Characterizing the variation in aggressiveness and sporulation of the NA1 and EU1 lineages of Phytophthora ramorum in Oregon

Phytophthora ramorum , the cause of sudden oak death, is an invasive pathogen present in parts of coastal California and south‐western Oregon forests. The majority of these forest infestations have been caused by the NA1 clonal lineage. In 2015, the EU1 lineage of P . ramorum was isolated from a tanoak ( Notholithocarpus densiflorus ) tree located in a mixed‐conifer forest of Curry County, Oregon. In order to evaluate the threat to Oregon forests of the EU1 lineage relative to the established NA1 lineage, a series of experiments was conducted comparing aggressiveness and sporulation of NA1 and EU1 isolates on logs and seedlings in the growth chamber and forest. There was no significant difference in lesion size on logs inoculated with NA1 and EU1 isolates for any of the tree species tested. Across all seedling experiments differences among isolates within lineage, in terms of both aggressiveness and sporulation, were more commonly observed than differences among lineages. Site to site variation in tanoak sporulation, as measured by rain bucket baiting, appears to be correlated with the number of P . ramorum ‐positive seedlings detected at each site.