Causal Agent: 

Fungus (Hyaloperonospora parasitica); Bremia lactucae (affects lettuce)

Characteristic Symptoms:
Leaf symptoms appear as yellowish, irregular lesions on the upper surface corresponding to a white fluffy or downy growth develops on the undersides of leaves.
Older lesions become dry, turn brown and papery in texture and may cause the entire leaf to drop.
The head of broccoli and cauliflower curds infected with black spots within or on the surface.
Conditions for Disease Development:
Downy mildew causes damage from the seedling (cotyledons) to harvesting stage.
The spore releases in the morning or after sunrise.
It is spread by the wind although leaf wetness is required for infection. Cool, moist conditions favour the disease. 

The fungus can survive on brassica weeds, in crop debris or soil for several months, even in the absence of a host.

Management and Control:
Plant resistant varieties (if available).
Plow in old crops as soon as harvesting is completed and ensure all crop residue is decomposed before planting.
Spray to fruit set with a protectant fungicide. In wet weather and after fruit set, use a systemic fungicide. If the disease is well advanced, it will be difficult to control.

Wider space in planting allow better airflow and spray penetration.

Avoid overhead irrigation
Spray with fungicides such as Mancozeb (Dithane M-45®), Chlorothalonil (Daconil 2787®) or metalaxyl+mancozeb (Ridomil®).  

To view other diseases,  click here.

Need more help?  Ask the Doctor.