(flowering dogwood) is a popular understory tree endemic to the eastern hardwood forests of the United States. In 1996, dogwood powdery mildew caused by , an obligate biotrophic fungus of large bracted dogwoods, reached epidemic levels throughout the growing region. In the late 1990s, both sexual and asexual stages of were regularly observed; thereafter, the sexual stage was found less frequently. We examined the genetic diversity and population structure of 167 samples on leaves using 15 microsatellite loci. Samples were organized into two separate collection zone data sets, separated as eight zones and two zones, for the subsequent analysis of microsatellite allele length data. Clone correction analysis reduced the sample size to 90 multilocus haplotypes. Our study indicated low genetic diversity, a lack of definitive population structure, low genetic distance among multilocus haplotypes, and significant linkage disequilibrium among zones. Evidence of a population bottleneck was also detected. The results of our study indicated a high probability that reproduces predominately via asexual conidia and lend support to the hypothesis that is an exotic pathogen to North America. Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.