Fitness of isolates of Erysiphe graminis f.sp. tritici with reduced sensitivity to fenpropimorph

Under crowded infection conditions, isolates of Erysiphe graminis f.sp. tritici with reduced sensitivity to fenpropimorph displayed a lower competitive ability than wild type isolates. The lower competitive ability could not be ascribed to changes in germination, latent period, colony forming ability or production of conidia, but seems to be a complex change in phenotype that cannot be assessed easily by characterization of individual fitness components. Using mildew isolates containing fungicide-sensitivity and virulence markers to study survival, some isolates with reduced sensitivity to fenpropimorph survived the winter. The competitive ability of these isolates seemed to be slightly lower than that of the mildew population that migrated naturally into the experimental field. Results described indicate that in practice, in periods without selection pressure, the frequency of isolates with reduced sensitivity to this compound may decrease.