FREQUENCY OF BENZIMIDAZOLE-RESISTANT AND DICARBOXIMIDE-RESISTANT STRAINS OF BOTRYTIS-CINEREA IN WESTERN OREGON SMALL FRUIT AND SNAP BEAN PLANTINGS

In 1990 and 1991, a total of 3,496 isolates of Botrytis cinerea were c ollected from strawberry, wine grape, raspberry, and snap bean plantin gs and from nonmanaged wild blackberry hedges located in Oregon's Will amette Valley. The percentage of isolates resistant to benomyl (5 mug/ ml) arid to vinclozolin (10 mug/ml) averaged 52 and 17%, respectively; 16% were resistant to both fungicides. The frequencies of resistance to benomyl and vinclozolin were significantly higher-(P less-than-or-e qual-to 0.05) in B. cinerea isolates obtained from strawberry than in isolates from the other crops. Of isolates of B. cinerea obtained from nonsprayed wild blackberry hedges, 38 and 6% were resistant to benomy l and vinclozolin, respectively. The frequency of benomyl- and vincloz olin-resistant strains of B. cinerea in a planting was significantly c orrelated (P < 0.05) with the number of applications in each respectiv e fungicide class. Vinclozolin resistance was estimated to increase by 20.8% per fungicide application in strawberry compared with 6.4% per application in wine grape, raspberry, and snap beans. In eight strawbe rry fields, frequency of vinclozolin resistance declined from an avera ge of 74% at harvest in June to 32% the following February or April. S trawberry fruit mummies harbored a higher frequency of dicarboximide-r esistant strains than did leaves.