In recent years, the eminent British pathologist Clive Brasier has drawn attention to the role of the plant trade in spreading certain forest pathogens. His studies indicate that Phytophthora might flourish in the nursery setting, and have opportunities to hybridize (Brasier 2003). Spread of the species or hybrids might be exacerbated by nurseries’ use of fungistatic (not fungicidal) chemicals that suppress disease symptoms but do not kill the pathogen (Brasier 2003). As a result, diseased plants appear healthy and are approved for shipment and sale.
Brasier (2003) noted further that pathogens in the Phytophthora genus became a major threat to European forests during the 1990s. During that decade, twelve previously unknown Phytophthora species were discovered in European forests or nurseries (Brasier 2003). Little is known about these pathogens’ ecological roles, or places of origin (Brasier 2003). Some North American forests, which have host plants and climates comparable to European forests, appear to be at risk to several of these Phytophthora pathogens. Alder dieback is one of these diseases. The threats of Phytophthora kernoviae and Phytophthora quercina are discussed elsewhere in this Gallery.
Concern about the rising threat has spurred Europe-wide research on several of these disease syndromes, including the role of Phytophthora cinnamomi in the decline of oaks in Mediterranean climates; and the role of various Phytophthora species in deciduous oak decline; chestnut mortality; and alder disease (Brasier 1999). The International Union of Forestry Research Organizations (IUFRO) created a working group on Phytophthora species in 1998 (Brasier 1999).
Europe is home to four species of alder: common alder (Alnus glutinosa), grey alder (A. incana), Italian alder (A. cordata), and green alder (A. viridis) (Forestry Commission 2004). Green alder and grey alder are also native to North America (NRCS Plants database 2006). They are joined by another for or five North American native species; nearly all of the continental states have one or more alder species (NRCS Plants database 2006). Alders are pioneer species that tolerate wet sites and are important components of riparian sites (Forestry Commission 2004). Furthermore, alder roots have nodules that fix nitrogen (Forestry Commission 2004).
In 1993, scientists in the United Kingdom determined that the alder dieback they had observed in recent years was a disease caused by a hybrid pathogen, Phytophthora alni (Forestry Commission 2004). Alder dieback is now present in eleven European countries: Austria, Belgium, France, Germany, Hungary, Ireland, Italy, Lithuania, Netherlands, Sweden, and the United Kingdom (Forestry Commission 2004). In the UK, the disease has spread steadily since 1994; by 2003, more than 15% of surveyed trees were affected or killed (Forestry Commission 2004). Alder dieback is also widespread throughout Austria (Cech 2004); and has affected more than one fourth of the alders along streams in Walloon area of Belgium (Abras 2005).
The disease agent, the pathogen P. alni, is a recently formed range of heteroploid-interspecific hybrids between a well-known pathogen of hardwood trees, Phytophthora cambivora, and a second unknown taxon similar to P. fragariae (which attacks strawberries) (Brasier et al. 1999). Neither parent has been reported as attacking alder. Both parent species were probably introduced to Europe (Brasier 2003). The hybrid exists in many forms with varying virulence (Forestry Commission 2004). Several subspecies have been recognized – P. alni subsp. alni, P. alni subsp. uniformis, and P. alni subsp. multiformis (Forestry Commission 2004).
The several alder species also vary in susceptibility: A. glutinosa is most susceptible; A. incana is most resistant (Forestry Commission 2004). The disease is found only on alders; tests of other riparian species have not detected Phytophthora alni (Forestry Commission 2004).
It is important this pathogen is not transmitted to North America. An alder dieback (of Alnus incana subsp. tenuifolia) has been detected in Alaska and in the Rockies, and it was considered possible that these diebacks were due to a Phytophthora pathogen (Worrall 2004). However, tests for Phytophthora have been negative (J. Worrall, USFS, pers. comm., 2006), and so it appears more likely these diebacks are due to some other cause, possibly Cytospora canker.
The severity of the disease varies from site to site within countries (e.g., the United Kingdom) and across Europe. In some areas of Germany and France, it has had severe impacts (Forestry Commission 2004). Environmental factors probably play a role; flooding is thought to promote infection (Forestry Commission 2004). Some trees appear to recover from the infection–although scientists do not yet know whether that recovery is permanent (Forestry Commission 2004).
There is evidence that the pathogen can be transmitted on nursery stock (Forestry Commission 2004). Scientists have developed a soil test for pathogen detection by tree nurseries in order to avoid further distribution by contaminated plants (Cech 2004).
The advent of alder dieback highlights the potential threat of hybridization in the rapid evolution of new plant pathogens. Inadvertent national or international movement of species on propagation material is a serious worry in this context (Brasier, et al. 1999). As Brasier has noted, routine diagnostic procedures currently in use are insufficiently sensitive to allow their detection (Brasier et al. 1999).
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.
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. 1999. Phytopthora pathogens of trees: Their Rising Profile in Europe. Forestry Commission. Information Note 30. HMSO, London, 6p. available at https://www.forestry.gov.uk/pdf/fcin30.pdf/$FILE/fcin30.pdf
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.
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.
Potter, K.M., Escanferla, M.E., Jetton, R.M., Man, G., Crane, B.S., Prioritizing the conservation needs of US tree spp: Evaluating vulnerability to forest insect and disease threats, Global Ecology and Conservation (2019), doi: https://doi.org/10.1016/
Forestry Commission, 2004. Phytophthora Disease of Alder. Accessed December 1, 2005.
USDA NRCS Plants Database. 2006. https://plants.usda.gov . Accessed 2006.
Worrall, J. 2004. Alder Dieback & Mortality: November 01, 2004. On-line posting at https://www.forestpathology.org/notes.html. Accessed March 23, 2006.