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The putative center of origin of Plasmopara viticola, the causal agent of grape downy mildew, is eastern North America, where it has been described on several members of the family Vitaceae (e.g., Vitis spp., Parthenocissus spp., and Ampélopsis spp.). We have completed the first large-scale sampling of P. viticola isolates across a range of wild and cultivated host species distributed throughout the above region. Sequencing results of four partial genes indicated the presence of a new P. viticola species on Vitis vulpina in Virginia, adding to the four cryptic species of P. viticola recently recorded. The phylogenetic analysis also indicated that the P. viticola species found on Parthenocissus quinquefolia in North America is identical to Plasmopara muralis in Europe. The geographicdistribution and host range of five pathogen species was determined through analysis of the internal transcribed spacer polymorphism of 896 isolates of P. viticola. Among three P. viticola species found on cultivated grape, one was restricted to Vitis interspecific hybrids within the northern part of eastern North America. A second species was recovered from V. vinifera and V. labrusca. and was distributed across most of the sampled region. A third species, although less abundant, was distributed across a larger geographical range, including the southern part of eastern North America. P. viticola clade aestivalis predominated (83% of isolates) in vineyards of the European winegrape V. vinifera within the sampled area, indicating that a single pathogen species may represent the primary threat to the European host species within eastern North America. [ABSTRACT FROM AUTHOR]