Recurrent Invasion of Oregon's Coastal Forests by the Sudden Oak Death Pathogen: The NA2 Lineage's First Escape into...

Phytophthora ramorum, the sudden oak death (SOD) pathogen, has invaded U.S. nurseries and western forests repeatedly. Invasions by three distinct clonal lineages (NA1, NA2, and EU1) have affected U.S. wildland forests since 2001. Most recently, NA2 established an invasive population in coastal forests surrounding Port Orford in southwest Oregon. This is the first instance of NA2 encountered outside of a nursery. We assessed this new invasion's diversity and population structure using single-nucleotide polymorphisms from whole genome sequences of 195 isolates. Genomes were sampled from the nursery trade and from the forest invasion surrounding Port Orford. The NA2 forest invasion lacks genetic diversity and likely originated from a single introduction. A subset of NA2 isolates (10.8%) were genetically distant from all other isolates including each other. This distance was correlated with frequency of genomic runs of homozygosity. No evidence of hybridization was found, though NA2 and EU1 mating types are compatible and their ranges in wild forests overlap. Aggressiveness and sporulation of NA2 isolates were measured on three host tree species and compared with isolates of other lineages. NA2 and EU1 phenotypes were similar. NA2 caused larger lesions on Douglas-fir and tanoak seedlings than NA1, the lineage of the original introduction into Oregon forests. Results indicate that the NA2 population is not distinct from the older EU1 Oregon population in terms of population diversity, disease aggressiveness, or potential for spread. However, the introduction of NA2 has increased the SOD epidemic's range, risk of hybridization, overall genetic diversity, and adaptive potential.