FREQUENCY-DEPENDENT AND DENSITY-DEPENDENT SELECTION IN WHEAT POWDERY MILDEW

The effects of different initial densities and isolate frequencies on three pairs of isolates of Erysiphe graminis f. sp. tritici, the fungu s which causes powdery mildew in wheat, were investigated in competiti on experiments. There was no evidence for frequency-dependent selectio n, which suggests that competition between colonies is not affected by genotype. Density-dependent selection was consistently found. For one pair of isolates it was shown that the logarithm of the selection coe fficient varied linearly with the logarithm of density, and the greate st changes in selection occurred over the lower density changes. Inves tigation of life history traits showed that the observed differences i n selection may have been caused by differences between isolates in th e effect of density on spore production.