Seasonal changes in pathotype complexity in French populations of barley powdery mildew

Airborne conidia of Erysiphe graminis f. sp. hordei were sampled in three regions and a single locality in the northern part of France for 2 years. Sampling was carried out in early spring, in late spring and in autumn, in order to separate the effects of winter bailey cultivars, carrying few specific resistance alleles, and of spring barley cultivars, carrying diverse resistance alleles, on the structure of the pathogen population. Although complex pathotypes with threes to 10 virulences were selected by spring cultivars. simple pathotypes. including a pathotype with the single unnecessary virulence allele Va22, which formed a clear majority of the samples. remained dominant in early spring, when winter but not spring cultivars were growing. In early spring. simple pathotypes were more prevalent in the north, where the winter cultivars represented 90% of the barley acreage, than in the east, where winter cultivars represented 65%. In thr west, the frequency of simple pathotypes was limited compared to the north, possibly because of the resistance allele Mlg in winter cultivars. The high frequency of simple pathotypes in eat-iv spring could bs explained hy a differential adaptation bet-v;een simple and complex pathotypes or by delayed epidemics on spring cultivars compared to winter cultivars.