Toward improved disease forecasting for Stemphylium leaf blight of onion

Stemphylium leaf blight (SLB), caused by the fungal plant pathogen Stemphylium vesicarium is an important foliar disease of onion in Ontario. The symptoms start as small, dark or tan water soaked lesions on the leaves that progress to severe leaf dieback. Resistance to common fungicides has been found in Ontario and could be increased with over application of fungicides. Improvements in disease forecasting are needed to identify when fungicide applications are not needed. A replicated field trial was established at the Ontario Crops Research Centre – Bradford to evaluate various spray timing models to reduce spray applications. Treatments were a Calendar spray (7–10 days), 2 TOMcast model variations, and BSPcast model. TOMcast is effective for SLB on tomatoes and BSPcast was developed for SLB on pear. Each treatment was assessed weekly after the three leaf growth stage and until the onions lodged. Assessments were completed by rating disease percentage of the three most mature leaves of 20 plants for each treatment. A final assessment was established on August 16 by pulling 20 plants from each treatment and assessing each leaf into percentage of disease categories. Disease severity was relatively low in 2021 (43% in the nontreated check). All disease forecasting models reduced disease severity, with 33% using Tomcast with a threshold of 15. This reduction in disease is relatively small and demonstrates that the models triggered more fungicide sprays than needed. Research is continuing to develop an improved disease forecasting model for SLB on onions.