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Title: First report of the EU1 clonal lineage of Phytophthora ramorum on tanoak in an Oregon forest

Author
item Grunwald, Niklaus - Nik
item Larsen, Meredith
item KAMVAR, Z - Oregon State University
item REESER, P - Oregon State University
item KANASKIE, A - Oregon Department Of Forestry
item LAINE, J - Oregon Department Of Forestry
item WIESE, R - Oregon Department Of Forestry

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2015
Publication Date: 4/8/2016
Citation: Grunwald, N.J., Larsen, M.M., Kamvar, Z.N., Reeser, P.W., Kanaskie, A., Laine, J., Wiese, R. 2016. First report of the EU1 clonal lineage of Phytophthora ramorum on tanoak in an Oregon forest. Plant Disease. 100(5):1024. doi: 10.1094/PDIS-10-15-1169-PDN

Interpretive Summary: The sudden oak death (SOD) disease is caused by the plant pathogen Phytophthora ramorum. P. ramorum exists as 4 clones named NA1, NA2, EU1 and EU2. Forest infestations in Oregon have been limited to the NA1 clonal lineage. In February 2015, P. ramorum was isolated from a dying tanoak tree in the South Fork Pistol River drainage of Curry County, Oregon. The isolated strain was identified as P. ramorum based morphology and molecular marker analysis. Subsequently, five more isolates were isolated from the original tree stump and the EU1 lineage was confirmed. These results provide further evidence that multiple distinct P. ramorum introduction events into the Curry County forest are a critical component of the epidemic. Additional monitoring is necessary to determine if the EU1 clonal lineage occurs elsewhere in Curry county forests.

Technical Abstract: Initially reported in California as the causal agent of sudden oak death (SOD), efforts to limit spread of Phytophthora ramorum in Oregon natural forests have concentrated on quarantine regulations and eradication of the pathogen from infested areas. P. ramorum has four clonal lineages NA1, NA2, EU1 and EU2. Forest infestations in Oregon have been limited to the NA1 clonal lineage whereas EU1, NA1 and NA2 clonal lineages have all been found in US nurseries. In February 2015, in response to an aerial survey, P. ramorum was isolated from a dying Notholithocarpus densiflorus tree in the South Fork Pistol River drainage of Curry County, Oregon. The isolated strain was identified as P. ramorum based on presence of chlamydospores, characteristic hyphae and sporangial morphology. Microsatellite genotyping at 14 loci and comparison to reference cultures revealed that these isolates belonged to the EU1 clonal lineage. Subsequently, five more isolates were isolated from the original tree stump and the EU1 lineage was confirmed. Microsatellite alleles of the forest EU1 isolates were nearly identical to EU1 isolates collected in 2012 from a nearby nursery during routine P. ramorum nursery monitoring, except for one allele at locus PrMS145a. Interestingly, several isolates differed at locus ILVOPrMS131a within both the 2015 forest and the 2012 nursery findings with identical allele frequencies in each population (except for locus PrMS145a). These data provide evidence for the introduction of EU1 into Curry County from the 2012 populations found in nurseries. However, no direct match was found, probably due to the paucity of EU1 in nursery samples. These results provide further evidence that multiple distinct P. ramorum introduction events into the Curry County forest are a critical component of the epidemic. Additional monitoring is necessary to determine if the EU1 clonal lineage occurs elsewhere in Curry county forests.