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item INGLIS, D
item DERIE, M
item Brown, Charles - Chuck
item Goodwin, Stephen - Steve
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: Where ever the potato is grown, a disease called late blight, caused by a fungus, is present. In certain growing areas it is so devastating that the potato crop would be destroyed every year if fungicides were not applied. Late blight was first noticed in Ireland in 1845 and lead to the complete failure of the potato crop and the starvation and emigration of a substantial portion of the Irish population. The fungus is thought to have originated in the Toluca Valley of Mexico where it is most aggressive and has the greatest genetic variability of anywhere in the world. During the last decade new forms of late blight have migrated out of Mexico and appeared all over the world. This new late blight has proved very hard to handle. It is resistant to a standard fungicide that used to be very effective. Using genetic markers to fingerprint the fungus researchers have found that completely new types of late blight have appeared in different locations in the US almost every year. Research carried out in Western Washington, which has high late blight pressure nearly every year, has shown that this region has a rapidly changing and very diverse set of late blight fingerprints. Potato growers have found that they have to intensify their efforts to combat the fungus by spraying more often. Late blight has two sexual compatibility groups. Both groups, A1 and A2, must be present for sexual

Technical Abstract: Western Washington represents a unique laboratory for studying Phytophthora infestans because it is here isolates having metalaxyl insensitivity and complex pathotypes were first detected in the US 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 throughout the region. An additional 45 isolates were collected from a single field. Based on mating type, metalaxyl-insensitivity, Gpi and Pep allozymes and RG57 DNA fingerprinting, 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 were isolated. This is the first report of A2 mating type in western Washington. Of the isolates collected, 90% in 1996, and 77% of the US-11, and all of the US- 8 and US-14 isolates in 1997, could infect four to seven potato R-gene differentials. The virulence pathotypes in this region have remained complex, even without the selection pressure of R- genes in the commercially grown cultivars. The 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.