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TREE
DISEASES

SPRING 1981

E.P. Van Arsdel
Texas A&M University

CHAPTER 12
WHITE PINE BLISTER RUST
(BASIDIOMYCETES)

Introduction:

White pine blister rust (caused by Cronartium ribicola Fischer) is important as a disease in many ways. It is the only disease that has had a large-scale disease control program in operation for a number of years. It is the only disease in which several means of control are being used or attempted in the forest.

The host trees, the white pines, are among the most valuable in the forest. In the northeastern one quarter of the United States white pine is an important tree and the best for reforestation purposes.

Since white pine is one of the most desirable tree species for the northeastern United States, it is unfortunate that fear of the blister rust disease has greatly limited the amount of white pine planted. Research has shown that, in many areas, loss from the disease has not been great even where pine stands have not been protected through ribes eradication. Conversely, in parts of the Lake States, present control through eradication has not worked as well as is desirable. Other controls have been devised.

Hosts:

All of the white pines (Shaw's Haploxylon 1911, = Critchfield and Littles subgenus Strobus, 1966) are involved as hosts of the blister rust fungus. There is a gradient of resistance from immune to highly susceptible; this relative susceptibility is shown in Table 1. The relative susceptibility of these pines is proportional to the distance from the home species of the rust, Pinus armandii of central China. There is a distinct break in the scale of susceptibility which represents the Atlantic Ocean. The Old World species are resistant; the New World species are susceptible. The New World pines were separated from the fungus from the Pliocene era to1900. They became immune to the American form of the fungus which infects the closely related pinyon pines and Ribes, but remained susceptible to the Old World form of the fungus (Van Arsdel, 1969).

The gooseberries and currents, members of the genus Ribes, are the alternate hosts to both the Asiatic and American forms of Cronartium ribicola (the American form is Cronartium occidentale). The ribes suffer little harm from the fungus.

Table 1. Relative susceptibility of white pines of the Cembrae and Strobi subsections of the Subgenus Strobus to Cronartium ribicola.

Species

Relative Susceptibility 2/

Type Reaction 3/

Continent of Natural Range 4/

Locale in Continent of Home Range

Pinus armandii

1

Immune

Asia

W. China, Taiwan, and Hainan

P. griffithii

2

Resistant

Asia

Himalayas

P. cembra

3

Resistant

Europe

Alps & Carpathian Mts.

P. koraiensis

4

Resistant

Asia

Amur, Korea,

East Siberia

P. peuce

5

Resistant

Europe

Balkans

P. parviflora

6

Resistant

Asia

Japan

P. pumila

7

Susceptible

Asia

Amur, Kamchatka, E. Siberia

P. sibirica

8

Susceptible

Asia

Siberia

============= ============ ============ ============ ============

P. strobiformis

9

Susceptible

North America

SW U.S. & Mexico

P. strobus

10

Susceptible

North America

Eastern U.S., Canada and Mexico

P. ayacahuite

11

Susceptible

North America

Mexico (Southern Mts.)

P. flexilis

12

Very Susceptible

North America

West U. S., Central Rocky Mts.

P. monticola

13

Very Susceptible

North America

NW U. S., Rocky & Cascade Mts.

P. lambertiana

14

Very Susceptible

North America

Calif., Cascade, Sierra Nevada

P. albicaulis

15

Very Susceptible

North America

Northwest U.S., Rocky & Cascade Mts.

1/ Classification according to Critchfield and Little 1966.

2/ Relative susceptibility according to interpretation of following works: Bedwell and Childs, 1943; Childs and Bedwell, 1948; Hirt,1940; Lachmund and Hansbrough, 1932; Moir, 1924; Pierce, 1932; Spaulding, 1922, 1925, 1929, 1956, 1958, 1961. This is similar to a listing made by Bingham in 1950 and 1962.

3/ Susceptibility classes used by Spaulding 1929.

4/ Ranges from Critchfield and Little 1966.

Symptoms:

On pine, the fungus first makes a yellowish discoloration on the greenish bark. Swelling is often noted the second year, and a typical spindle shaped swelling can usually be seen the first or second year. Often the bark breaks and orange aecia and aeciospores break out in the older parts of the spindle.

On Ribes (gooseberry and currants) the first conspicuous sign is the orange urediospores that form on the underside of the leaf. In cool weather the brown hair-like telia form.

