Persistence of complex virulences in populations of Phytophthora infestansin western Washington

Isolates of Phytophthora infestans, collected from bittersweet, hairy night shade, petunia, potato, potato vine, and tomato in western Washington, 1998 to 1999, were evaluated for virulence complexity as well as mating type, m etalaxyl insensitivity, allozymes of glucose-6-phosphate isomerase and pept idase, and DNA fingerprint with the RG57 probe. Results were compared with those from similar collections made in the same region during the 1990s. Ge nerally, virulence complexity was high for most of the isolates regardless of year, genotype, or host. No marked shift in virulence complexity was evi dent for the populations studied, and unnecessary virulences were maintaine d. During 1998 and 1999, isolates of the US-8 and US-ii genotypes had 4 or more virulence factors. US-8 isolates averaged 8.2 and 9.3, whereas US-II i solates averaged 5.4 and 6.3 virulence factors. The frequency of US-8 isola tes that were sensitive to metalaxyl ranged from 5% in 1998 to 72% in 1999. All of the US-11 isolates tested in 1998 and 1999 were insensitive to meta laxyl. From 1996 to 1999 on potato, the recovery of US-8 increased, whereas the recovery of US-II decreased. No evidence of new genotypes or sexual re combination was found. Western Washington was a desirable location for scre ening germ plasm for durable resistance to late blight due to the high freq uency and persistence of complex virulences.