This project aims to reducecrop loss for the New York onion industrythrough development of an integrated management strategy forthe dominant disease, Stemphylium leaf blight, caused by the fungus Stemphylium vesicarium. Stemphylium leaf blight is a re-emerging disease problem for growers in northern USA, and is now ubiquitous in commercial fields in New York. The disease causes premature death of foliage, preventing onion bulbsfrom achieving optimum size, leading tolosses in yield of30-40%.Control of SLB has relied heavily on the use of fungicides. However, the pathogen has developed widespread resistance to several fungicides, and fungicide options for control are now limited. This project will develop anintegrated management strategy guided through new knowledge on the pathogen population structure, the lifecycle of the pathogen, the influence of agronomic factors,and disease forecasting based on environmental variables, to advise growers of optimum timing of fungicide application.