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.