Resistance to Benzimidazole and Dicarboximide Fungicides in Greenhouse Isolates of Botrytis cinerea.

In 1996 and 1997, 325 isolates of Botrytis cinerea were collected from 35 commercial greenhouses growing ornamental crops in South Carolina to determine the incidence of resistance to benzimidazole and dicarboximide fungicides. Conidium germination was assessed on a defined agar medium amended with either thiophanate-methyl (a benzimidazole) or vinclozolin (a di-carboximide). A total of 53 representative isolates were evaluated further for conidium germination and mycelium growth on fungicide-amended medium and for infection of geranium seedlings treated with thiophanate-methyl or vinclozolin at label rates. Isolates were considered sensitive to thiophanate-methyl or vinclozolin when the effective concentration of the fungicide active ingredient resulting in 50% inhibition of germination (EC-germ) was ≤5 μg/ml or when the effective concentration of fungicide active ingredient resulting in 50% inhibition of mycelium growth (EC-growth) was ≤1 μg/ml. Of all isolates, 81% (262/325) were resistant to thiophanate-methyl and 69% (223/325) were resistant to vinclozolin. Four phenotypes were observed: sensitive to both fungicides (17%), resistant to both fungicides (67%), resistant only to thiophanate-methyl (14%), and resistant only to vinclozolin (2%). Isolates resistant to at least one fungicide were found in 33 of the 35 locations from which samples were taken. Disease incidences on geranium seedlings treated with 600 μg/ml of thiophanate-methyl and then inoculated with isolates sensitive and resistant to this fungicide were 1.4 and 96.1%, respectively. Disease incidences on geranium seedlings treated with 600 μg/ml of vinclozolin and then inoculated with isolates sensitive and resistant to this fungicide were 0.3 and 91.9%, respectively. With thiophanate-methyl, correlation coefficients (r) between disease incidence and log EC-germ or log EC-growth were 0.987 and 0.992, respectively. With vinclozolin, correlation coefficients between disease incidence and log EC-germ and log EC-growth were 0.975 and 0.893, respectively. Correlation coefficients between the two EC values for thiophanate-methyl were 0.989 and for vinclozolin were 0.900. Isolates sensitive to thiophanate-methyl had a mean EC-germ value of 0.93 μg/ml and a EC-growth value of 0.11 μg/ml. For isolates sensitive to vinclozolin the mean EC-germ value was 1.63 μg/ml and the mean EC-growth value was 0.26 μg/ml. Thiophanate-methyl-resistant isolates had mean EC-germ and EC-growth values greater than 500 μg/ml while vinclozolin-resistant isolates had a mean EC-germ value greater than 500 μg/ml and a mean EC-growth value of 3.18 μg/ml.