Chapter 1—Introduction

Arceuthobium (Viscaceae) is a clearly defined group of small (generally less than 20 cm high), variously colored (yellow, brown, black, or red) flowering plants that are aerial parasites only on members of the Pinaceae and Cupressaceae. The dwarf mistletoes, as they are commonly known, are of unusual biological interest because they are the most evolutionarily specialized genus of the Viscaceae. Some of the features that make Arceuthobium unique among mistletoes include:

The dwarf mistletoes are of immense economic importance because they are the single-most destructive pathogen of commercially valuable coniferous timber trees in several regions of Mexico, western Canada, western United States, and parts of Asia (Bakshi and Puri 1971, Hawksworth and Shaw 1984, Zakaullah and Badshah 1977).

With the increasing recognition of dwarf mistletoes as destructive parasites on commercially important forest trees, the need for additional systematic and other biological studies on the group became apparent. Control of dwarf mistletoes in some areas has been hampered by inadequate knowledge of the identity of the parasite involved and the relationship to its host(s).

Arceuthobium has been classically included in the subfamily Viscoideae of the Loranthaceae. Van Tieghem (1895) considered Arceuthobium so distinct from related genera that he proposed its classification as a separate family positioned phylogenetically between the Viscaceae and the Santalaceae. This proposal, however, has never been followed. The subfamilies Loranthoideae and Viscoideae are now generally agreed to warrant family status (Barlow 1964, Thorne 1992). Members of these groups differ in floral morphology, floral anatomy, pollen characteristics, embryology, and chromosome size and numbers (Calder 1983). The previously supposed similarities between the two groups are largely the result of evolutionary convergence for the aerial parasitic habit and seed dispersal by birds, rather than the consequence of a common phyletic origin. We, therefore, accept full family status for both groups.

Engler and Krause (1935) classified Arceuthobium as a monogeneric tribe in the Viscoideae. They placed Dendrophthora, Phoradendron, and Korthalsella in the tribe Phoradendreae and separated Korthalsella in subtribe Korthalsellinae and Phoradendron in subtribe Phoradendrinae. Regardless of its relationship to other Viscaceae, Arceuthobium has clearly defined limits, and its generic status has never been questioned. Kuijt (1970) suggested that the genus is separable into two natural groups on the basis of branching habit (verticillate versus flabellate).

The New World has a greater number of species (34) than the Old World (8). In the Old World, two species are recently described from China (Arceuthobium tibetense and A. sichuanense). Another two species, A. azoricum and A. juniperi-procerae, have been segregated from the widely distributed species A. oxycedri. In the New World, dwarf mistletoes are found from southeastern Alaska, northern Canada, and Newfoundland through most of the western United States and Mexico to Central America (Honduras), with an extreme outlying population on the island of Hispaniola (see fig. 5.2).

Species diversity is greatest in northwestern Mexico and the western United States, where 28 of the 34 New World species occur. The six New World species outside this area are Arceuthobium bicarinatum on Hispaniola; A. aureum, A. guatemalense, A. hawksworthii, and A. hondurense in Central America; and A. pusillum in southeastern Canada, the Great Lakes region, and the northeastern United States. Twenty species occur in Mexico, and eight of these are found also in the western United States. Nineteen species occur in the United States, five of which also occur in Canada (A. americanum, A. douglasii, A. laricis, A. pusillum, and A. tsugense). Arceuthobium douglasii has the widest distribution in the New World. This mistletoe is distributed from southern British Columbia, Canada, southward throughout most of the western United States to southern Durango, Mexico.


Objectives and Scope

Our initial interest in Arceuthobium was to clarify the confusion surrounding the A. campylopodum complex (see Hitchcock and Cronquist 1964), which centered primarily around Gillís (1935) designation of host-forms in this group. However, when we discovered the rich dwarf mistletoe flora in Mexico (Hawksworth and Wiens 1965, 1977, 1980, 1989), we expanded our investigations to include a comprehensive treatment of the entire genus. We have since studied naturally occurring populations of all 38 known New World taxa, as well as 4 of the 8 known Old World species. Beginning in 1962, our field studies of Arceuthobium have taken us over 800,000 kilometers by foot, hoof, wheel, and wing throughout North America, Central America, the Caribbean, the Azores, Europe, North and East Africa, and the Himalayas.