Causal organism:

Cronartium ribicola Fischer has long been considered an obligate parasite. It has been cultured tissue cultures of pine tissue, and on culture media containing "host tissue culture diffusates" (Harvey and Grasham 1969, Harvey and Grasham 1970) but in nature it grows only on living plants. The fungus must grow on two hosts to complete its life cycle (shown in figure 1.). It cannot spread from pine to pine. The mycelium growing in the pine is monokaryotic. A partial sexual fusion occurs in the pro-aecium in the early spring and dikaryotic aeciospores are released from the pine to infect ribes leaves. The mycelium in the ribes leaf is dikaryotic. The urediospores are the dikaryotic spores that are born on ribes leaves and infect ribes leaves. Teliospores remain attached to the ribes leaf. The dikaryotic nuclei fuse to make a 2n nucleus in the mature teliospore. The basidium grows out of the teliospore. Reduction division occurs in the basidium, and monokaryotic basidiospores are released to infect pines through stomata on leaves or young stems.

Transmission:

All of the spores are airborne in wind currents except the teliospore, which remains in place on the ribes leaf. Sporidia are released at night with the peak release occurring about 1:00 a.m. In most climates the spore must reach its new host in a single night and start penetration before sunrise. Aeciospores are released as dew dries off, usually about 10:00 a.m. for daytime transport. Urediospores are also released for daytime transport. The day-released spores are often lofted high and carried long distances to infect new ribes.

Effects of climate on spread:

In the northern Lake States where the climate is cool and moist, blister rust spreads from pine to ribes in the spring by means of aeciospores. There is little urediospore spread because the nights are less than 10C (50F) and therefore too cool for urediospores to germinate. A widespread fairly low level of infection occurs on the ribes, but the ribes are usually not defoliated. After two weeks of cool weather (no 3 days more than 28C (82F) fertile teliospores form. When 48 hours of wet weather cooler than 20C (68F) occur (July-Sept.), sporidia are released and infect pines. Pine infection is common.

In the southern Lake States, after a low level of aecial infection is present on the pines, the aeciospores infect the ribes. The temperatures are optimum for uredial infection on ribes and nearly all ribes leaves (especially R. cynasbaii) become heavily infected. The ribes bushes become defoliated by the fungus and there is no rust left to be carried back to the pine when the cool weather permitting infection occurs in the fall. Thus, the rust essentially controls itself by defoliating the alternate host. The rust is maintained at a low level, particularly on sites with locally cool wet microclimates, by certain rarer ribes (e.g. Ribes americanum) which are infected at a low level, do not defoliate, and spread the rust back to the pines on the favored sites. However, those sites where the most rust occurs (both in the northern and southern areas) have periods of weather cool enough for infection during the summer, and are not dependent on the ribes retaining their leaves into the fall.

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History of Control Development:

All control is based upon the relatively fragile system of spread by the basidiospore from the telia on the ribes to the pine.

In 1918 some of the earliest definite control recommendations telling how far ribes should be eradicated from the pines were made (Wheeler and Reynolds 1918). Wheeler's report emphasizes that rust cannot spread from pine to pine, but can come to the pine only from the currant and gooseberry bushes. It states that the only successful method of protecting pines is "the destruction of all currant and gooseberry bushes." He states "that the maximum distance to which it is necessary to remove currant or gooseberry bushes does not exceed one-third of a mile (1,760 feet) even under the most favorable conditions for infection. Under ordinary conditions a distance of 200 yards between white pines and currants or gooseberries will give much protection, probably sufficient to grow white pine commercially."

In 1920 the distance was shortened to 900 feet which is still the standard control distance today. "A distance of 200 to 300 yards between white pines and the nearest currant or gooseberry bushes is sufficient to protect the trees under ordinary conditions. Cultivated black currants are an exception to this rule because of their extreme susceptibility to the rust and their heavy spore production." (Gilbert and Reynolds 1920). Other modifications have been made from time to time. About 1945 the North Central Region (Milwaukee) of the Bureau of Entomology & Plant Quarantine recommended 900 feet zones in open areas with graduated differences to about 100 feet (as far as you could see) through dense woods, and no distance into swamps. Further modifications were suggested in my Station Paper 92, based on the University of Wisconsin Report for 1960 (Van Arsdel 1961).

