Causal Agent:
Peronospora parasitica

Distribution:
Worldwide

Symptoms:
Portions of the under surfaces of infected leaves become covered with a white to gray fluffy "down" comprised of the fungus spores. The upper surfaces of the leaves have purple, yellow or brown regions that correspond to the sporulating areas on the under surfaces of the leaves. Infected young plants may die. An early leaf infection may enter a plant's vascular system turning it black. Leaves with downy mildew may drop off and if the black rot bacterium entered through the detached leaves, Peronospora parasitica may not be suspected as a contributor to plant death. Cauliflower curd, broccoli florets, radish roots and cabbage heads may all become unmarketable if infected.

Conditions for Disease Development:
Heavy fogs, light rains, prolonged dews and night temperatures between 8-16°C (46-61°F) with day temperatures below 24°C (75°F) will greatly favor this pathogen.

Control:
Employ eradication of cruciferous weeds, low volume sprays or dusting with fungicides, no overhead irrigation, and good aeration in plant beds to reduce humidity, in order to control downy mildew.