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Evidence for at least two introductions of the sudden oak death pathogen into Oregon forests
Z. N. KAMVAR (1), M. M. Larsen (2), A. M. Kanaskie (3), E. M. Hansen (1), N. J. Grünwald (2). (1) Oregon State University, Corvallis, OR, U.S.A.; (2) USDA ARS HCRL, Corvallis, OR, U.S.A.; (3) Oregon Department of Forestry, Salem, OR, U.S.A.

In the mid 1990’s, a disease known as sudden oak death was discovered in California forests. The causal agent was formally described as <i>Phytophthora ramorum</i>. After first detection in Oregon forests in 2001, the epidemic was immediately subjected to an aggressive eradication effort by the Oregon Department of Forestry. A total of 512 isolates from symptomatic tanoaks were sampled from 2001 to 2014 and genotyped using five microsatellite markers.  Population genetic history was inferred using model-free methods appropriate for clonal populations. While three clonal lineages exist on the North American west coast, only the NA1 lineage has been discovered in OR forests. Since the initial introduction event into the Joe Hall area, the pathogen has spread North, West, and Southeast within Curry county. A discovery in the Hunter Creek area in 2011 appears to be a second introduction, as it does not cluster with the early introduction or subsequent infections. Including west coast nursery populations (n = 216) in the analyses provides support for at least two distinct introduction events from Oregon or California nurseries into OR forests. Continued vigilance and eradication of nursery populations of <i>P. ramorum</i> are important to avoid further emergence and potential introduction of other clonal lineages.

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