Identifying centers of origin of microbes that cause plant disease provides insights into latent genetic diversity, co-evolved hosts, and pathogen reservoirs. Here we investigate the population dynamics of Phytophthora pluvialis, an oomycete first isolated from soil, streams, and tree foliage in mixed Douglas-fir (Pseudotsuga menziesii) - tanoak (Notholithocarpus densiflorus) forests in the US Pacific Northwest (PNW) in 2002. It was then identified as the causal agent of red needle cast (RNC) of radiata pine (Pinus radiata) in New Zealand (NZ) in 2012. Given its low degree of pathogenicity in PNW trees, we hypothesize that the center of origin of P. pluvialis is in the PNW. We used genotyping-by-sequencing (GBS) to obtain 1,543 single nucleotide polymorphisms (SNPs) across 158 P. pluvialis isolates collected in the PNW and NZ. Our objective was to test the hypothesis that the center of origin of P. pluvialis and the source of the introduction that led to the RNC epidemic in NZ was in the PNW of the USA.
Results/Conclusions
We identified the PNW Coast Range as the center of origin of P. pluvialis. After grouping our samples into five genetic clusters, we found the Coastal cluster spanned the widest geographic range of all identified clusters and was the central cluster in a minimum spanning network. Rooted phylogenies showed Coastal samples located at the root of every PNW clade. These clades all emerged earlier than the NZ clades. Coastal had the highest degree of heterozygosity (Hs = 0.254) and median pairwise genetic distance (0.093) of any cluster. These results support the hypothesis that the Coastal cluster is the center of origin of P. pluvialis. Additionally, comparison of three different migration scenarios produced a unidirectional PNW to NZ model as the best fit for our data. This, combined with relatively low genetic distance, low population heterozygosity, and lack of geographic structure of the NZ genetic clusters, suggests a single colonization event followed by clonal expansion of P. pluvialis from the PNW to NZ. Future studies should increase sampling depth across the PNW to better understand P. pluvialis’ population dynamics within its proposed center of origin, which may overlap with the native range of P. radiata in northern California.