P-532 - Determining the relative importance of primary inoculum sources for Stemphylium leaf blight of onion in New York.

Stemphylium leaf blight (SLB), caused by the fungus Stemphylium vesicarium, can significantly reduce the yield of dry onions (Allium cepa L.) due to the premature defoliation that results in smaller bulbs. Despite being the most common foliar disease affecting onions in the northeastern United States, the lack of knowledge on the relative contributions of primary inoculum sources for SLB epidemics creates a major knowledge gap in the design of integrated management practices. Infested onion transplants and volunteers (plants regrowing from onion bulbs left in the field the previous season), may contribute to primary inoculum for SLB epidemics. In 2022 and 2023, isolates of S. vesicarium (n=180) were obtained from infested volunteers, transplants and symptomatic onion leaves in NY onion fields to assess the genetic diversity and relationship among pathogen populations using nine microsatellite markers. All S. vesicarium populations were highly diverse based on Evenness, Shannon-Wiener, and Stoddart and Taylor’s genotypic diversity indices. Populations from infected volunteers and onion leaves collected pre-harvest shared multiple multilocus genotypes (MLGs) while populations sampled from transplants shared no MLGs. No significant genetic differentiation among 2022 and 2023 populations was found, demonstrating the potential for S. vesicarium genotypes to persist between seasons. These results suggest that volunteers may be a more important source of primary inoculum for SLB epidemics in NY onion fields than transplants and highlight the importance of overwintering inoculum.