Climatic Limits to spread:

The zoned controls for blister rust in the Lake States were developed from the climatic limits to spread (Van Arsdel et. al. 1956, Van Arsdel et. al. 1961, Van Arsdel 1972).

We can look at any mass of infection incidence data and make a pretty good estimate of how much climatic variation is occurring in the infection process. As an example, we can take the masses of infection data collected in the Lake States by E. E. Honey, H. N. Putnam, Ray Weber and others that I presented at the First International Phytopathology Congress in London in 1968 (Van Arsdel 1968a 1972).

The variation is tremendous between the 29 stands prior to eradication (shown in figure 2). The pre-eradication incidence of rust ranged from no cankers per 100 trees to the greatest infection in a given year of 118 cankers per 100 trees. This variation can be ascribed to environmental differences since all stands had abundant ribes present. The number of repeatedly observed sample trees was adequate. The median number was about 250 trees per stand, with the range in individual stands from 75 to 1,500 trees. After eradication, nearly all of the variation was removed since all plots had less than 20 cankers per 100 trees and all but two had less than 5 cankers Per 100 trees (Van Arsdel 1972).

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The white pine blister rust fungus (Cronartium ribicola J.C. Fisch. ex Rabenh.) is sharply limited by meteorological factors in the eastern United States (Charlton 1963; Van Arsdel, Riker, and Patton 1956).

In the Lake States, climate largely determines the distribution of rust. For convenience, the disease distributions can be divided into three scales according to meteorological conditions: (1) a macro scale that is determined mainly by latitude and mass area elevation, (2) a meso-scale that is determined by elevation range of hills and river valleys, and (3) a micro scale that is determined by the structure of forest stands and the influence of small hills and slopes within the stand (Van Arsdel 1968b, 1972).

In the large-scale climatic gradation, rust is more general and found on white pines (P. strobus) on all sites in the more northern regions, at higher elevations, and near cold bodies of water such as Lake Superior (Van Arsdel 1961, 1964, 1972). This is shown as zone 4 in Fig. 3.

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Figure 3 Map showing differences in quantity of blister rust spread to white pines in the Lake States. Rust in tops of emergent pines carried from distant ribes are characteristic of Zone 4. The cooler the summer weather, the more favorable for disease spread.

In the mesoclimatic scale, the hill-valley structures are super-imposed on the macroclimatic scale and modify the large-scale distribution. Rust is more prevalent at high elevations and scarce in broad river valleys (Van Arsdel, Parmeter, and Riker 1957).

In the microclimatic scale, the stand structure and small topographic features are superimposed on the mesoclimatic scale and further modify the climatic distribution. The bases of slopes, small narrow valleys, and small openings in the crown cover of the forest have abundant rust. Zone 1 in the map in Fig. 2 shows where rust is found only in these cool wet sites. Shoulders of hills and large openings in the forest have less rust on the pines (Van Asdel et. al., 1961). The meteorological forces that cause locally cool wet places are illustrated in Fig. 4.

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Figure 4. Drainage of cold air at night and radiant heat loss make locally cool wet spots.

The patterns of climate-controlled rust distribution are determined by temperature and moisture effects on the production of spores, the duration of spore viability, the germination of spores, the penetration into the host, and the establishment in the host. The patterns of spore dispersal are controlled by night air circulation (Van Arsdel 1972).

Control:

Control measures for white pine blister rust include eradication of the alternate host, ribes (gooseberries and currants); the use of white pine trees resistant to fungus; the eradication of established cankers by pruning or antibiotics; and climatic escape. Of these, resistant trees are not yet ready for release, and practical methods for using antibiotic chemicals have not yet been developed. Until the past few years, control has depended almost entirely on eradication of the alternate host. Recent advances in research have resulted in cheaper and better methods by using climatic escape as an aid to control (Van Arsdel 1961).

Eradication of the alternate hosts, the gooseberries and currants, within the vicinity of white pines is an effective and economical means of controlling white pine blister rust under ordinary conditions (Gilbert and Reynolds 1920). This is as true today as it was when the committee proposed it in 1920. The control methods from the 1920 paper I quoted in the preceding history section is still a true statement. European black currants are still a great threat and should not be grown in white pine-producing regions. Even in much of the high hazard zone (zone 4 in Fig. 3) eradication reduces the level of pine infection to less than 10 percent (Van Arsdel et. al. 1956). This is a low enough incidence to permit commercial growing of a white pine crop.

