Downy Mildew (Cucurbits) Description: A period of at least 6 hours at 100% humidity on the leaf surface is required for infection. Downy mildew symptoms vary considerably, plant to plant. Lesions appear first on the oldest leaves and then spread to the younger leaves. Solution: Use plant spacings which reduce canopy density, and avoid overhead watering to help reduce the length of time of leaf wetness. Spray Soap-Shield® Fungicidal Soap as a protectant.