White pine blister rust Cronartium ribicola
White pine blister rust is a damaging disease of five-needle pines (genus Pinus section Quinquefoliae) caused by the rust fungus Cronartium ribicola. Epidemics of this introduced pathogen have produced severe economic and ecological losses to North American white pines. Silviculture and genetics can help conserve high-elevation white pines (Shaw and Geils, 2010).
For a comprehensive review and synthesis of the history, ecology, and management of white pines threatened by white pine blister rust see the August 2010 issue of Forest Pathology (Vol. 40:3-4).
All North American species of white pine are susceptible to white pine blister rust (Tomback and Achuff, 2010). In eastern North America, the pine host is eastern white pine (P. strobus). In the West, six species of white pine are infected in the wild. Infestations have not been reported for Great Basin bristlecone pine (P. longaeva) and the Mexican white pine species. The infected pines are western white pine (P. monticola), sugar pine (P. lambertiana), whitebark pine (P. albicaulis), Rocky Mountain bristlecone pine (P. aristata), foxtail pine (P. balfouriana), limber pine (P. flexilis), and southwestern white pine (P. flexilis var. reflexa). Western white pine, sugar pine, and southwestern white pine usually occur in moist, mid-elevation forests. The other pines occur in dry, timberline communities. White pines are important for biodiversity and wildlife.
White pine blister rust requires a living host and alternates between two host types named for distinctive structures that produce the infective spores (Zambino, 2010). White pines are aecial (cup-like) hosts; Ribes (currants and gooseberries), Pedicularis (lousewort), and Castilleja (paintbrush) are telial (hair-like) hosts. Host plants are infected though needles or leaves by aerially dispersed spores (Geils and others, 2010). Spores produced on an aecial host only infect a telial host. One type of spore from a telial host intensifies the pathogen on other telial hosts, and a different spore type spreads the pathogen to white pines. Parasitism of pine inner bark produces a necrotic canker that can eventually kill the pine host, while the disease usually only defoliates telial hosts.
White pine blister rust was introduced from Europe to North America in shipments of infected pine seedlings during the decades around 1900 (Geils and others, 2010). Initially, the pathogen spread rapidly in the humid-temperate climates of maritime and Great Lakes regions. Spread and intensification in the dry, extreme climates of continental regions and alpine environments has been slow and intermittent. White pines in Mexico and much of the southwestern US have escaped infection, even though aecial and telial hosts are present and other stem rust fungi tolerate the climate of these regions.
Control of white pine blister rust uses silvicultural and genetics to reduce infection and damage (King and others, 2010; Ostry and others, 2010; Zeglen and others, 2010). Early controls employed quarantine and eradication of Ribes; later, chemical and mechanical sanitation methods were developed. Current practices rely on vegetation management (site selection and preparation) and genetics (improved resistance). Numerous, site-specific factors affect the likelihood of infection and severity of infestation. Frequent episodes of cool, wet weather during the late summer favor inoculum production and dispersal. Proximity to telial hosts with abundant inoculum and abundant tree foliage close to the ground increase pine infection. Inherited traits affecting tolerance and resistance to infection vary by host species and population. Interactions between environmental and genetic factors are also important (Richardson and others 2010).
Summary of status of host species:
Before introduction of white pine blister rust, eastern white pine had been the major timber species in eastern Canada, New England, and the Great Lakes (Geils and others, 2010). Historically important products were ship masts and structural lumber; recently, the principal use has been for finish molding. Early campaigns in the East for Ribes eradication were supported and effective because pine trees were valued more than Ribes bushes. Blister rust is now a minor concern in growing eastern white pine for ornamental or wildlife use (Ostry and others, 2010).
The range of western white pine extends from sea level in British Columbia, through the forests of the Pacific Northwest (Washington and Oregon) and Idaho and Montana, to the subalpine forests of northern California (Tomback and Achuff, 2010). Because of site differences and history, blister rust incidence varies from stand to stand, but is everywhere a major concern. Western white pine once dominated the forest canopy in the Idaho and Montana; but logging removed the valuable sawtimber trees and disease severely reduced reproduction. Over 90% of the historical white pine range, species of fir, larch, and hemlock now dominate the forest (Schwandt and others, 2010). Nonetheless, western white pine remains important for timber and wildlife because of its adaptability to diverse sites, rapid growth, high quality wood and abundant seed production. Western white pine can be grown successfully on many sites using silviculture and genetically improved stock.
Sugar pine, the largest pine in North America, is noted for its timber, aesthetic, and ecological value (Tomback and Achuff, 2010). The distribution of sugar pine extends from southern Oregon to northern Baja California, but blister rust infestation is restricted to the Sierra Nevada and north. Where blister rust occurs, incidence varies from severe to moderate. Losses can be mitigated with silviculture and genetics (King and others, 2010; Zeglen and others, 2010).
