Barley as an Alternative Host of <i>Stemphylium vesicarium</i> in Michigan Onions and Susceptibility of Poaceae Cover Crops

Stemphylium leaf blight (SLB) poses a significant challenge to onion production. Tactics aimed at reducing the primary inoculum could enhance disease management, yet primary inoculum sources in the SLB-onion pathosystem remain unclear. This study identifies barley ( Hordeum vulgare), commonly used as an interseeded cover crop in onion muck production, as a potential source of an early-season conidia-based inoculum for SLB. Barley cover crop samples from three commercial onion production fields were collected weekly during the growing season. Field observations indicated that Stemphylium vesicarium conidia sporulation on barley cover crops appears to be associated with senescing tissue following the application of herbicides to terminate the cover crop. Koch's postulates confirmed that S. vesicarium infects barley, and cross-infection experiments demonstrated that S. vesicarium isolates from barley can infect onion. Among the tested alternative cover crops, only ryegrass ( Lolium multiflorum) was found to be a non-host of S. vesicarium. These findings highlight the need for informed cover crop selection to mitigate SLB risks in onion production.