White Pine Blister Rust Forest Health Protection USDA Forest Service

2013 Forest Health Protection and State Forestry Organizations

Management Guide for John Schwandt and Holly Kearns White Pine Blister Rust Forest Health Protection USDA Forest Service

Cronartium ribicola Fisch.

This fungus disease was  Conifer hosts: accidentally introduced from needled pines Topics All native five- Europe in 1910. Since then, it has devastated western white pine and Introduction 1  Alternate hosts: whitebark pine forests in the Life History 2 Ribes spp. (currants and northern and central Rocky gooseberries) are the main Mountains. Recognition 3 alternate hosts. Management 4 Resistance 4 Introduction Cultural practices 4 Management of western white pine This fungus has two host types that Pruning 5 has been confounded by the are both required to complete its life Exising 7 introduction of white pine blister cycle. One host is a five-needled pine Thinning 7 rust, caused by the fungus Cronartium and the other is a shrub or

Ribes management 8 ribicola Fisch. This disease was herbaceous host, referred to as the introduced into western North alternative host. Prioritizing stands 9 America from Europe in 1910 on Monitoring 9 On pines, the fungus causes branch infected eastern white pine seedlings flagging and stem cankers that Leave tree selection 10 grown in France and planted near eventually cause top kill or death. All Other Reading 10 Vancouver, BC. Western white pine sizes of trees are attacked and small proved to be highly susceptible to Field data sheet 12 seedlings can be killed rapidly. blister rust with mortality rates of 90 Generally, the larger the tree is at the percent or more in what were once time it becomes infected, the longer vigorous, well-stocked stands. The Key Points it survives after infection. On the disease is now distributed throughout  The cause of this alternate hosts, damage is usually the range of the pine hosts in the confined to small leaf spots but disease is an northern and central Rocky invasive species. minor defoliation ay occur if severely Mountain region. infected.  Forests have been dramatically altered by this WHITE PINE BLISTER RUST MANAGEMENT SUMMARY disease. Four major goals for integrated management of western white pines:  Pruning can save stands. 1. Breed for resistance. Use breeding programs that will incorporate desirable silvical characteristics as well as rust resistance while  Monitor to schedule minimizing selection pressure on the rust. treatment and 2. Prune and thin. Prune and thin to remove and minimize infections. assess resistance.  Hazard rate sites to 3. Hazard rate sites. Rate sites for potential blister rust damage. select the best 4. Maintain genetic diversity. Maintain genetic diversity by management. encouraging white pine leave trees.

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Life History White pine blister The white pine blister rust fungus is nearby. In late summer the fungus rust requires two host types to an obligate parasite which means it produces teliospores on small hair- complete its life can only survive on living host like projections on the underside of cycle. tissue. It has a complex life cycle leaves of the alternate hosts. These that includes five spore stages on soon produce very fragile spores Conifer hosts— two different hosts. Cankers on (basidiospores) which usually travel All native five-needled white pine produce characteristic very short distances to infect pines including western white pine (Figure 1), yellow-orange blister-like aecia that current- and one year-old needles of limber pine, whitebark erupt through the bark in spring. pine, sugar pine, and bristlecone pine.

Figure 5. Telia on underside of ribes leaf in the fall. [Photo by J. Schwandt] Figure 3. Aecia on a whitebark pine branch. [Photo by J. Schwandt] white pines. During spring, the fungus kills leaf Aeciospores from these blisters can tissue resulting in small leaf spots, travel long distances to infect leaves then grows into adjacent twigs and of the alternate hosts. Figure 1. Rust stem canker. branches. The fungus grows along During summer, pycniospores are branches about 2-4 inches per year Alternate hosts— produced near canker margins in toward the main stem, killing tissue Mostly Ribes spp.(currants watery droplets which insects carry as it advances. Cankers within six and gooseberries). from canker to canker resulting in Research has recently fertilization of the rust fungus. shown some species of Pedicularis (lousewort) and Castilleja (Indian paint brush) can also be infected, but their role has yet to be determined.