Blister rust control by ribes eradication has been evaluated in the North Central States, using unpublished data from 35 of the permanent plots maintained from 1935 to 1942 by Dr. Edwin E. Honey, Henry Putnam, and blister rust control workers, and from 39 plots I maintained from 1957 to 1967. Of Honey's plots, 29 had eradication treatment. I found that during the 4 years before eradication (eradication years varied), 5,174 cankers were initiated; during the 4 years after eradication 103 cankers were initiated. This was 72.4 cankers per 100 trees before eradication compared to 1.4 cankers per 100 trees after eradication. Of the 29 plots 23 showed a benefit from eradication, 3 showed no effect (these were rust free before eradication), and 3 showed negative effects. The negative effects were obtained in areas to which rust had not spread before eradication (1 case) or where continued infection could indicate long-distance spread (2 cases) (Van Arsdel 1968b). The foregoing data are presented in Figure 2.

In six pairs of Honey's plots where one of each pair was eradicated, the differences were more significant because some eradicated plots, as indicated above, showed an increase in infection after eradication, but to a much lesser degree than its non-eradicated partner. The non-eradicated plots had 2,575 cankers during the 4 years before their partners were eradicated and 1,663 cankers afterwards; the eradicated plots had 3,852 cankers before eradication, and 12 cankers afterwards. Expressed as cankers per 100 trees the ratios were 117:75:142:0.4 respectively. All paired plots showed a benefit from eradication (Van Arsdel 1968b).

Costs of eradication have been overestimated by people unfamiliar with the process in the field. In certain other areas unduly high administrative overhead costs are included in the eradication costs so it makes the project expensive. Using experienced crews in the zone extremely favorable to rust spread north of Lake Superior, where rocky areas and high populations increase costs, eradication costs about

$3.00 per acre (1967 dollars). Costs were about $6.00 per acre up until about 1960 when they abandoned camps of the CCC type and paid the boys' expenses in motels and restaurants. In Indiana farm lands where I did control work, eradication costs were $.25 to $.75 per acre in 1948.

Some people in the Lake States have advocated the use of red pine instead of white pine on the Superior National Forests because an economics evaluation report (King 1958) indicated blister rust control was not economical when the control costs were carried forward with compound interest to crop maturity time. The red pine establishment costs with brush clearing, rock raking, etc. were $70.00 per acre compared to $3.00 blister rust control costs (according to John Kernik, Timber Management, Superior National Forest) but there was no report out indicating that you couldn't afford to plant red pine at that high cost. It may well be that no forestry practice will pay a suitable rate of compound interest on invested dollars on the basis of sawtimber values alone on the poor, long rotation, northern Lake States forests. That's probably why the land was left with unpaid taxes and reverted to federal, state and county forests. The white pines reproduce vigorously on much of this land and control costs are relatively low-priced forest practices.

The discovery of Scleroderis canker as a serious and widespread disease on red pine has made red pine a much less desirable tree (Skilling and Cordell 1966).

Control of rust by alternate host eradication costs are higher in the northern Rocky Mountains where pine stands are inaccessible and road construction costs are charged against the disease control program.

Antibiotics have recently created great excitement and controversy as the new means of blister rust control. Part of the excitement is in the romance of using chemicals produced by microorganisms that have become so loved as cures for animal diseases. The areas where the chemicals could be used economically in the United States, particularly in the eastern part, are really limited. Costs are high (about $.05 per tree in dense stands) and the trees are too widely scattered in the high hazard zones.

The effectiveness of control by aerial application has yet to be demonstrated. Our 1961 tests on eastern white pine in the Lake States were unsuccessful. Comparable test have not been made in the western white pine regions.

Northern Rocky Mountain reports on the effectiveness of antibiotics to control blister rust were enthusiastic and claimed phenomenal control. They said spraying one part of the tree killed cankers all over the tree. The chemical also was supposed to prevent subsequent infection. Other observers of the western tests doubted this great effectiveness. Most agree the direct treated cankers are killed or set back Direct spraying and cutting or scratching of the canker is most effective. On January 5, 1966 I learned that aerial sprays with antibiotics had been discontinued in Region 1.

