Various types of downy mildew fungi cause disease in a number of crops across North America. Irregular brown or yellow spots develop on the upper leaf surface; the lower leaf surface beneath these spots is covered with a hairy white or purple mold during humid weather. Severely infected leaves die. The fungus that attacks cucumbers and muskmelons (and less frequently squash and pumpkins) occurs mainly in the East. Fruits of infected plants are small and bitter. Cabbage family members are often infected as seedlings, which rapidly yellow and die. Mature plants may have sunken black spots on their heads. Moist weather encourages the spread of this fungus.

Late blight strikes tomatoes and potatoes all over North America at any stage of growth. Irregular gray spots form on leaves. White mold grows on the undersides of these spots. Infected leaves turn brown and dry up. Gray, water-soaked spots on infected fruits later turn dark brown and corky. Infected tubers are covered with brown spots where rot begins. Wet weather with cool nights and warm days favors the spread of the disease. The fungus overwinters in infected plant debris.