Figure 6. New canker (orange discoloration) in a twig. [Photo by J. Schwandt]

Figure 4. Pycniospores exuding from an inches of the bole may continue to infected pine twig. [Photo by J. Schwandt] grow several years, even in branches that have no live foliage remaining. Cankers on branches and young The fungus builds up on the stems generally produce aecia 3-5 alternate hosts through the summer years after needle infection, but by producing spores (urediospores) Figure 2. Spots on a Ribes leaf. sporulation can be very sporadic. which reinfect alternate hosts White Pine Blister Rust Page 3

Recognizing rust infections

New infections fungus only has minimal effects on Figure 9. the rust, but may help to identify Wetting makes cankers Although the fungus causes small easier to see. needle spots the spring after rust cankers. infection, the earliest obvious Rodent chewing on western white symptom is usually discoloration, pine (figure 9) has almost always swelling, or pitch flow on an been an indicator of rust infection infected twig or branch. As the since rodents are attracted to high fungus girdles the branch, the sugar concentrations in cankers. needles beyond the canker die creating distinctive “flags” (figure 7). The bark at the canker center becomes sunken or cracked while the leading margin of a canker is

A branch canker showing more distinct discoloration after wetting with water. Figure 8. Rodent chewing on white pine bark is almost always an indication of a blister rust canker. [Photo by J. Schwandt] Figure 10. Abnormal Figure 7. Branch flag caused by a canker Stem cankers girdling the branch. [Photo by J. Schwandt] stem cankers. Infected trees may appear vigorous yellow to brown. (The canker until shortly before death (> 90% margin can usually be made more girdle), although stem cankers visible by washing and lightly usually have abundant resin flow on scrubbing with water). the outer bark. Sporulation Abnormal cankers may also be Characteristic sporulating yellow found, especially on trees with blisters near the canker margins are some level of natural resistance. produced in the spring, but may These can appear as slow growing not occur until many years after cankers with swollen callus ridges infection, and may not occur every and sunken centers or year. During the summer, cankers misshapened areas at the base of may produce watery droplets of small trees that can easily be pycniospores that ooze out from mistaken for mechanical damage inside canker margins and leave or root disease infection (figure dark brown spots (pycnial scars) 10). Trees with basal cankers may which can aid in canker be bent by snow or chewed by identification. rodents which may make identification difficult. Trees with Other fungi and rodent chewing root disease usually have an During periods of high moisture irregular “canker” margin without cankers may be partially covered any yellow discoloration at the top with a pink-purple weakly parasitic and copious resin that increases fungus called Tuberculina. This below ground line.

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Management Considerations

There are four major goals for practices such as pruning and integrated management of western thinning to remove and minimize white pines: (1) use breeding infections (3) develop hazard rating programs that will incorporate systems that will help rate sites for desirable silvical characteristics as infection potential, and (4) maintain well as rust resistance while genetic diversity by encouraging minimizing selection pressure on leave trees. the rust, (2) use silvicultural Figure 11. White pine blister rust resistance offers the best long Breeding for resistance term solution for the restoration of white pines. [Photo by S. Hagle] A cooperative western white pine overcome host resistance. Strains tree improvement program to of C. ribicola that have overcome capture and concentrate naturally resistance in western white pine occurring resistance mechanisms have been discovered in Oregon was started in 1959. The early and California. These populations crosses proved resistance traits are being closely monitored and so could be successfully passed on far have not spread far from their White Pine Tree through breeding and nearly 15 origin. Because of the potential for Improvement years later seed was operationally the rust to change, there is an Program Goals available from this first generation ongoing program to select and test (F1) of selectively bred trees additional parent trees exhibiting (Sandpoint, Idaho seed orchard). resistance. The testing is  Produce seedlings The best candidates from the first conducted at the Coeur d’Alene that have good breeding were crossed to create a nursery where seedlings from silvical second generation (F2) to further potentially resistant trees are characteristics improve resistance. F2 seed subjected to an intense spore load orchards were established at under favorable infection  Resistance levels Moscow, Lone Mountain, and conditions. Trees are checked for that will provide Coeur d’Alene, Idaho. Most of the resistance and good growth good survival while white pine planted since the mid characteristics for 3 years and the minimizing 1980s have been F2 stock from best performers are being added to selection pressure these seed orchards (Moscow was a new seed orchard established in on the rust. phased out in the early 1990s). A 1986 at Grouse Creek, Idaho. The single green house test of this stock goal is to produce seedlings that reported that 66% of this stock have good silvical characteristics as would remain canker free. well as resistance levels that will Genetic resistance to blister rust is provide good survival while not infallible since the fungus can minimizing selection pressure on Because of the make genetic adjustments which the rust. potential for the rust to change, the program to Silvicultural practices to minimize infection and mortality select and test Although white pine has only been infection levels in many additional parent trees planted on a small fraction of its plantations are high enough that exhibiting resistance is historical range, thousands of acres additional treatments may be ongoing. have been planted, and natural necessary to prevent white pine white pine regeneration is present stocking from falling below on many cutting units. However, White Pine Blister Rust Page 5

