The impatiens downy mildew epidemic in the U.S. is caused by new, introgressed lineages of Plasmopara destructor with prominent genotypic diversity and high evolutionary potential.

Impatiens downy mildew (IDM) caused by is currently the primary constraint on the production and use of impatiens () as bedding plants worldwide. Downy mildew has been documented since the 1880s from wild-grown spp., but epidemic outbreaks of the disease affecting the commercially grown, ornamental were only reported for the first time in 2003 in the United Kingdom (U.K.) and in 2004 in the United States (U.S.) Here we assess the genetic diversity, level of differentiation, and population structure from 623 samples associated with current and pre-epidemic IDM outbreaks, genotyping the samples with SSR markers. population structure following the emergence of IDM in the U.S. is subdivided into four genetic lineages characterized by high genetic diversity, mixed reproduction mode, inbreeding, and an excess of heterozygosity. genotypes are significantly differentiated from pre-epidemic IDM samples from hosts other than , but no geographical or temporal subdivision is evident. from different species show significant but very low levels of differentiation in the AMOVA test that did not hold in DAPC analyses. The same was observed between samples of and recovered from . The finding of shared genotypes in samples from different countries and lack of differentiation among U.S. and Costa Rican samples indicate the occurrence of the international movement of the pathogen. Our study provides the first high-resolution analysis of the diversity of populations and the IDM epidemic that may be instrumental for disease management and breeding efforts.