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item INGLIS, D.
item DERIE, M.
item Brown, Charles - Chuck
item GOODWIN, S.
item FRY, W.
item Deahl, Kenneth

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: During the past 8 years, potato and tomato crops in western Washington have suffered major disease losses due to the resurgence of an old enemy-late blight. More than 150 years after it devastated potato crops in the U.S. and Europe and led to the Irish potato famine, the plant pathogenic fungus, Phytophthora infestans, is again creating a major plant health problem. In this paper, we focus our attention on the characterization of recent pathogenic strains from western Washington since this was the site in North American where "new" genotypes of the fungus were first isolated. We document how contemporary strains of the pathogen are increasingly more fungicide-resistant and that a major portion of strains are more virulent and aggressive. We describe, for the first time, the isolation of a different mating type from the resident population. The virulence of pathotypes in this region remained complex yet the genetic variation encountered is thought to have occurred as a result of migration. This information will be used by growers, county agents and scientists to design new disease management strategies.

Technical Abstract: Western Washington represents a unique natural laboratory for studying Phytophthora infestans because it is where isolates having metalaxyl insensitivity and complex pathotypes were first detected in the U.S. during the early 1990's. A total of 115 isolates of P. infestans were obtained during 1996 from single lesions of infected tubers and/or foliage on potato, tomato, nightshade and bittersweet. An additional 45 isolates were collected from only one field. Based on mating type, metalaxyl-insensitivity, Gpi and Pep allozymes and RG57 DNA fingerprint, all of the isolates were US-11 and A1 mating type. However, during 1997 from a total of 120 isolates collected on potato, tomato and nightshade, US-11 was detected and an additional three genotypes: US-7, US-8 and US-14 which are A2 mating type. The virulence pathotypes in this region have remained complex, even without the selection pressure of R-genes in the cultivars grown commercially. The genetic variation encountered in P. infestans over the years in western Washington is thought to have occurred as a result of migration and not sexual recombination.