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Peer Reviewed

© 2008 Plant Management Network.
Accepted for publication 2 October 2008. Published 12 December 2008.

Phytophthora alni subsp. uniformis Found in Alaska Beneath Thinleaf Alders

Gerard C. Adams, Associate Professor, and Mursel Catal, Research Associate, Department of Plant Pathology, Michigan State University, East Lansing, MI 48824; Lori Trummer, Pathologist, Forest Health Protection, Alaska Region, US Forest Service, Anchorage, AK 99503; Everett M. Hansen, Professor, and Paul Reeser, Research Assistant, Department Botany & Plant Pathology, Oregon State University, Corvallis, OR 97331; James J. Worrall, Pathologist, Forest Health Management, Rocky Mountain Region, US Forest Service, Gunnison, CO 81230

Corresponding author: Gerard C. Adams.

Adams, G. C., Catal, M., Trummer, L., Hansen, E. M., Reeser, P., and Worrall, J. J. 2008. Phytophthora alni subsp. uniformis found in Alaska beneath thinleaf alders. Online. Plant Health Progress doi:10.1094/PHP-2008-1212-02-BR.

Phytophthora alni Brasier & S. A. Kirk 2004 causes a lethal root and collar disease of alder species in Europe. This emergent pathogen has multiple variants that have been placed in three subspecies (1). The species has not been previously found in North America, although an isolate tentatively referred to as P. alni was reported in a survey of nurseries in Minnesota (4). The potential establishment and spread of this complex of pathogens is perceived to represent a threat to all species of Alnus in the western hemisphere. A survey of Phytophthora species beneath Alnus was initiated following the discovery of widespread dieback and mortality in Alaska. Thirty riparian stands were sampled along a south-to-north transect from Kenai Peninsula through Anchorage and north to beyond Fairbanks (Fig. 1). Phytophthora species were baited from saturated rhizosphere soil and watercourses in situ using rhododendron leaves during July 2007. Three isolates of P. alni subsp. uniformis were isolated from soil beneath Alnus incana subsp. tenuifolia exhibiting dieback at two sample locations in remote, unmanaged stands hundreds of miles apart on the Kenai Peninsula and near Denali National Park. Species identification of the three isolates is based on DNA sequence homology of ITS and RAS-Ypt molecules (2), and subspecies identification by SCAR profiles (3), and morphology (GenBank EU371544-371553). Caducous sporangia, not previously observed in P. alni, were observed in the Alaskan isolates. USDA Forest Service and APHIS laboratories have additionally confirmed the identification. Phytophthora alni was not recovered from the other 28 sample sites in 2007. Excavations of nearly total root systems in 2008 revealed that symptoms of root and collar rot were rarely present at the two locations where the isolates of P. alni subsp. uniformis were obtained. Root rot which occasionally tested positive with ELISA for Phytophthora was limited generally to one root (approximately 1-cm diameter) per tree. Alder mortality and dieback were common at many of the 30 sites and were often associated with extensive stem cankers colonized by Valsa melanodiscus G.H. Otth 1870, Cryptosporella suffusa (Fr.:Fr.) L.C. Mejia & Castl. 2008, and other ascomycetes. Cytospora cankers also are prevalent in the European disease situation, but in Alaska Cytospora may be the primary agent. The discovery of P. alni in remote regions of Alaska does not match the scenario in Europe of introduction with infested nursery stock. Rather, putative introduction might have occurred from contaminated boots by fishing tourists; alternatively, the subspecies may be native. In European studies, P. alni subsp. uniformis is reported to be significantly less aggressive than P. alni subsp. alni, though still considered pathogenic. However, genetic studies indicate that P. alni subsp. uniformis may have given rise to P. alni subsp. alni on several occasions by hybridization with P. alni subsp. multiformis (2).


Fig. 1. Location of 30 riparian forests of Alnus spp. sampled during the 2007 survey of Phytophthora species beneath alders exhibiting dieback and mortality in Alaska.


Literature Cited

1. Brasier, C. M., Kirk, S. A., Delcan, J., Cooke, D. E. L., Jung, T., and Man in’t Veld, W. A. 2004. Phytophthora alni sp. nov. and its variants: Designation of emerging heteroploid hybrid pathogens spreading on Alnus trees. Mycol. Res. 108:1172-1184.

2. Ioos, R., Andrieuz, A., Marçais, B., and Frey, P. 2006. Genetic characterization of the natural hybrid species Phytophthora alni as inferred from nuclear and mitochondrial DNA analyses. Fungal Genet. Biol. 43:511-529.

3. Ioos, R., Husson, C., Andrieux, A., and Frey, P. 2005. SCAR-based PCR primers to detect the hybrid pathogen Phytophthora alni and its subspecies causing alder disease in Europe. Eur. J. Plant Pathol. 112:323-335.

4. Schwingle, B. W., Smith, J. A., and Blanchette, R. A. 2007: Phytophthora species associated with diseased woody ornamentals in Minnesota nurseries. Plant Dis. 91:97-102.