In 1993, British scientists determined that the alder dieback observed in recent years was a disease caused by a hybrid pathogen, Phytophthora alni (Forestry Commission 2004). The disease agent, P. alni, is a recently formed range of heteroploid-interspecific hybrids between a well-known pathogen of hardwood trees, P. cambivora, and an unknown taxon similar to P. fragariae (which attacks strawberries) (Brasier et al. 1999). Neither parent is reported to attack alder. Both parent species were probably introduced to Europe (Brasier 2003).
Initially three subspecies were identified: Phytophthora alni subsp. alni (= P. ×alni), P. alni subsp. uniformis, and P. alni subsp. multiformis (= P. ×multiformis) (Forestry Commission 2004). Scientists now use the term Phytophthora alni species complex (CAPS 2022). P. alni uniformis appears to be native to North America, from which it was accidentally introduced to Europe. One author suggested the introduction was in water accompanying fish stock for hatcheries (CAPS 2022).
One or more of the taxa in the complex are known to be present in Austria, Belgium, Croatia, Czechia, Denmark, Estonia, France, Germany, Hungary, Ireland, Italy, Latvia, Lithuania, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland, United Kingdom (CAPS 2022).
The complex has a narrow host range, attacking Alnus only. Other riparian species tested have been negative (Forestry Commission 2004).
Four species of alder are native to Europe: common or black alder (Alnus glutinosa), grey alder (A. incana), Italian alder (A. cordata), and green alder (A. viridis) (Forestry Commission 2004). A. viridis and A. incana are also native to North America (NRCS Plants database 2006). Four or five species are native to North America. See below.
Alders are pioneer species that tolerate wet sites so they are important components of riparian sites. They provide ecological corridors for flora and fauna, help to stabilize riverbanks, and filter pollutants before they enter the water (Vincent et al. 2025; CAPS 2022).
Equally important, alders establish symbioses with nitrogen-fixing bacteria of the Frankia genus. Alder leaf litter therefore constitutes a major component of the trophic chain in riparian ecosystems. Vincent et al. (2025) call the Alnus-Frankia symbiotic complex an “engineering system” in the functioning of these ecosystems. [They discuss the tree species’ trade-off between defense against the disease and N-fixing function.]
Interactions between alder species and the various pathogen taxa vary. Phytophthora alni alni is the most aggressive and pathogenic on European alder species. About 89% of European populations belong to this taxon. Phytophthora alni uniformis is the taxon most widespread in Scandinavia because it better tolerates cold temperatures (Brasier et al. 2004; CAPS 2022). Among the tree species, A. glutinosa is the most susceptible while A. incana is the most resistant (Forestry Commission 2004).
Disease severity varies from site to site. Since 2000, heavy losses have been reported in some of the large alder forests in Scotland, western England and Wales; northeastern France and neighboring Walloon area of Belgium; Bavaria and Austria; and southern Sweden (Forestry Commission 2004; CAPS 2022; CDFA).
Environmental factors probably play a role; flooding is thought to promote infection. Infection risk is higher in warmer, slow moving waters, and in fine textured soil, especially clay loams. However, the pathogen has also damaged alder in forest plantations on sites far away from water courses. In riparian stands, the pathogen typically infects the tree through lenticels and roots at the collar and less frequently at the surface of exposed large roots. In non-riparian sites, infection occurs through the tap roots, causing part of the root system beyond the observable collar rot to be dead. The infections at non-riparian sites suggest that the alder plants were infected at the nursery before planting (CDFA).
Several sources document evidence of pathogen transmittal on nursery stock (Forestry Commission 2004; Cech 2004; CAPS 2022). It also probably spreads via water: irrigation of fields with infected river water, fish farming, and even movement of cattle between river systems [CAPS 2022].
Some trees appear to recover–although as of 2004 scientist didn’t know whether recovery was permanent (Forestry Commission 2004). One factor appears to be cold temperatures that suppress the pathogen.
Detection is difficult. Reliable diagnosis in the field is impossible because symptoms are similar to those caused by other root and collar rot Phytophthora species (CAPS 2022). Infected plants in nurseries have no visible symptoms (Jung and Blaschke, 2004; confirmed by CAPS 2022).
North America
One or more alder species grow in nearly all of the continental states (NRCS Plants database 2006). Alnus incana is widespread in western states, the upper Midwest, and northeast; A. viridis is present in the upper Midwest and New England (CAPS 20220. California lists additional species: Alnus rubra (red alder) is found in the state’s coast ranges and the Klamath mountains; A. rhombifolia (white alder) across most of the state. The green ash subspecies A. incana tenuifolia (creek alder) grows in the coastal ranges and the Sierra Nevada. Two subspecies of Alnus viridis (Siberian alder) grow along the state’s northwest coast (Calfora, 2021; cited by CDFA).
