Comparison of the genetic diversity and population structure of Stemphylium vesicarium from New York and Georgia onion fields

Stemphylium leaf blight, caused by the fungus Stemphylium vesicarium, has emerged as one of the most damaging foliar diseases of onion in northeastern North America. The pathogen is distributed globally and has adversely affected onion production in more than 20 countries, but information surrounding S. vesicarium population genetic structure and differentiation is limited. In the United States (US), studies evaluating S. vesicarium population genetic structure between fields within New York (NY) state have been conducted, but a more comprehensive study across onion producing states is needed. In this study, 340 S. vesicarium isolates were collected in 2022 from two major onion-producing states (NY, n = 160 and Georgia (GA), n = 180) to characterize population genetic diversity and structure using nine polymorphic simple sequence repeat markers. The results revealed a higher genetic diversity in populations in NY compared to GA. Population structure analyses showed a clear clustering of individuals based on state of origin, but low genetic structure was observed within each state, indicating a uniform distribution of haplotypes. These findings highlight the importance of considering geographic scale when evaluating the population structure of S. vesicarium and provide a framework for understanding the pathogen’s dispersal and adaptation dynamics across onion production systems.