Introduced pathogens have led to devastating epidemics in naïve host populations that lack evolved defences, as demonstrated by the plant pathogen Cryphonectria parasitica, the fungus that causes chestnut blight. Its introduction from Asia  practically eliminated the American chestnut (Castanea dentata) and markedly altered the species composition of forests throughout eastern North America. Source pathogen populations are expected to be more diverse than introduced populations because introduced populations have smaller effective population sizes due to losses in genetic diversity from population bottlenecks and genetic drift associated with small founder population sizes [2, 3]. However, this pattern could be reversed if multiple divergent lineages from separate sources colonize an area [4, 5]. Where introductions are few, haplotypes in introduced populations should be a subset of those in the source population [6, 7]. Additionally, for sexually reproducing organisms, recombination from sexual reproduction may be more prevalent in source or native populations, whereas clonal reproduction may dominate in introduced or marginal populations since multiple mating types necessary for sexual reproduction may not be present [8–10]. However, lack of variation in introduced populations can make it difficult to detect recombination.
The focus of this research is the invasion history and population structure of the grape powdery mildew fungus, Erysiphe necator (formerly Uncinula necator), an obligate parasite of Vitis species that was introduced into Europe and, eventually, all other wine-producing regions of the world. Historical records support the hypothesis that the source of the introduction is eastern North America . Powdery mildew was described on grapes in North America in 1834, prior to its discovery in Europe in 1845 . Eastern North America is the centre of origin for many wild species of Vitis that have relatively high levels of resistance to many diseases and pests of grapevines, including powdery mildew [13, 14]. After its introduction to Europe, grape powdery mildew was observed throughout all wine-producing regions of the world, including California in 1859  and Australia in 1866 . E. necator most likely dispersed long distances by the movement of grapevines, which were frequently traded between continents in the mid-1800's and later. E. necator remains dormant as mycelium in dormant buds, or as sexual spores in cleistothecia in the bark of vines [17, 18].
Population genetic studies on E. necator to date have been limited to introduced populations in Europe and Australia where two distinct, yet sympatric, genetic groups have been consistently found [19–25]. The groups, designated as A and B (or groups I and III in earlier studies), were originally identified using anonymous markers assayed by RAPDs, ISSRs and AFLPs. Subsequent gene sequence analysis detected fixed nucleotide differences between groups at several nuclear loci, including 14 α-demethylase (CYP51), the internal transcribed spacer (ITS) regions of ribosomal DNA (rDNA) , and beta-tubulin (TUB2) . In India, a third genetic group was found, defined by RAPDs and a unique ITS sequence [19, 26]. Small differences in reproductive fitness  and temporal variation have been found between groups A and B [22, 23, 25, 26] leading to the hypothesis that temporal variation between the groups may be maintaining the differentiation by preventing interbreeding . Group A is genetically less diverse than group B, thus it has been suggested that it is clonal, whereas group B is sexually reproducing [19, 23]. Groups A and B produce viable sexual progeny (ascospores) in laboratory crosses [21, 22, 29], but recombinants have not been found in nature.
We had two major objectives for this study. First, to understand the evolutionary processes that led to the existence of groups A and B of E. necator in introduced populations, we tested the hypothesis that A and B were derived from separate introductions, as opposed to diverging after their introduction. To address this question, it was essential to study the population structure in eastern North America, the putative source population. Because no information was available on the population genetics of E. necator in North America, our major second objective was to describe the diversity and population structure in the eastern Unites States (US). We tested the hypothesis that if the eastern US population was a potential source of introductions, haplotypes found in introduced populations of Europe, Australia, and the western US would also be found in the eastern US. Moreover, we predicted that populations in the eastern US would have greater haplotype and nucleotide diversity than introduced populations. Finally, we tested the hypotheses that the population in the eastern US is structured by geography, Vitis host species, or host habitat (wild or cultivated Vitis).