The more than 3,000 dwarf mistletoe specimens we have collected in our field work, plus many early U.S. Department of Agriculture collections by J. R. Weir, G. G. Hedgcock, J. S. Boyce, and L. S. Gill, are filed at the USDA Forest Serviceís Mistletoe Herbarium at Fort Collins, Colorado. They provide the basis for our taxonomic understanding of the genus. Duplicates of our collections are deposited in various North American herbaria, particularly those of the University of Colorado (Boulder), Missouri Botanical Garden (St. Louis), U.S. National Museum (Washington, DC), and Instituto de Biologia of the Universidad Nacional Autonoma de Mexico, Escuela Nacional de Ciencias Biologias of the Instituto Politecnico Nacional, and Instituto Nacional de Investigaciones Forestales y Agropecuarias (Mexico City). In addition to our own collections, we have examined the specimens at the major herbaria in North America and Europe.

Our goal was to develop a natural and useful classification of dwarf mistletoes based on a broad eclectic approach involving morphology, ecology, physiology, biochemistry, and genetics. Other features of evolutionary interest included biogeography, paleobotany, reproduction, and life cycles. Finally, to provide utility for practicing foresters, discussions of pathological effects on host trees and control measures were included.

A glossary of some of the less familiar terms and special usages is included, as well as lists of scientific and common names of species mentioned in the text.


Taxonomic History

The first dwarf mistletoe described, Arceuthobium oxycedri, was originally included in Viscum (Clusius 1576). It was later segregated from Viscum by Hoffman (1808) as Razoumofskya. Arceuthobium was first proposed as a genus by Marschall von Bieberstein (1819), and it was generally used until the early 1900ís. The Vienna Botanical Congress in 1905 conserved Arceuthobium over Razoumofskya. Because the American Code emphasized strict priority, however, most botanists in the United States continued to use Razoumofskya. Arceuthobium finally displaced Razoumofskya as a result of the 1930 Cambridge Botanical Congress and Gillís (1935) taxonomic revision of the species in the United States.

Humboldt and Bonplandís collection of a dwarf mistletoe on Cofre de Perote (Veracruz, Mexico, 1804) was apparently the first in the New World. This specimen was designated as the type for Arceuthobium vaginatum described in 1806 by Willdenow (as Viscum vaginatum). In 1826, David Douglas (1914) discovered two dwarf mistletoes on his botanical explorations of the Pacific Northwest: A. campylopodum (on Pinus ponderosa) and A. americanum (on P. contorta). William Hooker (1847) first discussed the taxonomy of Arceuthobium in North America north of Mexico and compared Douglasí specimens and another by Drummond (of A. americanum) with the European A. oxycedri, but he detected no differences except for color variations. George Engelmann was the first to publish (in Gray 1850) a formal description of a dwarf mistletoe found in the United States (A. americanum). As the taxonomic architect of the genus in North America, Engelmann (associated with the Missouri Botanical Garden) named most of the American species of Arceuthobium during the latter half of the 19th century (Gray 1850, Watson 1880).

Between 1910 and 1920, G. G. Hedgcock and J. R. Weir of the Division of Forest Pathology (Department of Agriculture) independently initiated studies on the taxonomy and host relationships of Arceuthobium in the United States. Both published a number of articles on dwarf mistletoes, but for unknown reasons their taxonomic conclusions remained unpublished as manuscripts on file with the USDA Forest Service, Fort Collins, Colorado. This was unfortunate, because Hedgcock had a keen understanding of the genus. For example, he first recognized that the dwarf mistletoe on Pinus lambertiana (our A. californicum) was distinct from A. cyanocarpum, and that A. campylopodum was distinguishable from A. occidentale. Hedgcock (1915) published a host list of Arceuthobium for the United States and later reported some new hosts of A. campylopodum and A. occidentale resulting from artificial inoculations (Hedgcock and Hunt 1917).

Weir (1918a) artificially inoculated various hosts with several species of dwarf mistletoes in the Northwest under both field and greenhouse conditions, and he discussed the taxonomic implications of the results. He began a systematic treatise on American dwarf mistletoes but never completed the work. He did, however, publish observations of hosts associated with several dwarf mistletoes (Weir 1915a, 1915b, 1916c, 1917, 1918b).

