Population genetics of P. ramorum in Washington State

During the past four years (2005-2008), Phytophthora ramorum has been detected in 27 western Washington nurseries. To date, 314 P. ramorum isolates have been collected from the 27 nurseries, as well as two streams, a retention pond, and one landscape site, and genotyped using four microsatellite markers (PrMs5, Pr9C3, PrMs39 and PrMs45).

All three previously described lineages (EU1, NA1, and NA2) were detected in each of the four years. In this population, the EU1 lineage is represented by one genotype, the NA2 lineage is represented by one genotype, and the NA1 lineage is represented by four genotypes. The NA1 lineage was the most common, occurring in 24 nurseries, one landscape situation, and both streams. The NA2 lineage was detected at 10 nurseries and one stream, while the EU1 lineage was detected at five nurseries. The occurrence of the EU1 lineage in Washington has increased in frequency over the past four years while the overall number of P. ramorum sites and isolates has declined. In four of the five sites where the EU1 lineage occurred, the NA1 lineage was also detected at the same time. At one nursery in 2007, these two lineages were isolated from different branches on the same rhododendron plant. At another nursery in 2008, they were isolated from the same soil bait. Although no genotype detected to date possesses a hybrid of alleles from both the European (EU1) and North American (NA1 and NA2) lineages, the presence of the EU1 and NA1 lineages on the same plant and soil bait illustrates the need for a better overall understanding of the population structure of P. ramorum in nurseries to reduce the risk of hybridized species occurring as a result of sexual recombination.