Cross‐resistance relationships between benzimidazole fungicides and diethofencarb in Botrytis cinerea and their genetical basis in Ustilago maydis

Abstract

AbstractAfter nitrosoguanidine‐ or UV‐mutagenesis, three different benzimidazole‐resistant phenotypes were isolated on media containing benomyl or a mixture of carbendazim and diethofencarb from wild‐type strains of Botrytis cinerea Pers. ex Fr. and Ustilago maydis (D.C.) Corda. Mutants of B. cinerea with moderate (MBr) or low (LBr) resistance to benzimidazoles and high resistance to diethofencarb (Dr) were isolated from the fungicide‐mixture‐containing medium in low frequency (7–1 × 10−8). Only benzimidazole‐resistant strains highly sensitive to diethofencarb (HBrDs) were identified on benomyl‐containing medium at a frequency of 6.6 × 10−6. Fitness‐determining characteristics such as sporulation, germination and germ‐tube elongation, were found to be reduced significantly in the mutants of B. cinerea that were resistant to both benzimidazoles and diethofencarb. However, pathogenicity of a MBrDr mutant strain on cucumber seedlings was equal to that of the wild type and a carbendazim + diethofencarb mixture was found to control grey mould caused by the wild type, but was not effective when the plant cotyledons were infected by the mutant strain.Three benzimidazole‐resistant phenotypes (HBrDs, HBrDr, MBrDr) were isolated easily in U. maydis from a benomyl‐containing medium. In contrast with B. cinerea, only one‐tenth of the benzimidazole‐resistant strains were sensitive to diethofencarb. Genetic analysis of benzimidazole resistance in U. maydis showed that the three benzimidazole‐resistant phenotypes were due to three allelic mutations in a single gene and one of them was responsible for the negative cross‐resistance between benzimidazoles and diethofencarb.