(acceptable levels. In spite of the There are several management early test indicating 66% resistance practices that can help minimize for the life of the F2 stock, we levels of infection and mortality in Survey of F2 found about half of 60 surveyed white pine plantations. These plantations has plantations had over 50% practices use biological concepts to found higher rust infection, and some had over 90% create an environment less infection rates than infection after only 15-20 years. favorable to the rust, so the trees expected. However, the F2 stock at all sites have a better chance of surviving. had lower infection and mortality These practices include lower  Half of 60 levels than F1 or natural branch pruning, canker excision, surveyed regeneration. We are continuing to judicious use of thinning, and plantations had monitor stands to see if the higher practices that minimize ribes over 50% infection, populations. than expected infection levels  Some had over result in higher than expected 90% infection after mortality. only 15-20 years. Pruning Basal and lower stem cankers are and F1, we expect pruning to be at most often the cause of mortality least as effective in F2 as in naturals. of blister rust-killed white pines. (We are currently establishing plots Since blister rust can only infect to monitor effects of prune only as through needles, these infections well as pruning and thinning on F2 must occur through needles on stock.) small branches when trees are young. Therefore, pruning lower It is important to remember that branches before cankers can pruning does not change the rust develop or spread into the bole susceptibility of white pine. A tree may prolong survival and reduce with a branch canker is still mortality (figure 12). susceptible to future infections, but pruning has greatly increased its A test of pruning lower branches chances of survival. In fact, some from young white pine greatly geneticists are concerned that if reduced mortality and infection these trees live long enough to from blister rust over the next contribute to future natural thirty years. Thirty years after regeneration, resistance levels in treatment, survival of white pine in off-spring may be watered down. 15-year old stands of unimproved Careful monitoring of stands is stock that were pruned and recommended in order to apply thinned, was 70% versus 40% in pruning at the most beneficial time. Figure 12. Pruning removes the thinned only and untreated control This depends largely on branches nearest the ground where most infections occur. plots. In addition, the number of management objectives regarding [Photo by J. Schwandt] trees without obvious infections the amount of white pine desired in (“clean”) in the pruned and thinned a stand so it is crucial to look at plots was nearly twice the number both the infection levels and of clean trees in the controls and stocking levels when making thinned only plots. We have no pruning decisions. It is preferable to long-term pruning efficacy data allow the rust to have time to infect available for F2 stock, but since F2 the most susceptible trees, but to has always out-preformed naturals

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prune a stand while enough trees the canker with water will usually are still prunable or clean to assist in defining the margins of the maintain acceptable stocking levels. canker). For this reason, decisions to prune or excise should always be preceded Even if a stand has some blister Stands that are at by a rust status survey. Rust status is rust infection, it may not need to be least 10 years of the current status of white pine pruned. The rust status survey will age, and average blister rust infection in a stand. tell you how many trees per acre less than 35 feet tall Stands that are at least 10 years of are uninfected as well as number of trees with prunable and lethal are the best age, but average less than 35 feet in height offer the greatest cankers so you can make a much candidates for opportunity for pruning to reduce wiser decision about pruning. In a pruning. infection and mortality. A rust dense stand, there may be many status sample should include at least infected trees, but the number of 100 white pines. Infection can vary clean trees may still be high enough across a stand, so the survey should to satisfy management objectives, cover the entire stand and fixed area so pruning may be unnecessary (or plots are required to calculate trees delayed). In lightly stocked stands, per acre. Additional tallies of other every white pine may be needed to species may also be beneficial to maintain management objectives determine total stand density of all for trees desired per acre, so it may species. A sample rust status survey warrant pruning even if the number form can be found at the end of of infected trees is small. this section. A single pruning to a height of eight feet or the lower 50 percent of the tree height (whichever is Each white pine is rated by the less) is recommended. Individual most lethal canker observed. Trees infected branches above this height with no obvious cankers are should also be pruned considered “clean”, while those (“pathological pruning”). Pruning with only branch cankers more than above eight feet in trees greater 24 inches from the bole are than 16 feet tall has not been considered to be “safe”, since the justified for blister rust, but may be branch will likely die before the considered if pruning for clear canker can grow into the bole. If wood. Both dead and live branches the canker is on or within six inches should be cut since infections on of the bole, it is considered lethal. If “dead” branches may continue to the most lethal canker is on a grow towards the bole for several branch between 6 and 24 inches of years. Cuts can be flush with the Figure 14. (top) Damage the bole and within easy reach from branch collar since white pine has has occurred early in stand the ground, trees are considered very little tendency to develop development. Figure 15. prunable. Since the fungus grows 2- decay from wounds. Pruning is (bottom) Pruned seedling. 4 inches per year, if the margin of a accomplished using hand pruners, Early pruning may be the branch canker is four to six inches shears with 2-foot handles, or only option for saving young trees on hazardous sites. from the bole, it could probably be pruning saws, and there have never [Photos by J. Schwandt] safely pruned immediately, but if been reports of transferring the pruning will be not be conducted fungus on pruning tools. for at least a year, six inches is a Prune the desired number of white safer distance to use for surveys. pine crop trees per acre whether

