Technical Abstract: Exotic plant invaders are a major global threat to biodiversity and ecosystem function. Here I present multiple lines of evidence suggesting that soil microbial communities affect the population growth rates of Prunus serotina in its native range and affect its invasiveness abroad. Research often reveals complex regulatory effects of oomycetes (specificially Pythium spp.) on P. serotina populations in its native range. Prunus serotina appears highly susceptible to some Pythium spp. even when they are at relatively low densities. Although research indicates oomycetes (and often more specifically Pythium) negatively affect P. serotina throughout its native range, results exhibit a high degree spatial complexity. For example in its native range, field and laboratory experiments reveal that pathogenic effects and Pythium densities vary among sites, among individual P. serotina trees, and with distance from P. serotina trees. At a biogeographical scale, plant-soil biota interactions appear more negative in the native than the non-native ranges, and biogeographical comparisons of the virulence of Pythium associating with P. serotina in its native vs. non-native ranges reveal that the Pythium species in its native range are considerably more virulent than those in its non-native range. A phylogentic comparison of Pythium isolates associating with P. serotina revealed taxonomic shifts with virulent taxa associating with P. serotina in its native but not its non-native ranges. In total, these various lines of evidence support the idea that oomycetes, specifically Pythium species, are an important biological factor regulating the density of numerous populations of P. serotina in its native range but not in its invasive range. This biogeographical variation in regulation by Pythium pathogens is likely an important factor affecting its invasive success abroad. I also highlight the general challenges scientists face when determining whether soil microbes affect plant population growth rates and invasions and attempting to uncover the taxa driving these effects.