Influence of spray programmes on development of fungicide resistance in the eyespot pathogen of wheat, Pseudocercospovella herpotrichoides

Abstract

A site with no known history of cereal growing or carbendazim fungicide use was inoculated in 1983 with an equal mixture of spores of R‐type and W‐type isolates of Pseudocercosporella herpotrichoides. all sensitive to carbendazim. Following three consecutive years of carbendazim sprays, more than 90% of isolates recovered were carbendazim‐resistant. A majority of R‐type isolates was recovered after 3 years and resistance was found to be more frequent in R‐type than in W‐type isolates. A mixture of carbendazim with prochloraz initially delayed the development of carbendazim resistance, but after 3 years the incidence of resistance was similar to that where carbendazim alone was applied. The best control of eyespot was given by the mixture and by a split application of prochloraz in the first year, by the mixture in the second year, and by the split prochloraz application in the final year. A greater proportion of R‐type isolates was recovered from prochloraz‐treated plots than from unsprayed plots; the proportion of these that were resistant to carbendazim was lower than that from carbendazim treatments. Throughout this study isolates resistant to prochloraz were not detected.