The role of the sexual stage in the over‐summering of Erysiphe graminis DC. f.sp. hordei Marchal under semi‐arid conditions

SUMMARY The sexual stage of E. graminis f.sp. hordei was found to be functional in Israel, and adapted to the climate of the region. Cleistothecia serve as the principal, and perhaps the only means for persistence of the pathogen in the host‐less, hot and rainless summer, and induce initial infection on wild and cultivated barleys in late autumn. The cool and rainy weather, which induces germination of seeds of the host in the field, also permits ascospore differentiation and ejection. Details are given of the conditions favouring or delaying cleistothccial maturation during and after the growing season.