). The species probably occurs in Afghanistan, but we have no documentation. Tutin and others (1964) list its occurrence in Portugal as doubtful, and it is not listed in recent floras of that country.

Distribution maps for Arceuthobium oxycedri have been published by Hawksworth and Wiens (1976), for Spain (Bolos and Vigo 1990), for the Vaucluse Department in France (Girerd 1978), for Europe (Jalas and Souminen 1976), for the Mediterranean area (Markgraf 1934), for Himachal Pradesh, Jammu, and Kashmir States in India (Bakshi and Puri 1971), for the Upper Kanag Valley in Pakistan (Zakaullah and Badshah 1982), and for southwestern China (Kiu and Ren 1982). Elevational range is generally 1,000&150;2,500 m. It occurs as low as 550 m in northern Italy (Brilli-Cataerina and Gubellini 1983), as high as 2,700&150;3,100 m in Pakistan (Zakaullah and Badshah 1977), and up to 3,000&150;3,500 m in southwestern China (Kiu 1984b).

Discussion: Our field and laboratory studies (Hawksworth and Wiens (1976) have shown that Arceuthobium oxycedri, as originally interpreted, actually consists of three allopatric species: A. oxycedri (sensu stricto), A. azoricum from the Azores, and A. juniperi-procerae from Ethiopia and Kenya. All are parasitic on Juniperus. Some characteristics of these taxa are summarized in table 16.7 and discussed under the individual species.

Arceuthobium oxycedri is distinguished from the other species primarily by its deep green color, higher frequency of whorled branching, elongated internodes (especially in staminate plants), and more glaucous fruits. Zefirov (1955) described a new variety (var. cupressi) occurring on an introduced tree (Cupressus macrocarpa). This variety was distinguished from the typical form on Juniperus by its longer, thinner, greenish shoots. Giving formal taxonomic status to individuals or populations based on characters associated with plants occurring in unnatural habitats is unlikely to be accepted by most taxonomists. We place this variety into synonymy.

Arceuthobium oxycedri is reported to be damaging to juniper forests in many areas, but there are few quantitative reports of losses, except in Bluchistan, Pakistan, where this dwarf mistletoe caused 50% tree mortality in Juniperus macropoda (Jamal and Beg 1974).

 

Arceuthobium pini
Alpine Dwarf Mistletoe

40. A. pini Hawksworth & Wiens, Brittonia 22: 267, 1970. TYPE COLLECTION: CHINA: Yunnan Province, East flank of Lichiang Range, latitude 27°35' N, elevation 3,650 m, on Pinus, Forrest 10169 in 1913 (Lectotype BM! Isotype K!).

Description: Plants 5&150;22 cm high, olive to dull green; with verticillate branching (fig. 16.127). Basal diameter of dominant shoots 1.5&150;3 mm. Third internode 5&150;14 (mean 12) mm long, 1.5&150;2.5 (mean 2) mm wide (5 collections); length/width ratio 6.2:1. Staminate flowers 2.0&150;2.5 mm across; perianth mostly 3-merous (rarely 4-merous); lobes 1.0&150;1.5 mm long, 1 mm wide, rounded. Pollen polar diameter 15&150;18 (mean 16) µm; equatorial diameter 16&150;18 (mean 18) µm; polar/equatorial diameter ratio 1:1.13; spine height 1.5 µm, wall thickness ca. 1.0 µm (3 collections). Mature fruit ca. 3.0&150;3.5 mm long, 2 mm wide, distal portion probably glaucous. n = ?

Phenology: Time of meiosis unknown. Anthesis usually April&150;June, but one specimen collected in July still had a few flowers with pollen. Fruit maturation time not precisely known, but apparently in August or September; maturation period presumably averages ca. 16 months.

Hosts: Known only from Pinus densata and P. yunnanensis.

Distribution: Southwest China (Yunnan, Sichuan, and Xizang). Distribution map given by Kiu and Ren (1982). Elevational range is 2,600&150;4,000 m (Kiu 1984b).

Discussion: This species was formerly included in Arceuthobium chinense; but it is easily distinguished by its much larger shoots (10&150;20 cm versus 3&150;5 cm) and its occurrence on Pinus, as opposed to Keteleeria. Kiu (1984b) suggests that the species does not induce witches’ brooms, but the isotype at Kew clearly indicates systemic brooming (Hawksworth and Wiens 1972).

 

Arceuthobium sichuanense
Sichuan Dwarf Mistletoe

41. A. sichuanense (H. S. Kiu) Hawksworth & Wiens, Novon 3:156. 1993. TYPE COLLECTION: CHINA: Sichuan: Dawu, elevation 4,200 m; parasite of Picea balfouriana (as P. likiangensis var. balfouriana), Wu & Gao 111629; 23 May 1974 (Holotype CDBI). A. pini var. sichuanense Kiu, Acta Phytotaxonomica Sinica 22:205, 1984.

Description: Shoots 2&150;6 cm high, greenish yellow. Basal diameter of dominant shoots 1.0&150;1.5 mm. Flowers axillary or terminal on short lateral branches; staminate flowers 1.5&150;2.0 mm across; perianth 3-merous. Fruits 3&150;4 mm long, 1.5&150;2.0 mm wide. n = ?

Phenology: Anthesis June&150;July. Period of fruit maturation unknown.

Hosts: The known hosts are Picea balfouriana (= P. likiangensis var. balfouriana) (China and Bhutan) and P. spinulosa (Bhutan).

