Invasive Pathogens
Current Status
No serious exotic tree pathogens of native tree hosts have been confirmed in Alaska.
Forest Health Protection is gathering information about a newly detected disease of spruce trees in Alaska, spruce bud blight. This disease is now known to be caused by three separate fungi that can be distinguished based on microscopic spore structures. Of the three fungi, the native/non-native status of Gemmamyces piceae is unknown but it is well-distributed in Alaska and has been established for some time if it was introduced. Genetic work has been conducted with partners: visit the spruce bud blight page to learn more.
Black knot of cherry (Apriosporina morbosa) is damaging ornamental chokecherry trees and contributing to mortality of European bird cherry trees on the University of Alaska Anchorage campus and in adjacent woodlands. The disease causes black, knobby swellings (knots) on host trees and may girdle the stems and boles. Both chokecherry and European bird cherry are recognized as extremely invasive in Alaska, prompting interest in this pathogen’s potential impact on these hosts. While native trees and shrubs in Alaska are unsusceptible to black knot, some related desirable ornamental and orchard trees are susceptible to the disease.
Phytophthora austrocedri, an aggressive soilborne plant pathogen, was identified on symptomatic common juniper plants at two Oregon nurseries in 2024. This disease of unknown origin has been introduced to Argentina and the United Kingdom, where it continues to cause extensive mortality of Chilean cypress and common juniper, respectively. Preventing unintentional spread and eradicating infected plant material are critical steps to make sure this pathogen does not impact yellow-cedar or other related species in Alaska.
Alaska is not safe from invasive pathogen introductions, particularly with increased trade and transportation and changing climate. Continued importation of live plant material and firewood are major potential pathways for invasive pathogen (and insect) introductions. Factors that may have protected Alaska from pathogen introductions in the past actually heighten our vulnerability. For example, low tree species diversity translates to potentially substantial, statewide impacts if introduced pathogens cause damage or mortality of our few dominant tree species; so far, the most devastating invasive forest diseases in North America have not affected our native trees. There can be lengthy delays between introduction and detection on our vast landscape, potentially allowing pathogens lag time to spread long distances via microscopic spores. Worldwide, there are no examples of the successful eradication of invasive plant pathogens established in forest ecosystems. Preventing invasive pathogens from entering Alaska must be the top priority.
Plant pathogens that are inconspicuous and minor in their native range can have major impacts in new habitats due to differences in host susceptibility and climate, which impedes our ability to anticipate new introductions. That said, Forest Health Protection and cooperators in Alaska have been working to identify potential invasive tree pathogens in Alaska, primarily based on host genus susceptibility (see Table). A thorough assessment of potential exotic tree pathogens requires a comprehensive list of native species for context. As tree pathogens are found and identified, they are compared to known native species to determine whether they are native or suspected of being introduced. Unfortunately, mycology and pathology in Alaska is not advanced to the point where such lists include all or most organisms. Field surveys and identification of tree pathogens should be a continuing, long-term goal of the forest health program. Importation of plants closely related to our native species is the most likely mode of invasive pathogen introduction.
Ashiglar, S. M.; Hanna, J. W.; Ross-Davis, A. L.; Klopfenstein, N. B. 2014. The USDA Forest Service-RMRS forest fungi collection: Resource for fungal identification, developing biological controls, predicting invasive pathogens, and predicting potential impacts of climate change. In: Chadwick, K.; Palacios, P., comps. Proceedings of the 61st Annual Western International Forest Disease Work Conference; October 6-11, 2013; Waterton Lakes National Park; AB, Canada. Washington, DC: U.S.D.A Forest Service, Forest Health Protection. p. 127-130. Available here.
ÄŚernĂ˝, K., Pešková, V., Soukup, F., Havrdová, L., Strnadová, V., ZahradnĂk, D., & Hrabetová, M. 2016. Gemmamyces bud blight of Picea pungens: A sudden disease outbreak in central Europe. Plant Pathology, 65(8): 1267-1278. Available here.
Jaklitsch, W. M. and H. Voglmayr. 2017. Three former taxa of Cucurbitaria and considerations on Petrakia in the Melanommataceae. Sydowia, 69: 81-95.
Klopfenstein, N. B.; Juzwik, J.; Ostry, M. E.; Kim, M.; Zambino, P. J.; Venette, R. C.; Richardson, B. A.; Lundquist, J. E.; Lodge, D. J.; Glaeser, J. A.; Frankel, S. J.; Otrosina, W. J.; Spaine, P.; Geils, B. W. 2010. Invasive forest pathogens: Summary of issues, critical needs, and future goals for Forest Service Research and Development. In: Dix, M. E.; Britton, K., editors. A dynamic invasive species research vision: Opportunities and priorities 2009-29. Gen. Tech. Rep. WO-79/83. Washington, DC: U.S. Department of Agriculture, Forest Service, Research and Development: 23-33. Available here.
Content prepared by Forest Pathologists Robin Mulvey and Dr. Loretta Winton, Forest Health Protection, robin.mulvey@usda.gov and loretta.winton@usda.gov