In the southeast at Ashville, Powers and Stegall (1965) report no effects of the chemicals on cankers. Field tests in the Lake States of Van Arsdel, Phelps, and Weber showed killing of direct treated cankers with "Actidione", Upjohn's brand of cycloheximide. Aerial sprays were ineffective. Phytoactin acts fungistatically; it stops canker growth for one year then they resume growth (usually accelerated). Low levels of phytoactin stimulated cankers to grow faster. Transport on main stems has been observed, but none to kill branch cankers. Bioassays do show transport throughout the crown. Complete cures (overgrowth started) of cankers sprayed with phytoactin was found on two 6' D.B.H. trees (of seven) that had been drenched on the basal 6' with 800 ppm in fuel oil emulsion.

The only controlled condition experiments made were my own made in the University of Wisconsin greenhouses. The summary of this paper is quoted here:

"Preliminary results of foliage sprays on cankered (graft transfers) trees indicated that cycloheximide could be transported in the tree, that it could kill cankers by means of foliage sprays on new shoot and needle growth (not hardened off), that sprays on old-growth needles were not effective on small trees, and that phytotoxic effects were serious in foliage sprays at 100 ppm on new shoots (flush growth). Oil sprays were most effective. Phytostreptin discolored cankers and had some effect, but the blister rust continued growth. Purified phytoactin in ethyl alcohol and water was more promising because it had a fungistatic as well as a great discoloring effect" (Van Arsdel 1962).

Pruning out the cankered branches or cutting out the cankers with a knife is a superior, simpler, and more effective method of removing cankers than antibiotics are today. Since more that 99 percent of the cankers are on the lower 6 feet of the stem, early pruning all the branches is a very effective means of blister rust control. Weber (1964) showed this to be effective. It is one of the most effective means of control we can use today in the long distance spread areas of zone 4 in the Lake States.

"Pruning was done and blister rust infection data were recorded in the spring of 1957 and in the falls of 1958, 1960, and 1962. Hand clippers were used to remove one or two whorls at each pruning. At least one-half the live crown was left in each tree. Blister rust infection data as of 1962 are as follows:

 

Pruned

Unpruned

Healthy trees

178

105

Fatally infected 1

43

152

Total trees

221

257

Percent fatally infected

19

59

1With a fatal stem canker or a branch canker that would probably be lethal if not pruned.

Tree height averaged 1 foot in 1957 and 9 feet by 1962. No appreciable loss of height growth has been noted in pruned trees as compared to the un-pruned checks. Thus far, a total of 5 man-hours have been spent on the four biennial prunings --less than 2 minutes per tree." (Weber 1964).

Donald Stewart had also suggested that pruning limited rust spread (Stewart 1957). Pruning methods for various types of cankers are presented by Martin and Gravatt (1954).

Resistant white pine trees offer the best control in the long run. In both western and eastern white pines resistant trees are found that have withstood heavy onslaught of the rust in the highest hazard zones of the northern Lake States and northern Idaho. Such resistant trees are rare, but their use in a breeding program is promising.

Some information about resistant trees is summarized below. The results are from 2 resistant pine programs, Dick Bingham's on western white pine in Moscow, Idaho and Robert Patton's on eastern white pine in Madison, Wisconsin. Both programs are now well financed and progressing well.

DATA

WESTERN WHITE PINE

EASTERN WHITE PINE

 

(gain in resistance- Hanover, 1963)

(Van Arsdel's memory of Patton's comments)

Number of resistant trees selected

200

250

Proportion of resistant seedlings from open pollinated resistant trees.

4.0%

practically none

Proportion of resistant seedlings from controlled crossed between resistant selections

12.6%

15%

Proportion of resistant seedlings from controlled crosses between resistant selections reselected on basis of desirable progeny.

26.8%

No data

Proportion of resistant seedlings from controlled crosses between resistant selections with high combining ability

30.0%

35%

Proportion of resistant seedlings from selfs of best combining tree (the selfs are giants).

 

60% with field resistance in remainder.

