To understand the invasion history of the grape powdery mildew fungus, Erysiphe necator, I investigated the evolutionary relationships between introduced populations of Europe, Australia and the western US and populations in the eastern US. Additionally, I tested the hypothesis that populations of E. necator in the eastern US are structured based on geography and Vitis host species. Multilocus sequencing analysis of three nuclear gene regions from 146 isolates of E. necator is consistent with the hypothesis that introduced populations are derived from two separate introductions from the eastern US. The invasion history of E. necator follows a pattern consistent with plant-mediated dispersal. E. necator shows geographic structure, but no genetic structure across Vitis host species, except with respect to V. rotundifolia. In ascomycetes, mating compatibility is regulated by the mating-type locus, MAT1. I identified and sequenced genes at the MAT1 locus in Erysiphe necator and developed a PCR-based marker for determining mating type. I designed degenerate primers that amplify conserved regions of MAT1-1 and MAT1-2 in other powdery mildew fungi. The mating-type genes in E. necator are similar to those of other Leotiomycetes; however, the structure of the MAT1 locus in E. necator, like the MAT1-2 idiomorph of Blumeria graminis, is markedly different from other ascomycetes in that it is greatly expanded and may contain a large amount of repetitive DNA. Random mating and recombination in heterothallic fungi should result in high genotypic diversity, 1:1 mating-type ratios, and random associations of alleles at different loci, i.e., linkage equilibrium. I sampled isolates from vineyards in Burdett, NY and Winchester, VA. Isolates were genotyped for mating type and 11 SSR markers. After clone correction, mating-type ratios in the three populations did not deviate from 1:1. Genotypic diversity was high, but even with clone correction, I detected significant linkage disequilibrium in all populations. Vineyard populations were spatially structured, which likely results from short dispersal distances. Overall, these results suggest that selection for clonal genotypes and spatial genetic aggregation during the asexual phase of the epidemic contribute to persistent linkage disequilibrium even though populations undergo an annual sexual cycle.