(Moistening and lightly scrubbing they appear to be infected or not. White Pine Blister Rust Page 7

Other species should not be duff or brush and may be overlooked so crews should be pruned as their shade will help instructed to check carefully for prevent sunscald and reduce ribes, hidden branches and basal cankers. plus impede movement of rust (More detailed information spores. Live branches near the regarding pruning is available in base are often partially buried in Schnepf and Schwandt, 2006).

Excising Cankers

Excising cankers Excising is best accomplished from Small bole cankers on valuable mid-April through early June when trees or branch cankers within six bark is soft and canker margins are inches of the bole can be “excised” most obvious. Excisable cankers Figure 16. Excising around a to eliminate individual cankers. girdle no more than 50 percent of small branch canker will Excision consists of cutting a the tree circumference, have their effectively kill this canker. channel completely through the upper edge no more than six feet bark and cambium at two inches from the ground, and lower edge beyond the visible margin of the no less than six inches from the canker. This essentially creates an ground. (This includes the island of tissue that dies along with discolored area beyond the obvious the infection. Excision is very time edge of the canker.) Branch consuming (expensive), and far cankers that are within six inches more difficult than pruning, so it is of the bole can be treated by generally not recommended for pruning the branch and then operational forest use. excising the bole around the branch collar.

Thinning

Precommercial thinning is often precommercially thinned 10-15 used in the white pine type to year old white pine stands were Figure 17. A successful excision may leave a large scar that will release dominant and co-dominant significantly higher than unthinned eventually heal. trees by removing trees in the lower plots in the same stands 30 years crown classes. Recent research has after treatment. The higher losses found that while white pine in the thinned only treatments are responds to thinning, it does not probably due to increased retention need release unless over-topped by of lower live branches where many other species. We recommend infections occur and increased We recommend leaving all white pine regardless of vigor of Ribes populations which leaving all white infection or spacing (often referred may increase local inoculum levels. pine regardless of to as “ghosting”) and allow blister Plots that were pruned as well as infection or rust to thin the white pine. thinned had significantly lower rust infection and mortality than both spacing to allow Timing and intensity of the thinned only and untreated blister rust to thin precommercial thinning of other plots, so pruning of lower branches the white pine. species may also affect blister rust appeared to be more important infection. Blister rust infection than the increase in Ribes levels and mortality in five

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populations due to thinning. the most susceptible trees, and to Thinning should be delayed as enhance shading out of lower branches along with the shade Figure 18. long as possible to allow the rust to intolerant Ribes plants. Two important local thin Ribes species. Ribes Management Although spores from white pine that light partial cutting can activate can travel many miles to Ribes the dormant seed, but close before plants, spores produced on Ribes the plants can produce seed. Site that infect white pines are very preparation can also be a factor; fragile and do not survive very long. Recent surveys have not found as high a correlation between local ribes and infection levels as was once thought, but high populations of Ribes could still be a major factor in local inoculum levels.

