Lowered virulence to rice plants and decreased biosynthesis of gibberellins in mutants of Gibberella fujikuroi selected with pefurazoate

Mutants of Gibberella fujikuroi were obtained by selection from a large number of conidia of a wild type field isolate of the fungus on agar plates containing lethal concentration of a sterol demethylation inhibitor, pefurazoate. The mutants isolated from the colonies grown on the plates had higher minimal growth-inhibitory concentrations of pefurazoate and the EC95 values were higher than that for the mother isolate but only a small increase was observed in the EC50 values. Though the slope of the dosage-response curve was changed in the mutants, the mechanism of action of the fungicide in a mutant proved to be its inhibition of sterol 14α-demethylation as shown by an experiment on inhibition of ergosterol biosynthesis using 14C-acetate. Gibberellin production in the mutants in liquid culture was tested by bioassay of gibberellins and assay of gibberellin A3 using HPLC. Mutants were also tested for their virulence to rice plants. Decreased virulence due to decreased production of gibberellins was observed in the mutants. A mutational change in a site related to 14α-demethylation in ergosterol biosynthesis and to kaurene oxidation in gibberellin biosynthesis in common is discussed.