Infections of downy mildew begin as irregular yellow patches on leaves; these chlorotic lesions later turn brown. If conditions are favorable, white fluffy growth of the fungus develops on the undersides of leaves. Leaves may take on a blighted appearance. Systemic infections can cause internal black streaks and patches to form in stems and floret branches of broccoli and cauliflower.
Identification
Downy mildew gets its name from the downy masses of spores it produces on the undersides of affected leaves. These downy growths initially may be confused with those of powdery mildew. However, the two diseases differ in several important ways. Downy mildew produces spores mostly on the undersides of leaves and only after rain or very heavy fog. Spores disappear soon after leaves dry out. Powdery mildew does not require water on the leaf surface for spore formation, and the powdery growth appears on both sides of leaves. Powdery mildew thrives in warm weather whereas downy mildew is a cool weather disease. Dry, desiccating winds and clear, warm days inhibit growth and spread of downy mildew.
Life cycle
Peronospora parasitica requires cool, moist weather for infection and disease development to take place. The pathogen survives between crops on weed hosts or as resilient oospores in crop residue. Spores are airborne. This disease is most serious on young seedlings; if cotyledons and the first true leaves are severely infected, the young plant may die.
Solutions
The best way to prevent downy mildew is to avoid the environmental conditions that favor it. Freely circulating air, allowing plants to dry between irrigations, and keeping leaves as dry as possible are good ways to limit the disease. Cool, moist weather in early spring and late fall favor the disease, so adjusting planting times may help in some areas. Destroy infected crop debris and susceptible related weeds before planting a new crop. Resistant broccoli varieties are available.