Aven Nelson of the University of Wyoming described two new dwarf mistletoes: Arceuthobium cyanocarpum in Wyoming (Coulter and Nelson 1909) and A. blumeri in Arizona (Nelson 1913). Karl von Tubeuf (1919) published a review of the genus based on available literature, his extensive knowledge of A. oxycedri, and results of a brief trip in 1913 to the western United States. Unfortunately, this work did little to resolve taxonomic relationships. Heil (1923), another German worker, described A. abietis-religiosae, a parasite of Abies in central Mexico.

Following Nelsonís and Tubeufís studies, taxonomic understanding of the genus did not advance until a revision of the species in the United States was published in 1935 by L. S. Gill while employed by the U.S. Department of Agriculture. Gill re-evaluated Engelmannís taxonomic conclusions, recognized flowering periods as an important taxonomic character, and reduced the rank of several closely related species to host-forms of Arceuthobium campylopodum and A. vaginatum. Although Gill clearly established a taxonomic framework for the genus, he considered his work to be provisional "pending a complete revision of the genus based on further field and experimental evidence."

Job Kuijt (1955, 1960a, 1960b, 1963, 1964) rejected Gillís host-form concept for Arceuthobium campylopodum and A. vaginatum and considered each as a single variable species. He also concluded, as did Gill (1935), that A. bicarinatum of Hispaniola should be included in A. campylopodum.

Hawksworth and Graham (1963) first discussed the difficulties of applying Gillís host-form concept to the dwarf mistletoes that parasitize Picea in the western United States. They concluded that many of Gillís "forms" in the Arceuthobium campylopodum complex could be distinguished morphologically and should be accorded higher taxonomic rank.

Hawksworth and Wiens (1964) described a new species, Arceuthobium gillii (a parasite of Pinus leiophylla var. chihuahuana in southern Arizona and northern Mexico) that had been previously confused with the more widespread A. vaginatum subsp. cryptopodum (a parasite of P. ponderosa). Hawksworth and Wiens (1965) published the first comprehensive study of the genus in Mexico and described five additional taxa. Wiens (1968) analyzed the chromosome numbers in Arceuthobium and organized the species into distinct flowering groups. Next, Hawksworth and Wiens (1970a) published a subgeneric classification of the genus with descriptions of four new species and two new combinations from Mexico, Guatemala, and the United States. We then reviewed the biology and classification of the entire genus (Hawksworth and Wiens 1970b) and published our monograph on Arceuthobium 2 years later (Hawksworth and Wiens 1972).

Following the publication of our monograph, we segregated two species from the widely distributed Arceuthobium oxycedri: A. azoricum from the Azores and A. juniperi-procerae from East Africa (Hawksworth and Wiens 1976). Shortly thereafter, we described three additional taxa from Mexico and Guatemala (Hawksworth and Wiens 1977). This was followed by a study of the flavonoid chemistry of the genus (Crawford and Hawksworth 1979) and description of another new species (A. pendens) on pinyons from central Mexico (Hawksworth and Wiens 1980). Mark and Hawksworth (1981) analyzed the branching patterns in two California dwarf mistletoes. An update and review of the taxonomy of the genus was completed by Hawksworth and Wiens (1984). A re-evaluation of the classification of the dwarf mistletoes on Tsuga was then undertaken (Hawksworth 1987a), and the taxonomy and evolution of the genus was reviewed by Hawksworth (1987b). Clarification of the host relationships of three dwarf mistletoes that parasitize white pines in northern California and southern Oregon was completed by Mathiasen and Hawksworth (1988). Two additional new species were described, and two new combinations were proposed for Mexican dwarf mistletoes by Hawksworth and Wiens (1989). Hawksworth (1991a) reviewed the genus in Mexico and Central America. Three species from California and southern Oregon were described, and the subspecies and host races of Arceuthobium tsugense were characterized by Hawksworth and others (1992b). Finally, differences between herbarium collections of A. aureum subsp. aureum from Guatemala and Belize lead us to the discovery of A. hawksworthii (Wiens and Shaw 1994).

Five dwarf mistletoes are now known from southwestern China (Kiu 1984b): Arceuthobium chinense, A. oxycedri, A. pini, A. sichuanense, and A. tibetense. Kiu and Ren (1982) described A. tibetense, a parasite of Abies. Kiu (1984a) described A. pini var. sichuanense, a parasite of Picea; and Hawksworth and Wiens (1993) raised it to specific rank.