Whitebark pine is found at high elevations in the Rocky Mountains, Cascades, Coast Range, and Sierra Nevada (Tomback and Achuff, 2010). Because whitebark pine (and limber pine) usually are inaccessible and produce poor quality timber, these and other high-elevation pines have historically received little attention (Tomback and Achuff, 2010). However, whitebark pine is especially critical for many wildlife species—notably bears and nutcrackers (Tomback and Achuff, 2010). Although blister rust occurs over most of the host distribution, incidence and damage are greater in northern ranges (Schwandt and others, 2010). Mortality from mountain pine beetle, ecological changes from fire suppression, and regeneration failure attributed to blister rust contribute to population declines. Climate change in combination with beetles and blister rust further threatens whitebark pine. Silviculture is limited where these pines occur in remote, wilderness areas. Gene conservation is important, but screening and breeding for disease resistance is only recently begun (King and others, 2010).
Limber pine is distributed from the Canadian Rockies to the Southern Rocky Mountains (New Mexico) and California desert ranges. Blister rust is common throughout the northern ranges and generally absent in southern regions (Schwandt and others, 2010). Alberta populations are severely infested; epidemics occur in the South Dakota Black Hills, Nevada Jarbidge Mountains, and several Colorado forests. The pathogen is not reported for either California or Utah. Closely related to limber pine, southwestern white pine extends from southern Colorado to Mexico where it is replaced by P. ayacahuite. The oldest epidemic of C. ribicola on southwestern white pine dates to 1969. Since then, the epidemic in the cool, wet Sacramento Mountains of southern New Mexico has greatly intensified. Isolated epidemics occur in Arizona and New Mexico but neither Texas nor Mexico. Limber pine is similar to whitebark pine in ecology, value, and management; southwestern white pine resembles western white pine.
The other high-elevation white pines occur as small, isolated populations; only some are infested (Schwandt and others, 2010). The foxtail pines in the Klamath Mountains of northern California are infested, but the foxtail pines in the Sierra Nevada are not. The only infestation of Rocky Mountain bristlecone pine is in the Sangre de Cristo Mountains of southern Colorado. Diseased bristlecone pine trees are restricted to areas near infected limber pine and Ribes. Great Basin bristlecone pine, including the ancient trees in the California White Mountains, are not infested.
Although blister rust alone can damage white pine, other diseases, insects, and disturbances also pose serious threats (Schwandt and others, 2010). These threats vary by region and stand age and include mountain pine beetle, dwarf mistletoe, and shoot, cone or foliage insects and pathogens. Bear and deer strip bark from the bole; squirrel and other seed predators prevent regeneration. Other factors reducing white pine populations are wildfire and biotic succession.
North American white pine species share a long evolutionary history with the native mountain pine beetle. Blister rust and climate change, however, could greatly affect the character of beetle outbreaks (Hunt and others, 2010). Although most of the mature white pine trees in a stand were killed during an outbreak, enough trees had escaped in the past that populations of white pine could persist. Climate change, however, might allow for more frequent and severe outbreaks, and blister rust might reduce regeneration so much that white pine populations would not recover. The ecological and genetic interactions of beetles with the blister rust pathogen and hosts are so complex that the consequences of management intervention are difficult to predict. For example, a low-intensity burn intended to facilitate pine regeneration might also stimulate Ribes re-growth and thereby increase blister rust incidence (Zambino, 2010).
Genetic variation among American populations of C. ribicola is relatively small (Richardson and others, 2010). Several recent and interesting discoveries have been made, including—genetic differences between eastern and western populations, capacity for a broad telial host range, hybridization, and similarity among blister rust fungi of white pines from Asia, Europe, and North America. Therefore, even though C. ribicola is already widely distributed across North America, new introductions from Eurasia could have major undesirable genetic consequences.
Until the 1960s, Ribes eradication was the principal method for protecting white pine stands from blister rust (Zeglen and others, 2010). Although eradication was at least partially effective, it was discontinued because of increased costs, lower incidence, and conversion to multispecies forest management. Silvicultural methods, including hazard mapping, are still used; but they are augmented with genetic programs to produce improved planting stock. Traditional tree improvement techniques have identified how white pines react to infection (King and others, 2010). The susceptibility of white pine to infection decreases with age (ontogenic resistance). Some trees survive with disease better than others (partial resistance or tolerance). Actively resistant trees (R-gene or major gene resistance) are immune to all but specifically virulent races. Improved stock is available for western white pine and sugar pine; but few stands are now re-planted so opportunities to increase disease resistance during regeneration are lost (Schwandt and others, 2010). New molecular techniques provide the detailed understanding of the biochemistry of resistance needed for conserving high-elevation white pines (Richardson and others, 2010).
