Competitive ability and ethirimol sensitivity in strains of barley powdery mildew

Strains within field populations of barley powdery mildew (Erysiphe graminis f. sp. hordei) varied greatly in their response to ethirimol. Each strain remained stable whether the fungicide was present or not, and no evidence for adaptation was obtained. Strains of intermediate sensitivity were the most frequent within the pathogen population, and these also dominated model populations maintained in the laboratory. Ethirimol eliminated sensitive strains from laboratory mixtures, and increased the relative fitness of insensitive ones, but not sufficiently to oust the intermediate strains. Mildew from treated field plots was less sensitive than that from untreated plots, but only early in the epidemic. Insensitivity was not related to the level of ethirimol used and, at four times the rate used commercially, insensitive strains were no more frequent than at lower rates. As the complexity of mildew populations increased, changes in ethirimol sensitivity in response to selection became less pronounced, and it is suggested that strains of intermediate sensitivity to ethirimol exert a stabilising effect within natural populations. This could alter if the fitness of insensitive strains were to increase, perhaps through recombination. Consideration should be given to the effect ethirimol might have on the composition of the pathogen population if applied when sexual recombination occurs, and to the role ascospores play in disease development.