Several fungal species affect different crops to produce similar symptoms, all known as downy mildew.
Pseudoperonospora cubensis (affects curcubits), Plasmopara viticola (affects grapes), Bremia lactucae (affects lettuce), Peronospora destructor (affects onions).
In moist weather, a white downy fungal growth develops on the underside of the leaf.
Spots turn brown and dead areas develop.
|How is it spread|
It is mainly spread by wind from older crops, but spores can survive in the soil and on crop residue. Worse on lower, shaded leaves that stay moist.
Warm, moist weather favours the spread of the disease.
Curcubits, grape, lettuce, onion.
Plant resistant varieties. Plough 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 spaced plantings allow better air flow and spray penetration.
Chemical registrations and permits