NEW YORK IS THE FIFTH LARGEST PRODUCER OF ONIONS IN THE UNITED STATES, PRODUCING 142,246 T FROM 2,832 HA (USDA NASS 2017). THE MAJORITY OF ONIONS ARE GROWN IN LOCALIZED REGIONS ON MUCK (HIGH ORGANIC MATTER) SOILS, WHERE FOLIAR DISEASE MANAGEMENT IS A MAJOR DETERMINANT OF ONION PRODUCTIVITY AND PROFITABILITY. OVER THE LAST TENYEARS STEMPHYLIUM LEAF BLIGHT CAUSED BY THE FUNGUS STEMPHYLIUM VESICARIUM HAS BECOME UBIQUITOUS IN FIELDS THROUGHOUT NEW YORK, CAUSING PREMATURE DEATH OF FOLIAGE AND PREVENTING ONION BULBS FROM ATTAINING OPTIMUM SIZE.WHERE THE DISEASE IS NOT ADEQUATELY CONTROLLED, YIELD LOSSES OF OVER 30-40% HAVE BEEN RECORDED. THE FUNGUS HAS RAPIDLY DEVELOPED WIDESPREAD RESISTANCE TO MANY OF THE COMMONLY USED FUNGICIDES, AND OPTIONS FOR CONTROLLING THIS IMPORTANT DISEASE ARE BECOMING LIMITED. THIS PROJECT WILL PROVIDE GROWERS WITH AN INTEGRATED MANAGEMENT STRATEGY THROUGH GAINING A BETTER UNDERSTANDING OF PATHOGEN DIVERSITY, UNDERTANDING THE INFLUENCE OF AGRONOMIC PRACTICES ON THE SEVERITY OF DISEASE EPIDEMICS, AND TESTING AND FINE-TUNING A DISEASE FORECASTER BASED ON WEATHER VARIABLES AS A PREDICTOR OF DISEASE OUTBREAKS AS A MEANS OF ACHIEVINGBETTER TIMING OF CONTROL STRATEGIES.