Genetic characterization and population structure of <i>Erysiphe pulchra</i> the causal agent of powdery mildew on <i>Cornus florida</i> in the eastern United States

Cornus florida L. (flowering dogwood) is a deciduous understory tree endemic to the eastern United States. Known for its attractive bracts, C. florida is very popular for its ornamental qualities. After 1995, dogwood powdery mildew caused by Erysiphe pulchra Cooke & Peck reached epidemic levels throughout the C. florida growing region. Initially, both sexual and asexual stages of E. pulchra were regularly observed, but in recent years, the teleomorph has not been seen as often. Fifteen microsatellite loci were used to analyze the genetic diversity and population structure of 174 E. pulchra samples from 10 eastern states. The results of this study indicated low genetic diversity, lack of definitive population structure, and significant linkage disequilibrium within the sampled population. Evidence of a recent bottleneck was also observed. Our results suggest that E. pulchra has become clonal in eastern United States and may be an exotic plant pathogen to North America.