Downy mildew, caused by Pseudoperonospora cubensis, is one of the most important foliar diseases of cucurbits. It has been reported worldwide in production areas where humidity and temperature favor its establishment. Downy mildew occurs in temperate and tropical areas with sufficient leaf wetness periods. Without adequate control measures, downy mildew can result in major crops losses in cucurbits in both open fields and greenhouses. In Illinois, this disease usually occurs toward the end of the growing season, as the pathogen is blown from the Southern states northward.