Distribution and Severity of Alder Phytophthora in Alaska

In Alaska, an unprecedented dieback and mortality of Alnus incana ssp. tenuifolia has occurred which stimulated an effort to determine causal agents of the disease. In Europe, similar dieback and mortality of Alnus incana and Alnus glutinosa has been attributed to root rot by a spectrum of newly emergent strains in the hybrid species Phytophthora alni. The variable hybrids of P. alni were grouped into three subspecies: P. alni ssp. alni (PAA), P. alni ssp. multiformis (PAM), and P. alni ssp. uniformis (PAU). From 2007 to 2008, we conducted a survey of Phytophthora species at 30 locations with stream baiting as used in the 2007 national Phytophthora ramorum Early Detection Survey for Forests in the United States. Additionally, Phytophthora species from saturated rhizosphere soil beneath alder stands were baited in situ using rhododendron leaves. We discovered PAU in rhizosphere soils in 2007 at two sample locations in unmanaged stands hundreds of miles apart, on the Kenai Peninsula and near Denali National Park. PAA was reported to be the most aggressive and pathogenic to alders and PAM and PAU were significantly less aggressive than PAA, though still pathogenic. To ascertain whether PAU was of restricted distribution due to recent introduction, or widespread distribution, we extended the survey in 2008 to 81 locations. Intensive sampling was conducted at five alder stands exhibiting dieback and 10 alder genets per location were excavated to expose nearly the entire root system for evaluation of the severity of root rot, ELISA detection of Phytophthora in diseased roots, and isolation of Phytophthora species. At intensive sites, four bowls each containing 500 ml samples of saturated rhizosphere soil were baited by floating three detached leaves of Rhododendron spp. for a 2-week period. Leaves were rinsed and sealed in plastic bags and shipped to the laboratory where leaf tissues were placed in PARPH-V8 agar selective for Phytophthora spp. Phytophthora spp. were identified from DNA sequence of the ITS-rDNA region. The survey yielded some species newly reported for the U.S., including P. aff. gallica,and an undescribed species in Clade 8C closely related to P. ramorum and P. foliorum, and other undescribed species. The Clade 8C species was of restricted range in our isolations, and all 20 isolates were from one location. The species was of interest to researchers developing systems for detection of P. ramorum. Thirty-three isolates of PAU were identified out of approximately 600 isolated and sequenced Phytophthora spp. PAU was collected from 11 geographically distributed stands. Only one isolate was obtained from bait floating in a water course (the Tanana River) out of 81 watercourses sampled. Soil isolates were from four plots in southcentral Alaska along the Kenai and Russian Rivers, and seven plots in the interior, including a plot in Fairbanks, three plots between Delta Junction and Fairbanks along Highway 2, two between Slana and Tok along Highway 1, and one near Denali National Park on Highway 3. PAU was widely distributed and difficult to isolate. Severity of root rot was low, with less than one diseased root discovered per genet, on average. Root rot does not appear to be a significant contributor to the dieback and mortality of alder in Alaska. 1 A version of this paper was presented at the Fourth Sudden Oak Death Science Symposium, June 1518, 2009, Santa Cruz, California. 2 Michigan State University, East Lansing MI 48824. 3 USDA Forest Service, Anchorage, AK 99503. Corresponding author: [email protected]. GENERAL TECHNICAL REPORT PSW-GTR-229