SELECTION AND GENETIC ANALYSIS OF LABORATORY MUTANTS OF BOTRYOTINIA FUCKELIANA RESISTANT TO FENHEXAMID

Ten monoascosporic strains of Botryotinia fuckeliana were used in mycelium growth tests to evaluate baseline sensitivity to Fenhexamid, a fungicide recently introduced in the control of grey mould. Their response to the fungicide was EC₅₀ = 0.1-0.3 µg ml⁻¹ and MIC = 0.3-3 µg ml⁻¹. Eight laboratory mutants resistant to the fungicide were obtained from UV-irradiated or unirradiated conidia plated on Fenhexamid-amended medium (3 µg ml⁻¹). Mutation ratios were 1.7·10⁻⁶ of survivor conidia for UVinduced mutations and 0.6·10⁻⁸ for spontaneous mutations. Two levels of resistance to Fenhexamid were distinguished in mycelium growth tests: low (110 µg ml⁻¹) and high resistance (EC₅₀>100 µg ml⁻¹). In conidial germination tests, Fenhexamid proved to be a powerful inhibitor of germ tube elongation (EC₅₀ ~1 µg ml⁻¹) even for mutants displaying high resistance in mycelium tests. Resistant mutants were crossed with Fenhexamid-sensitive reference strains to derive meiotic progeny and to assess the mode of inheritance of resistant phenotypes. Segregation of resistant/sensitive phenotypes in ascospore offspring indicated that resistant phenotypes were due to mutations in single major genes inherited in Mendelian fashion and unlinked with the Mbc1 and Daf1 genes, responsible for resistance to benzimidazole and dicarboximide fungicides, respectively.