Benzimidazole resistance in Cercospora beticola sampled from sugarbeet fields in Michigan, USA

Sugarbeet (Beta vulgaris L.) leaves infected with Cercospora beticola Sacc. were collected in 44 and 48 grower fields in Michigan in 1998 and 1999, respectively, following reports of the inadequacy of benzimidazole fungicides in controlling leaf spot disease. Spores resistant to thiophanate methyl (TM), a representative benzimidazole fungicide, could be obtained from the leaves. Resistance was determined by radial mycelial growth on potato dextrose agar amended with 5 mug/mL TM. In 1998, viable conidia of C. beticola were isolated from 556 individual leaf spots; 115 of these isolates, from 21 of the 44 growers' fields, were resistant to TM. In 1999, viable conidia were isolated from 441 leaf spots; 365 of these isolates were resistant to TM, and resistant isolates were obtained from all but one of the fields sampled. This is the first report of detection of TM resistance in C. beticola, in Michigan, indicating that alternative measures should be implemented to reduce resistance to benzimidazole and other fungicides in Michigan C. beticola populations.