Frequency- 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 fungus which causes powdery mildew in wheat, were investigated in competition experiments. There was no evidence for frequency-dependent selection, 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 coefficient varied linearly with the logarithm of density, and the greatest changes in selection occurred over the lower density changes. Investigation of life history traits showed that the observed differences in selection may have been caused by differences between isolates in the effect of density on spore production.