Grape downy mildew is a destructive disease of both wine and processing grapes in New York and Pennsylvania. Its occurrence is sporadic, and seemingly unpredictable. We developed a computer model (DMCAST) which forecasts both initial infection, and cycles of secondary disease development. Our objectives were (1) to simplify the original DMCAST (which was a model designed for research) for practical use in regional IPM programs, and (2) to evaluate the utility of model forecasts of downy mildew in a management program for multiple diseases.