Heavy loss of alders could result in significant ecological effects including changes in forest and soil composition and wildlife food and habitat (CAPS 2022). California’s analysis (CDFA) determined that introduction of the Phytophthora alni species complex to California would cause “high” ecological impacts, especially through reducing biodiversity, disrupting natural communities, or changing ecosystem processes, and disrupting endangered species’ critical habitats. Furthermore, loss of alders could trigger additional official or private treatment programs and significantly impact cultural practices, home/urban gardening or ornamental plantings. In addition, A. rubra is one of the few commercial hardwood species in the American West (CAPS 2022).
Forest managers in North America became concerned immediately upon detection of the pathogen in the United Kingdom. The initial focus was on dieback of Alnus incana tenuifolia in Alaska and the Rocky Mountains (Worrall, 2004). It was later determined that Phytophthora alni uniformis is common in streams and alder stands in Alaska and Oregon. While some sources (e.g., CAPS 2022) say it does not cause noticeable damage, California authorities report that in Oregon P. alni uniformis causes some – “limited” – lesions on red alder roots (CDFA).
The Phytophthora alni species complex has been listed by USDA’s Federal Interagency Committee on Invasive Terrestrial Animals and Pathogens (ITAP.gov) as one of the worst pathogens not yet introduced to the United States (CDFA 2021). North American Alnus species’ susceptibility is currently unknown. One study (Cech 1998) found that strains of P. alni were pathogenic to A. rubra. One model determined that the climate and wide presence of potential hosts make the United States suitable for the most pathogenic taxon, Phytophthora alni alni (Downing et al., 2010).
The most likely pathway for the complex to reach the U.S. is through importation of contaminated soil, nursery stock, or wood with bark. Currently, the import of Alnus species. plant material (except seed) is prohibited from all countries except Canada specifically to prevent introduction of the Phytophthora alni species complex (CAPS 2022).
Project CAPTURE
USFS scientists and managers developed a conservation priority-setting framework for forest tree species at risk from pest & pathogens and other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The scientists prioritized 419 tree species native to the North American continent. The analysis identified 15 taxonomic groups requiring the most immediate conservation intervention because of the tree species’ exposure to an extrinsic threat, their sensitivity to the threat, and their ability to adapt to it. Each of these 15 most vulnerable species, and several additional species, should be the focus of both a comprehensive gene conservation program and a genetic resistance screening and development effort. Phytophthora alni (causes alder dieback) is not known to be a threat to any of these 15 most vulnerable species. P. alni is not currently known to be a threat to any of these 15 most vulnerable species.
Sources
Abras, S. 2005. Reporting in Walloon Agricultural Research Centre No. 7 Summer 2005. On-line at www.cra.wallonie.be/english/doc/warc-info-072005.pdf; accessed December 9, 2005.
Brasier, C.M., Cooke, D.E.L., and Duncan. J.M. 1999. Origin of a new Phytophthora pathogen through interspecific hybridization. Proc. Natl. Acad. Sci. Vol. 96, 5878-5883.
Brasier, C.M. 2003. Phytopthoras in European forests: Their rising significance. In Sudden Oak Death: How Concerned Should You Be? An International Online Symposium April 21.
https://www.apsnet.org/online/SOD/Papers/Brasier/. Accessed December 9, 2005.
California Department of Food and Agriculture (CDFA) Pest Rating Proposal for Phytophthora alni species complex November 2021.
Cech, T.L. 2004. Phytophthora disease (Phytophthora alni) of Alder – current situation in Austria. in Forstschutz Aktuell Nr. 29 – Abstracts Waldschutz / Suche Forstschutz Aktuell Nr. 29 – Abstracts at https://bfw.ac.at/400/2256.html. Accessed December 1, 2005.
Forestry Commission, 2004. Phytophthora Disease of Alder. Accessed December 1, 2005.
Potter, K.M., Escanferla, M.E., Jetton, R.M., Man, G., Crane, B.S., Prioritizing the conservation needs of US tree species: Evaluating vulnerability to forest insect and disease threats, Global Ecology and Conservation (2019), doi: https://doi.org/10A.1016/
United States Department of Agriculture Cooperative Agricultural Pest Survey (CAPS) Datasheet Version 3 December 30, 2022
United States Department of Agriculture Natural Resources Conservation Service Plant Database 2006. https://plants.usda.gov. Accessed 2006.
Vincent, M. H. Boubakri, P. Fournier, N. Parisot, P. Pétriacq, C. Cassan, A. Flandin, G. Miotello, J. Armengaud, A-E. Hay, A. Herrera-Belaroussi. 2025. Phytophthora alni Infection Reinforces the Defense Reactions in Alnus glutinosa-Frankia Roots to the Detriment of Nodules. MPMI Vol. 38, No. 3, 2025, pp. 463–478, https://doi.org/10.1094/MPMI-12-24-0160-R
Worrall, J. 2004. Alder Dieback and Mortality: November 01, 2004. On-line posting at https://www.forestpathology.org/notes.html. Accessed March 23, 2006.