Most Ribes species are shade intolerant, so any management activity that increases shade will Figure 19. Early blister rust control efforts Ribes lacustre (top) and Ribes help reduce Ribes abundance, and included eradication of wild currants and viscosissimum (bottom) are two any activity that increases sunlight gooseberries from forest lands, but cultivated common and important will encourage Ribes populations. plants also were removed from gardens. alternate hosts for white pine Since Ribes seed can remain blister rust. [Photos by S. Hagle] dormant if undisturbed in the forest burning generally increases Ribes floor many decades, activities that production (except for hot burns), disturb the duff layer can activate compared to unburned sites. dormant seed. Studies have found

Rust Hazard— The favorableness of a site for the development of the rust. Rust hazard rating is still under development, but several factors have been implicated by a University of Idaho study of 60 white pine plantations (citation).

 Infection levels increased with elevation above 3500 feet, slopes greater than 15 percent, Ribes bushes/acre greater than 100, and stand age.  Infection levels also were higher on sites with tall brush (> 4.5 feet), southern aspects, cedar- wild ginger habitat types or sites that had been broadcast burned.  Additional testing of these relationships is needed.  Local weather patterns and micro-site environment are also important factors contributing to rust hazard but are currently poorly understood. White Pine Blister Rust Page 9

Prioritizing Stands for Treatment If several stands are being density of all species. Grand fir, considered for pruning or western hemlock, and western Priority for treatment thinning, it may be necessary to redcedar more effectively suppress depends on: prioritize them to insure the Ribes than do Douglas-fir, western proper stands are treated. larch, and pines. Therefore, Ribes  Thinning plans, Prioritization will depend on many populations will decline most factors such as access, thinning rapidly in heavily stocked stands  Other species plans, and other species present, as with high proportions of grand fir, present, well as average age, height, and western hemlock, and western  Average age and rust infection levels of white pine. redcedar. Stands with large height, Prescriptions involving stocking amounts of grand fir and Douglas-  Rust infection reduction in young mixed stands fir may suffer increasing mortality levels, should take into account white from root disease so retention of  Relative site pine stock type (natural, F1 or F2), less susceptible white pine may be hazard. rust hazard, and level of rust increasingly important. infection as well as the stocking

Monitoring The importance of monitoring in 1992, 1998, and 2004. Infection stands for blister rust can not be levels in five F2 stands increased stressed enough. Even after dramatically in the first six years silvicultural treatments, stands and all but one doubled in 12 years should be monitored to evaluate while infection levels in the two F1 treatment effectiveness. Rust plantations doubled to 80% during infection and mortality levels can the first six years of monitoring. Monitor stands increase dramatically in a few years, Mortality rates in all but the even in plantations with initially youngest F2 plantation were  For changes in rust low levels of rust. already 30-46% and over 70% in infection and both F1 plantations during a 12 mortality levels. Table 1 shows survey results in year period 3-8 years after planting. seven young plantations surveyed  For treatment effectiveness after Table 1. Change in percent infection and mortality in seven thinning, pruning plantations of improved stock over a 12-year interval. and excising.

Stock Year % Infection % Mortality Stand Name Type Planted 1992 1998 2004 1992 1998 2004

F2 1986 Copper 1 37.7 49.7 60.0 10.3 32.0 46.3

F2 1986 Copper 2 11.5 34.4 45.9 9.8 18.0 29.6

F2 1984 Copper 21 29.9 42.7 47.6 13.4 28.7 38.4

F2 1989 Varnum 2 1.9 14.2 28.3 0.0 3.8 7.5

F2 1988 Varnum 11 1.8 36.0 45.9 1.8 10.8 45.0

F1 1988 Varnum 23a 43.1 80.2 88.8 6.0 61.2 79.5

F1 1988 Varnum 23b 43.8 83.8 90.8 19.2 46.2 71.5

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Leave Trees In stands with mature live white Guidelines for leaving up to 10 pine, it is advisable to leave some of trees per acre are provided in the best trees to help maintain the Schwandt and Zack (1996). The genetic diversity. These trees have Inland Empire Tree Improvement been exposed to many years of Cooperative encourages foresters to spores, and may possess resistance look for and document possible traits not yet incorporated into the rust free “plus” trees for addition to breeding program. the breeding program.

Figure 20. Top-kill from two blister rust cankers at different levels on the stem.

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