Matthew John

In 2023, Michigan’s onion production contributed an estimated 85 million pounds to U.S. production, valued at $18.7 million. Yellow and red dry storage onions are primarily grown for their higher sulfur content and lower moisture compared to sweet onions. However, in Michigan and other North American production regions, Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium, is a major foliar disease and a significant threat to onion production. This disease leads to premature senescence, reduced bulb size, and lower marketable yield. The survival and spread of S. vesicarium in onion fields are largely influenced by ascospores and conidia, each playing a distinct role in disease epidemiology. However, the source of inoculum and its role in disease development remain unclear. Onion growers typically apply fungicides weekly or biweekly to control SLB, as no commercially resistant cultivars are available. SLB management heavily relies on repeated applications of single-site fungicides, posing a high risk of resistance development in the pathogen. My research focuses on understanding the epidemiology of S. vesicarium, including the identification of alternative hosts. Additionally, to mitigate fungicide resistance, new fungicide products will be tested, and alternative forecasting models will be optimized to assess their effectiveness under local environmental conditions, improving SLB management strategies in Michigan onion.