Downey mildew: Pale spots on onion leaves
Pathogen: Peronospora destructor
Symptoms
Pale spots or elongated patches on leaves; gray-purple fuzzy growth on leaf surface; leaves turning pale then yellow; leaf tips collapsing.
Disease Development
The fungus survives in volunteer onion plants, onion sets, plant debris or in the soil. The fungal spores are disseminated onto plants by winds and splashing rain during cool wet weather, which is essential for disease development. Rain, dew or high humidity (>95%) is required for fungal spore germination and infection. The fungus grows internally and continues to produce spores as long as the weather remains cool and wet.
Disease Management
A regular fungicide spray program based on climatic conditions can reduce crop losses. Avoid planting onion sets that are contaminated with the fungus. Eliminate plant debris and cull piles. Plant rows in the direction of the prevailing winds and use furrow irrigation rather than sprinkler irrigation. A 3-4 year rotation out of onions in areas where the disease is present can help reduce losses.
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