| Abstract: | Mutants of Phytophthora parasitica Dast. more or less resistant to dimethomorph or metalaxyl were obtained by treating mycelium with ultraviolet radiation. Some metalaxyl-resistant mutants exhibited levels of resistance (i.e. the ratio between the EC50 values for the mutant and the wild-type strain) greater than 100, but those observed in the dimethomorph-resistant mutants never exceeded 25. Most mutants retained their resistance after sub-culturing on unamended agar medium. Single zoospore clones derived from a metalaxyl-resistant and a dimethomorph-resistant mutant, selected as amongst the most resistant, had levels of resistance similar to those of the parental strains. The metalaxyl-resistant mutants were also resistant to the related phenylamide fungicides, furalaxyl and benalaxyl, but remained sensitive to dimethomorph. The dimethomorph-resistant mutants were not resistant to phenylamide fungicides. The pathogenicity of some metalaxyl- or dimethomorph-resistant mutants on tobacco leaves was similar to that of the wild-type strains. |
