THE INDUCTION AND CHARACTERIZATION OF ISOLATES OF PSEUDOCERCOSPORELLA-HERPOTRICHOIDES WITH ALTERED SENSITIVITY TO THE FUNGICIDE PROCHLORAZ

Isolates of Pseudocercosporella herpotrichoides (Fron.) Deighton, causal agent of eyespot disease of cereals, with reduced sensitivity to the imidazole fungicide prochloraz were produced by spontaneous selection on fungicide-amended media or by successive rounds of ultra-violet (UV) irradiation of conidia. Resistance frequencies were similar to those obtained for other sterol-biosynthesis-inhibiting (SBI) compounds. Stability testing indicated that a large proportion of the UV-induced mutants reverted rapidly to wild-type sensitivity levels. However, the spontaneously selected strains, and strains produced after the third round of mutation proved more stable. Strains produced by spontaneous selection and from a single round of UV irradiation showed relatively low levels of resistance, in most cases within the range shown by the European field population. Subsequent rounds of UV irradiation raised resistance levels by factors of between 5 and 16 times. Growth rate in culture and sporulation ability were impaired in some, but not all, strains. Spontaneous mutants showed no decrease in pathogenicity to cereal hosts while UV-induced strains were more variable, with some giving significantly reduced infection scores. The spontaneous mutants, and some of the UV-induced strains, gave higher infection scores on plants treated with prochloraz than on untreated plants. This may have been due to changes in the stem base microflora resulting in reduced competition. The findings from this study suggest a multi-factorial basis for resistance, resulting in directional rather than disruptive selection for field resistance.