Shift in Phytophthora ramorum populations in Washington State

The EU1 strain is now the most common P. ramorum lineage detected in WA state nurseries and non-nursery sites, with a concurrent rise in NA2 and decrease in NA1. Evaluation of genotypes over the past 5 years by Gary Chastagner, Katie Coats, and Marianne Elliott at Washington State University shows the reversal in genotype frequency occurred in 2008 and persisted into 2009. They detected EU1 in 99 positive samples at seven locations, NA1 in 17 samples at four sites, and NA2 in 48 samples at three sites in 2009. In contrast, the first year of the study (2005) identified four EU1 samples at one site, 103 NA1 samples at 14 sites, and 12 NA2 samples at five sites.
Of the 46 Washington nurseries found positive since 2003, five had infected plant detections in 2009. Twenty nurseries have been confirmed positive at least two years. Two of these sites were found to have positive plants three different years and two sites were positive four years. Genotype analysis has been performed on samples from 31 nurseries since 2005. Of the nurseries with multiple confirmed years, six had various combinations of two lineages for one or two years and one had a year with all three lineages followed by two other years with two lineages present each year.
Nine Washington non-nursery sites (landscapes, water, soil or trace-forward landscape plants) have also been identified P. ramorum positive since 2005, with seven sites having been positive in 2008 and/or 2009. Of the seven sites, the Rosedale Stream (Pierce County) has been found positive for four years (NA1) and the Sammamish River (King County) has been confirmed positive for three years (several lineages). A Pierce County site with NA2-positive salal plants was also found adjacent to nursery property, and four residential sites have been found in the past two years, all of which have been identified with the EU1 strain (two residences were found with the EU1 strain in 2008 and 2009, one with plants and soil, and one with just soil). These four residential sites were trace-forwards from the same EU1-positive nursery.
In 2009, a new systematic sampling method was implemented, which involved not only genotyping the original DNA sample, but also a second round of sampling using the same plant and/or bait material from which the DNA positive was detected. In addition, through a cooperative effort with the Washington State Department of Agriculture and the USDA Animal and Plant Health Inspection Service (APHIS), a third round of sampling was performed on whole plants or plant material collected from most of the nurseries prior to destruction of the plant material. This duplicate and triplicate sampling allowed for a better representation of the genetic distribution of the P. ramorum population.
In total, 450 samples from 31 nurseries, three water sites, and six landscape locations have been analyzed. For more information on the study and its findings, contact Gary Chastagner at chastag@wsu.edu.

From the COMTF March newsletter