The data are rather similar but there are some differences. In both cases resistance to the disease seems to be polygenic. The eastern white pine has not had a number of F2 progenies produced but there is not a great deal more information on them yet. If the program on eastern white pine was aimed in certain ways) seeds from controlled crosses of high combing resistance parents could now be used in establishing seed orchards. However, there are only 6 such high combining trees available now, and Dr. Patton wants 18 genetic sources of resistance available before trees are released. He feels this is necessary because of the possibility of races developing in the fungus that could infect the resistant trees.

At this time when chemicals are not as successful as was hoped for, and genetically resistant trees are not widely available, the best way to grow white pine in the Lake States is with zoned control based on the climatic factors affecting the rust distribution.

For the best control of blister rust, the Lake States have been divided into four zones as was shown in figure 3 (from Van Arsdel 1968b).

Not all of the details of climatic zoning can be shown on a map of this scale. In Wisconsin and southern Minnesota all of the lower and broader valleys are in zone 1; the highest hills and plateaus are in zone 3; the remainder of the land is in zone 2. In the zone 3 and 4 region of the northern part of the State, any place with more than 1/4 of the trees with the yellow-orange or copper-brown blister rust flags in the tops of the tallest trees is likely to be getting long distance spread into the area. Those areas with the treetop rust flags are in zone 4, those without are in zone 3. Each of the four zones has a separate set of rust control aids tailored to fit the needs of that part of the State. These zones and their recommended controls are as follows:

ZONE 1 - SOUTH AND LOWLAND CENTRAL LAKE STATES.

(Un-shaded area in map in figure 3 contains all land less than 1,000 feet above sea level, except along the Great Lakes shores.) For this zone no rust prevention methods are recommended. Some losses will occur at the bases of slopes and in small openings in an overstory crown canopy, but these will not be enough to impair the development of the forest. Small openings in an overstory canopy are those openings in the crowns with a diameter less than the height of the surrounding trees. Such openings are cool and wet. These conditions favor rust infection of white pine (Van Arsdel 1968b).

ZONE 2 - SOUTH CENTRAL MIDLANDS.

(Stippled area in Figure 3 contains most land 1,000 to 1,200 ft. elevation and northern Lake Michigan shore.) For this moderate-hazard area the following controls are recommended: a.) Eradicate gooseberries and currants in pine areas and within 70 ft. of the stand or prune the branches from the lower one-half of the crown starting the third year after planting (about age 7) and continuing every two years until there are no branches within 8 ft. of the ground. b.) Avoid planting sites where cold air collects at night and outward infrared radiation continues while side shade. Avoid the bottoms of V-shaped valleys, kettle holes, bases of slopes, small openings in forests, and selectively logged or shelterwood cut stands. c.) Do plant under solid oak canopies on sandy or dry ridgetop sites, in open fields, on steep slopes, on hilltops and on hill shoulders.

ZONE 3 - NORTHERN ZONE

(Diagonally shaded area in Figure 3; northern area less than 1,400 feet above sea level away from cold lake shores and southern Wisconsin and Minnesota above 1,200 ft.)

a. Eradicate or poison Ribes within the pine stands and within 600 feet of the pines.

b. Prune the lower branches from the lower one-half of the crowns, starting at age 5-7 and continuing every two years until there are no branches within 8 ft. of the ground.

c. Prune out any cankers that appear on the tree at locations other than on the lower branches.

d. Plant only under overstories of aspen, oak on sand or other poor sites, or under lightly thinned birch or pine. In the thinned stands, spaces between the crowns must not exceed one-fourth the height of the surrounding trees. The overstories should be thin enough so the trees grow more than two feet in height each year after the trees are three feet tall.

e. Maintain overstories over natural stands in the manner described above in 3-d. Do not release your pines until they are thirty-five feet tall.

f. Plant only where the Ribes have been eradicated.

ZONE 4 - NORTHERN HIGHLAND, LAKE SUPERIOR SHORE.

(Horizontal shading in Figure l.) Blister rust spreads long distances in this zone, and losses to blister rust will be greater.

Control recommendations are exactly as for zone 3, but you may expect losses to blister rust up to 25 percent (usually less than 10 percent) of your stand, even with the controls. The amount of loss will vary greatly over different parts of zone 4 depending on how many rust spores are being carried into the particular area. Night circulation downdraft areas from counter lake breezes and valley winds will have the most rust. Two such areas are large enough to show in the map in Figure 1. They are the isolated sections of zone 4 found in Polk and St. Croix Counties, and in southeastern Marinette County.