Management and research to maintain threatened white pine communities is justified by their ecological and economic importance (Tomback and Achuff, 2010; Schwandt and others, 2010). Some of the biological challenges to sustaining these communities arise from the diversity, complexity, and potential rapid evolution of blister rust pathogens and hosts (Hunt and others, 2010). Constraints on management intervention include costs, policy conflicts, and uncertainty.
Schwandt and others (2010) identify various silviculture and genetics techniques useful for sustaining viable white pine populations on the landscape. For high-elevation white pines, maintaining genetic diversity involves monitoring and protection of designated populations in the wild. Genetic options include selecting for resistance in North American populations and crossing with related Eurasian stone pines with resistance. Although silviculture could enable natural regeneration on many sites, planting would be required on sites lacking a desirable seed source. Adaptive management incorporating participation by a wide range of stakeholders, experimentation, and learning could mitigate the risks of action or inaction and help promote resilient ecosystems.
Several priority conservation actions identified by Hunt and others (2010) are:
o Obtain and incorporate into breeding programs the information needed for genetic resource management of white pines, especially high-elevation species
o Continue work on improving pine regeneration, protecting seedlings and poles, and developing better models to assess site hazard and treatment efficacy.
o Research the ecology, genetics and evolutionary biology of Cronartium ribicola fungi, Ribes and other telial hosts
o Assess and mitigate risk from expanded Ribes cultivation
o Research the genetic and proteomic factors controlling disease expression
o Gain public support for coordinated efforts
o Assess white pine ecosystem conditions, trends and threats, set priorities at regional and local levels; apply mitigation
o Continue providing expertise in forest pathology and other specialized fields.
Geils, B.W.; Hummer, K.E.; Hunt, R.S. 2010. White pines, Ribes, and blister rust: a review and synthesis. Forest Pathology (3/4): 147–185. [Online]. doi: 10.1111/j.1439-0329.2010.00654.x Available: http://www.treesearch.fs.fed.us/pubs/36222
Hunt, R.S.; Geils, B.W.; Hummer, K.E. 2010. White pines, Ribes, and blister rust: integration and action. Forest Pathology 40 (3–4): 402–417. [Online]. doi: 10.1111/j.1439-329.2010.00665.x Available: http://www.treesearch.fs.fed.us/pubs/36221
King, J.N.; David, A.; Noshad, D.; Smith, J. 2010. A review of genetic approaches to the management of blister rust in white pines. Forest Pathology 40 (3–4): 292–313. [Online]. doi: 10.1111/j.1439-0329.2010.00659.x
Ostry, M.E.; Laflamme, G.; Katovich, S.A. 2010. Silvicultural approaches for management of eastern white pine to minimize impacts of damaging agents. Forest Pathology 40 (3–4): I332–346. [Online]. doi: 10.1111/j.1439- 0329.2010.00661.x Available: http://www.treesearch.fs.fed.us/pubs/36134
Richardson, B. A.; Ekramoddoullah, A. K. M.; Liu, J.-J.; Kim, M.-S.; Klopfenstein, N.B. 2010. Current and future molecular approaches to investigate the white pine blister rust pathosystem. Forest Pathology 40 (3–4): 314–331. [Online]. doi: 10.1111/j.1439-0329.2010.00660.x Available: http://www.treesearch.fs.fed.us/pubs/36219
Schwandt, J.W.; Lockman, I.B.; Kliejunas, J.T.; Muir, J.A. 2010. Current health issues and management strategies for white pines in the western United States and Canada. Forest Pathology 40 (3/4): 226–250. [Online]. doi: 10.1111/j.1439-0329.2010.00656.x
Shaw, C.G.; Geils, B.W., guest eds. 2010. Special Issue: White Pines, Ribes and Blister Rust. In: Woodward, S., editor-in-Chief Berlin, Germany: Blackwell Verlag GmbH. [Online]. doi: 10.1111/efp.2010.40.issue-3-4
Tomback, D.F.; Achuff, P. 2010. Blister rust and western forest biodiversity: ecology, values and outlook for white pines. Forest Pathology 40 (3/4): 186–225. [Online]. doi: 10.1111/j.1439-0329.2010.00655.x
Zambino, P.J. 2010. Biology and pathology of Ribes and their implications for management of white pine blister rust. Forest Pathology 40 (3/4): 264–291. [Online]. doi: 10.1111/j.1439-0329.2010.00658.x Available: http://www.treesearch.fs.fed.us/pubs/36992
Zeglen, S.; Pronos, J.; Merler, H. 2010. Silvicultural management of white pines in western North America. Forest Pathology 40 (3–4): 347–368. [Online]. doi: 10.1111/j.1439-0329.2010.00662.x