Susceptibility of Long-day Onion Cultivars to Stemphylium Leaf Blight in New York

Stemphylium leaf blight (SLB), which is caused by the fungus Stemphylium vesicarium, is the most common foliar disease affecting onions ( Allium cepa ) in the northeastern United States. It causes severe necrosis and premature defoliation, thus reducing the photosynthetic area, and may deleteriously affect bulb size. Onion production in New York relies on multiple fungicide applications for foliar disease control because no information regarding the susceptibility of the locally suitable and commercially available long-day onion cultivars to SLB is available. In this study, both replicated controlled environment and field trials were conducted to evaluate the susceptibility of selected long-day onion cultivars to SLB using S. vesicarium isolates representative of the New York population. The severity of SLB was estimated at regular intervals and used to calculate the area under disease progress curve (AUDPC) to reflect epidemic progress. Yield, average bulb size (ABS), and average bulb weight (ABW) of eight cultivars tested in the field trial in 2024 were also recorded. In the controlled environment trials, there were small differences in susceptibility among cultivars, with the lowest AUDPC for SVNY1610 and Redwing. In the field trial, AUDPC was also significantly lower for all cultivars except for Patterson and Candy. Yield, ABS, and ABW were significantly higher for ‘SVNY1608’ than for ‘Candy’ and ‘Red Spring’. These findings may assist the design of integrated disease management strategies for SLB and foliar disease control programs and inform breeding programs for SLB resistance in onions.