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

Isolates of Pseudocercosporella herpotrichoides (Fron.) Deighton, caus al agent of eyespot disease of cereals, with reduced sensitivity to th e imidazole fungicide prochloraz were produced by spontaneous selectio n on fungicide-amended media or by successive rounds of ultra-violet ( UV) irradiation of conidia. Resistance frequencies were similar to tho se 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, s trains. Spontaneous mutants showed no decrease in pathogenicity to cer eal hosts while UV-induced strains were more variable, with some givin g significantly reduced infection scores. The spontaneous mutants, and some of the UV-induced strains, gave higher infection scores on plant s treated with prochloraz than on untreated plants. This may have been due to changes in the stem base microflora resulting in reduced compe tition. The findings from this study suggest a multi-factorial basis f or resistance, resulting in directional rather than disruptive selecti on for field resistance.