Irregular brown or yellow spots on leaves are an early symptom of this fungal disease, which can infect many vegetable crops. Check the undersides of affected leaves; presence of a hairy white or purple mold confirms the diagnosis. Severely infected leaves may die, and the disease can eventually spread to stems, flowers, and fruit.
The fungus overwinters on plant debris. Downy mildew is a host-specific disease. Thus, if your cucumber plants become infected, that doesn’t necessarily mean your onions will succumb.
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