(synonym: Peronospora parasitica)
This disease manifests as yellow, purple or brown irregularshaped areas on upper leaf surfaces, which correspond to white to gray, “downy” fungal spore masses on abaxial leaf surfaces. Under heavy disease pressure, sporangia develop on the upper leaf surfaces, as well, which may lead to seedling death. Early infections by this obligate pathogen may invade the vascular system, turning it black. Cauliflower curds, broccoli florets, radish roots and cabbage heads may all be unmarketable if infected.
Conditions for Disease Development
Heavy fog, light rains, prolonged leaf wetness and night temperatures between 8-16°C (46-61°F), with day temperatures below 24°C (75°), generally favor disease development.
Eradicate cruciferous weeds and volunteers. Furrow or drip irrigate, and transplant at densities that promote good aeration and reduced humidity. Apply fungicides early and often to achieve adequate disease control.