Spring 2026 PLP Seminar: Matthew John
March 16, 2026 1:30PM - 2:30PM
Atmospheric Inoculum Concentrations of Stemphylium vesicarium and New Hosts in Michigan Onion Systems
Abstract
Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium, is a relatively new constraint to onion production in Michigan and other production regions despite fungicide applications. The pathogen has become resistant to the strobilurin fungicides, a commonly used product. Information on inoculum source(s) and potential hosts can optimize disease management strategies. The objective of this study was to quantify the atmospheric ascospore and conidial concentrations in a commercial onion field and to evaluate the role of barley cover crops in SLB development. Burkard 7-day volumetric spore traps were deployed from March to November in Hamilton, MI, and disease progression in untreated plots was assessed. Ascospores were first detected in mid-March, prior to onion establishment, and declined as the season progressed. Atmospheric concentrations of conidia were observed from late June through September, with peak concentrations coinciding with increased disease severity. Barley cover crops established as wind breaks and sampled during the growing season harbored S. vesicarium conidia. Molecular analyses confirmed isolates from onion and barley samples clustered within S. vesicarium clades. The number of cover crop samples with S. vesicarium conidia peaked when there were high atmospheric conidial concentrations during critical onion growth stages. Atmospheric inoculum sources were also monitored in a nearby celery field following onion production. Celery is a newly described host of S. vesicarium. Atmospheric concentrations of conidia peaked during crop harvest but were overall lower than those observed in the onion production field. These findings demonstrate that airborne inoculum is present earlier than typically assumed and that non-onion hosts may serve as inoculum sources in Michigan onion production systems, informing disease management strategies.
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