Selection for increased spore efficacy by host genetic background in a wheat powdery mildew population.

ABSTRACT A field experiment was designed to test the hypothesis of for increased reproductive ability on different host genetic backgrounds within a wheat powdery mildew population. Studies have suggested that, in host mixtures, such selection could increase the reproduction rate of simple patho-types that always develop on the same host genetic background, whereas complex pathotypes should not be affected because they infect different host genotypes. In our experiment, the Erysiphe graminis population reproduced for successive generations on cvs. Orkis and Etecho, either grown as pure stands or in a mixture. In an additional treatment, the host cultivar changed after each generation. Isolates were sampled in April and, after seven pathogen generations, in July. At the second sampling date and for pure stands only, mean spore efficacy was greater on the host from which isolates were sampled than on the other one. This was attributed to selection within the pathogen population for better spore efficacy on the host genetic background. This selection was independent of the virulence genes carried by the isolates. The possibility of a phenotypic plasticity effect was tested and rejected.