Phytophthora alni (Brasier et al.)
Significance: A lethal root and collar rot disease of alder (Alder sp.) caused by a then undescribed species of Phytophthora, was first observed in the UK in the early 1990s. Subsequent surveys have found the pathogen to be very damaging and spreading on alder in many other parts of Europe, particularly central Europe. The disease poses a significant threat to natural and managed alder stands in Europe and threatens the stability of riparian ecosystems. Also, the disease likely poses a similar threat to alders on other continents, such as within North America and Asia. In 2001 the PAS posted an alert about the alder Phytopthora. At that time the fungus was still undescribed. In 2004 Brasier et al. formally described the causal agent as Phytophthora alni and considered it to be a probable hybrid between two other Phytophthora species. Brasier et al. subdivided the species into three variants; a standard hybrid designated as P. alni subsp. alni, a Swedish variant called P. alni subsp. uniformis, and a Dutch, German and UK variant called P. alni subsp. multiformis. In 2004, Jung and Blaschke published an update on the pathogen relative to alders in Bavaria and provided information on distribution, modes of spread, as well as possible management strategies. The pathogen was shown to be associated with spread in water courses, as well as with infected nursery stock. A 3-year fallow period between bare-rooted alder crops was recommended for control in infested areas as the pathogen appears to have a relatively poor long-term survival rate in soil. In 2005, Ioos et al. developed Polymerase Chain Reaction primers derived from Sequence Characterized Amplified Regions which could be used to detect P. alni in infested river water, in soil, and from necrotic alder bark. Since being found in the UK, Phytophthora alni has only been reported under natural conditions from Alnus species (Alder cordata, A. glutinosa, A. incana, and A. rubra). However, in 2003, Santini et al. showed in greenhouse inoculation trials, that other tree species, including walnut (Juglans regia) and chestnut (Castanea sativa), may be susceptible to the pathogen. More recently, Santini et al. (2006) published similar work showing that seedlings of wild cherry (Prunus avium) are also susceptible to Phytophthora alni in greenhouse inoculation trials.
Comments: Phytophthora alni appears to pose significant phytosanitary concerns for those countries where the pathogen does not yet occur and where alders may be at risk. Phytosanitary organizations should maintain an awareness about the pathogen and in particular any hosts or potential hosts that might act as a pathway for its introduction or spread.
Brasier, C.M., S.A. Kirk, J. Delcan, D.E.L. Cooke, T. Jung, and W. A. Man In't Veld. 2004. Phytophthora alni sp. nov. and its variants: designation of emerging heteroploid hybrid pathogens spreading on Alnus trees. Mycological Research Vol. 108(10):1172-1184. Ioos, R., C. Husson, A. Andrieux, and P. Frey. 2005. SCAR-based PCR primers to detect the hybrid pathogen Phytophthora alni and its subspecies causing alder disease in Europe. European Journal of Plant Pathology Vol. 112:323-335. Jung, T. and M. Blaschke. 2004. Phytophthora root and collar rot of alders in Bavaria: distribution, modes of spread and possible management strategies. Plant Pathology Vol. 53:197-208. NAPPO. 2001. New record of Phytophthora root rot disease of Alder from Italy. Phytosanitary Alert system posting dated June 01, 2001. Santini, A., G.P. Barzanti, and P. Capretti. 2001. A New Phytophthora Root Disease of Alder in Italy. Plant Disease Vol. 85:560. Santini, A., G.P. Barzanti, and P. Capretti. 2003. Susceptibility of some mesophilic hardwoods to alder Phytophthora. Journal of Phytopathology Vol. 151:406-410. Santini, A., F. Biancalani, G.P. Barzanti, and P. Capretti. 2006. Pathogenicity of four Phytophthora Species on Wild Cherry and Italian Alder Seedlings. Journal of Phytopathology Vol. 154:163-167.