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

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Commodities

• Vegetables: onions

Practices

• Pest Management: integrated pest management

Stemphylium leaf blight (SLB), caused by the fungus Stemphylium vesicarium, is one of the most important foliar diseases affecting onions (Allium cepa L.) in the northeastern United States. Infested transplants and volunteers (plants regrowing from onion bulbs left in the field the previous season), may play an important role in SLB epidemics serving as primary inoculum. However, the lack of knowledge surrounding the relative contributions of primary inoculum sources for SLB epidemics is a significant knowledge gap to the design of integrated management practices. Isolates of S. vesicarium obtained in 2022 and 2023 from infested volunteers, transplants and symptomatic onions in the field will be used in this study. Relationships among pathogen populations convey important implications for epidemiology, including sources of inoculum, changes in diversity, and patterns of dispersal. The genetic diversity and differentiation in each S. vesicarium population will be characterized using nine microsatellite markers. Microsatellites are broadly used for population genetic studies due to their codominance, hypervariability, locus-specificity, and reproducibility. Multilocus genotypes will be compared among populations to establish whether transplants or volunteers are contributing genotypes to the S. vesicarium populations in NY onion fields. Multiple diversity and differentiation indices will also be calculated to assess the relationships among individuals of the populations. The outcome of this project will be the development of hypotheses surrounding multiple facets of SLB epidemiology to facilitate the design of integrated disease management strategies. Research findings will be communicated through various platforms and formats to effectively engage stakeholders and growers throughout NY.

1. Objective: To determine the genetic diversity within and among S. vesicarium populations collected from infested transplants and volunteers at the beginning of the season and field populations collected at the end of the season.

1.1 Hypothesis: S. vesicarium populations from infested transplants and volunteers have high genetic diversity but similar frequencies of alleles among populations and share the same genotypes.

2. Objective: To determine the population structure of S. vesicarium in NY onion fields.

2.1 Hypothesis: There are no distinct patterns or clusters of genotypes among the S. vesicarium populations indicating that populations are part of one interbreeding and genetically uniform population.