Distribution: Southwestern China (Sichuan and Xizang) and Bhutan. A distribution map of the species in Xizang is given by Kiu and Ren (1982). The species has been recently reported from Zong-lela in western Bhutan, where it is causing serious damage to Picea balfouriana and P. spinulosa (Naithan and Singh 1989). Elevational range is 2,800&150;4,200 m in southwestern China (Kiu 1984b) and 3,000&150;3,200 m in Bhutan (Naithan and Singh 1989; Rushforth, personal communication).

Discussion: This taxon was originally described as a variety of Arceuthobium pini, but we raised it to the specific level because it is a much smaller plant (2&150;6 cm versus 5&150;22 cm), has distinct host differences (Picea versus Pinus), and probable differences in witches’ broom formation. The two species are allopatric. This dwarf mistletoe occurs at the highest elevation of any species in the genus (4,200 m). Arceuthobium globosum subsp. grandicaule, however, attains an elevation approaching this (4,100 m) in central Mexico. Kiu (1984) notes that A. sichuanense causes systemic witches’ brooms.

 

Arceuthobium tibetense
Tibetan Dwarf Mistletoe

42. A. tibetense H. S. Kiu and W. Ren, Journal Yunnan Forestry College 1: 42, 1982. TYPE COLLECTION: CHINA: Xizang (Tibet): Mainling; elevation 3,400 m; parasite of Abies forrestii (= A. georgei); Chao 14, 21 May 1981 (Holotype, Herbarium of the Yunnan Forestry College, Kunming. Isotypes: SCBI, FPF!).

Description: Shoots 0.5&150;2.2 cm high, yellowish green, branching "opposite." Basal diameter of dominant shoots ca. 1.5 mm. Flowers terminal or axillary; staminate flower 2 mm across; perianth mainly 3-merous, rarely 4-merous, segments subtriangular, ca. 1.2 mm long. Fruit ca. 2 mm long, proximal portion glaucous. n = ?

Phenology: Anthesis May&150;June. Period of fruit maturation unknown.

Host: Known only on Abies forrestii (= A. georgei).

Distribution: Known only in the Mainling area in eastern Xizang (Tibet) at elevations of 3,200&150;3,400 m (Kiu and Ling 1988). A distribution map is given in Kiu and Ren (1982). The dwarf mistletoe reported on Abies pindrow in India may be this taxon (Rajagopaliengar 1955).

Discussion: This recently described species is characterized by its small size and exclusive occurrence on Abies. The species causes systemic witches’ brooms (Kiu 1984). The fir forests in the Mainling area are severely damaged by this dwarf mistletoe, which occurs on about two-thirds of the trees (Kiu and Ren 1982).

 

Rejected Species

  1. Arceuthobium bonaniae Grisebach, Cat. Plant. Cuben.: 121, 1866 = Dendrophthora bonaniae (Griseb.) Eichler, Flora Brasil. 5(2): 103, 1868.
  2. Arceuthobium cubense Leiva & Bisse, Revue Jardin Botanic. Nac. Cuba 4: 60, 1983 = Dendrophthora cupressoides (Macf.) Eichler, Flora Brasil. 5(2): 103, 1868. (Kuijt 1987)
    = Dendrophthora podocarpicola Leiva, Revue Jardin Botanic Nac. Cuba 7: 16, 1986.
  3. Arceuthobium cupressoides (Macf.) Grisebach, Flora of British West Indies: 315, 1864 = Razoumofskya cupressoides (Macf.) Kuntze, Revision of Genera of Plants. 2: 587, 1891
    = Dendrophthora cupressoides (Macf.) Eichler, Flora Brasil 5(2): 103, 1868.
  4. Arceuthobium dacrydii Ridley, Journal of Fed. Malay States Museum 6: 170, 1915 = Korthalsella dacrydii (Ridley) Danser, Recueil des Travaux Botaniques Neerlandai 31: 759, 1934.
  5. Arceuthobium domingensis (Spreng.) Grisebach, Cat. Plant. Cuben: 121, 1866 = Dendrophthora domingensis (Spreng.) Eichler, Flora Brasil. 5(2): 103, 1868.
  6. Arceuthobium epiviscum Grisebach, Memoires American Academy of Arts and Sciences N.S., 8: 192, 1861 = Dendrophthora epiviscum (Griseb.) Eichler, Flora Brasil. 5(2): 103, 1868.
  7. Arceuthobium glauca Grisebach, Cat. Plant. Cuben.: 121, 1866 = Dendrophthora glauca (Griseb.) Eichler, Flora Brasil. 5(2): 103, 1868.
  8. Arceuthobium gracile (DC.) Grisebach, Flora British West Indies: 315, 1864 (non-Engelmann, Memoirs American Academy of Arts and Sciences, N.S., 4(4): 59, 1849). = Razoumofskya gracilis (DC.) Kuntze, Revision of Genera of Plants 2: 587, 1891
    = Dendrophthora flagelliformis (Lam.) Krug & Urban, Berichte der Deutschen Botanischen Gesellschaft 14: 287, 1896.
  9. Arceuthobium mancinellae Grisebach, Cat. Plant. Cuben.: 121, 1866 = Dendrophthora mancinellae (Griseb.) Eichler, Flora Brasil 5(2): 103, 1868.
  10. Arceuthobium opuntioides (L.) Grisebach, Flora British West Indies: 315, 1864. = Razoumofskya jamaicensis Hoffman, Hortus Mosquensis (unpaged), 1808.
    = Razoumofskya opuntioides (L.) Kuntze, Revision of Genera of Plants 2: 587, 1891
    = Dendrophthora opuntioides (L.) Eichler, Flora Brasil 5(2): 102, 1868.

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