The recent investigations of electrophoretic characteristics and DNA sequencing analyses of the genus by Daniel Nickrent and colleagues at the University of Illinois and Southern Illinois University (Nickrent 1986, 1987; Nickrent and Butler 1990; Nickrent and Stell 1990; Nickrent and others 1984; Schuette 1992; Schuette and Nickrent 1992) have provided exciting new insights into the evolutionary history of the group and the relationships among the various taxa (chapter 15).


Ethnobotanical and Medicinal Uses

An extensive folklore surrounds the European mistletoe, Viscum album, in northern Europe and England. As new lands were discovered and colonized by these Europeans, other Viscaceae such as Phoradendron were encountered that resembled V. album. Inevitably, the folklore associated with V. album was transferred to these plants (Calder 1983). Because of their relative inconspicuousness and general dissimilarity to Viscum and Phoradendron, little folklore, however, is associated with species of Arceuthobium. According to Fernald (1900), French and English women in northern Maine used spruce twigs infected with A. pusillum in their hair at the mid-winter ball. He was unable to determine, however, if the plant had the traditional seasonal significance of the European mistletoe. Young women of the Okanogan–Colville tribe of British Columbia and Washington boiled branches from witchesí brooms of Douglas-fir infected with A. douglasii to make a hair wash that they believed gave them long, thick hair (Turner and others 1980).

Mistletoes are utilized for medicinal purposes by aboriginal peoples in many parts of the world, and the dwarf mistletoes are no exception (Moerman 1977). California Indians prepared a decoction of Arceuthobium occidentale to treat stomach ache (Chestnut 1902). Indians in Butte County, California, used an undetermined species of dwarf mistletoe for treating hemorrhage of the lungs and mouth, tuberculosis, emaciation, stomach ache, cough, colds, and rheumatism (Taylor 1981). Bella Coola Indians of coastal British Columbia employed shoots of A. tsugense in the treatment of several of the same maladies (Smith 1928). Navajo Indians of New Mexico and Arizona used both A. divaricatum and A. vaginatum subsp. cryptopodum for unspecified medicinal purposes (Vestal 1952, Wyman and Harris 1941).

Dwarf mistletoes were utilized in the treatment of several ailments in Veracruz, Mexico (Cházaro and Oliva 1988), and Martínez (1959) reports that Arceuthobium vaginatum is used for the treatment of cough in Mexico. Sra. S. Gonzales of Villa Guerrero, Durango, Mexico (personal communication, 1987), informed us that indigenous peoples in the vicinity of Tepehuanes, Durango, used a decoction of A. vaginatum subsp. vaginatum for the treatment of rheumatism and lung disorders and that A. globosum subsp. globosum was utilized for the treatment of diarrhea and nervous, pulmonary, and rheumatic disorders. Arceuthobium globosum subsp. globosum is also burned as incense in religious ceremonies, but Sra. Gonzales did not know the basis for this practice.

Various mistletoes, especially Viscum album, have been studied for pharmaceutical purposes, but the dwarf mistletoes have received relatively little attention in this regard. Extracts of Old World Arceuthobium oxycedri have a hypotensive action similar to extracts of V. album (Livon 1913); constituents of A. americanum show some anti-tumor activity (Sealwry and others 1959). Toxic proteins in V. album and Phoradendron have been examined for cancer therapy (Luther and Becker 1987). Several dwarf mistletoes have also been tested for toxic proteins (A. americanum, A. campylopodum, A. divaricatum and A. vaginatum subsp. cryptopodum), but only A. americanum tested positive for toxic acetone precipitate. This indicates a moderate level of toxic proteins, but too low to be considered for commercial use (Samuelsson 1969). Recent tests for pharmacologically active lectins in several North American species of Arceuthobium taxa have all been negative (H. Fraanz, personal communication, 1988).

There are reports of minor uses of dwarf mistletoes as dye plants (Bliss 1980) and as pollen sources for honey bees in California (Coleman 1921) and Arizona (OíNeal and Waller 1984).

The juniper dwarf mistletoe, Arceuthobium oxycedri, is a preferred food of sheep and goats in Mediterranean countries, the Near East, and in the Himalayas (Acatay 1954, Zakaullah and Badshah 1977). Acatay (1954) expressed concern over the forest damage caused by cutting down juniper trees to provide dwarf mistletoe fodder for sheep and goats. Various mistletoes are utilized as food for domestic animals in different parts of the world, and both the Viscaceae and Loranthaceae appear to have few chemical defenses against predation (Barlow and Wiens 1977).

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