Start seeds in commercial potting soil, not in garden soil. If disease has been severe enough in the past to warrant chemical control, metalaxyl can be used before or at the time of seeding. Use for outdoor seeding only, not for transplants. Apply all chemicals according to directions on the label. This disease is caused by the fungus Peronospora parasitica and can attack both seedlings and mature vegetable plants.
Infected plants develop a gray mold on the lower leaf surface.
The upper leaf surface of infected plants first turns yellow and then may turn brown or necrotic. Leaves wither and die. Symptoms differ from powdery mildew in that the downy mildew fungus grows only on the lower surface of the leaf. Development of the disease is favored by moist conditions. Rotate with crops other than cole crops or greens.