Comparative epidemiology of NA1 and EU1 Phytophthora ramorum isolates From Curry County, OR

The 2015 detection of the Phytophthora ramorum EU1 lineage in Oregon forests poses a new threat to sudden oak death management in Curry County. EU1 may be more aggressive and spread at a faster rate than has been observed for NA1 over the 17 years it has been managed in Oregon forests. EU1 may also infect some hosts, notably conifers, at a greater frequency. To assess any additional risk posed by EU1, we performed field surveys assessing the distribution and frequency of understory infection surrounding SOD-infested trees. We also conducted laboratory assays testing for epidemiologically relevant differences between Curry County NA1 and EU1 isolates. To determine if the EU1-infested sites were larger upon detection, or if EU1 was infecting hosts at a greater rate, we established transects 20 m uphill, downhill and perpendicular to a confirmed, SOD-infected tanoak tree presumed to be the primary inoculum source contributing to understory infection at a site (7 sites per lineage). In 5 m2 blocks we recorded the presence of understory hosts and collected samples for plating in selective media to confirm infection by P. ramorum. Recovery of P. ramorum from understory vegetation declined with distance from the primary source of inoculum in both EU1 and NA1 sites. EU1 sites were the same size as NA1 sites upon detection, having similar disease incidence at a given distance from the site center (Wilcoxon rank-sum test; p = 0.38). Tanoak was abundant and was the most commonly infected host at both NA1 and EU1 sites. There was no difference between the recovery rates of either lineage for all hosts (Pearson’s test for independence at α = 0.05). P. ramorum was not recovered from conifers.