Downy mildew infects many vegetable crops including lettuce, spinach, onions and cole crops. Each crop is infected by a different pathogen that causes downy mildew. Cool to moderate temperatures coupled with extended periods of leaf wetness and high humidity are perfect conditions for disease.
Upper leaf surface symptoms show yellow angular lesions. A fungal growth will occur on the bottom side of the leaf. Grapes also can become infected with downy mildew.