Following these rules for escaping rust will permit freer use of white pine in the state and encourage the use of white pine for reforestation with only light losses.

RELATIVE EFFECTIVENESS

In zone 4, of these control methods) eradication alone reduced the number of cankers to l/8 of those found in the non-eradicated open field site; maintaining an overstory reduced the number of cankers to l/9 the number found without an overstory; and pruning the trees reduced the number to l/3 the number of cankers on the un-pruned trees (2/3 of trees survived.)

Literature Cited:

Bedwell, J. L. and Thomas W. Childs. 1943. Susceptibility of Whitebark pine to Blister Rust in the Pacific Northwest. J. of  For. Vol. 41:12.

Bingham, R. T. 1972. Taxonomy, Crossability and Relative Blister Rust Resistance of 5-Needled White Pines. Biology of Rust Resistance in Forest Trees: Proc. of a NATO-IUFRO Advanced Study Institute. USDA For. Svc. Misc. Pub. No. 1221, pp. 271-280.

Charlton, John W. 1963. Relating Climate to Eastern White Pine Blister Rust Infection Hazard. Coop. Study. East Reg., For. Svc. USDA, Upper Darby, Pa.

Childs, Thomas W. and J. L. Bedwell. 1948. Susceptibility of Some White Pine Species to Cronartium ribicola in the Pacific NW. J. of  For. 46:8.

Critchfield, William B. and Elbert L. Little, Jr. 1966. Geographic Distribution of the Pines of the World. USDA For. Svc. Misc. Pub. 991. 91 pp.

Gilbert, A. W. and Harris A. Reynolds. 1920. Report on white pine blister rust control 1920. Pub. by Amer. Plant Pest Committee, 4 Joy St., Boston.

Hanover, J. W. 1963. Breeding blister rust resistant western white pine. A mimeographed program for breeding. 6pp.

Harvey, A. E. and J. L. Grasham. 1969. Growth of the rust fungus Cronartium ribicola in tissue cultures of Pinus monticola. Canad. Jour. Bot. 47:663-666.

Harvey, A. E. and J. L. Grasham. 1970. Survival of Cronarium ribicola on a medium containing host tissue culture diffusates. Mycopathologia et. mycologia applicata. (In press when received in 1970).

Hirt, Ray R. 1940. Relative Susceptibility to Cronartium ribicola of 5 Needled Pines Planted in the East. J. of For. Vol. 38. No. 12.

King, D. B. 1958. Incidence of white pine blister rust infection in the Lake States. U.S. Forest Service Lake States, Forest Experiment Station, Station paper 64. 12 pp.

Martin, J.F. and G. F. Gravatt. 1954. Saving white pines by removing blister rust cankers, U.S. Dept. of Agriculture. Circ. No. 948.22 pp.

Moir, W. Stuart. 1924. White-Pine Blister Rust in Western Europe. USDA, Dept. Bull No. 1186, 32 pp.

Moss, Virgil D. 1957. Acti-dione treatment of blister rust trunk cankers on Western white pine. Plant Disease Reptr. 41:709-714.

Moss, Virgil D. 1958. Acti-dione stove oil treatment of blister rust trunk cankers on reproduction and pole western white pine. Plant Disease Reptr. 42:703-706.

Moss, Virgil D. 1961. Antibiotics for control of blister rust on western white pine. For. Sci. 7:380-396.

Pierce, R. G. 1932. A comparison of Five-Needled Pines of the World With Reference Particularly to the Foreign White Pines. Bureau of Entomology and Plant Quarantine Report. Processed 70 pp. On file, Texas ASM University Plant Science Department.

Powers, H. R., Jr. and W. A. Stegall, Jr. 1965. An evaluation of cycloheximide (actidione) for control of white pine blister rust in the southeast. Plant Disease Reporter. 49:342-346.

Riker, A. J., T. F. Kouba, and B. W. Henry. 1947. The influence of temperature and humidity on the development of white pine blister rust on Ribes leaves. (Abs.) Phytopathology 37:19.

Skilling, Darroll D. and Charles E. Cordell. 1966. Scleroderris canker on National Forests in Upper Michigan and Northern Wisconsin. U.S.D.A. Forest Service Research Paper NC-3. 10 pp.

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