Cucumber, Pumpkin, Squash: Powdery mildew

Biology

Powdery mildew is a common fungal disease of squash. Upper and lower leaf surfaces and stems are typically affected. Small, white spots quickly enlarge into patches of characteristic powdery-white fungal growth. Older leaves, shaded leaves, and leaf undersides are favored locations for powdery mildew development, although it can quickly spread to involve large portions of the plant. Tiny, speck-like, black fungal structures may be present in the white mats. Severely affected leaves may yellow and die prematurely. Occasional fruit infections occur on cucumber, although most commercially-grown varieties are resistant. Disease development is favored by humid weather, cool nights, and cloudy days. Dense plantings are more likely to be affected.

Management Options

Use Integrated Pest Management (IPM) for successful plant problem management.

Non-chemical Management

Select non-chemical management options as your first choice!

• Plant resistant varieties when available. Resistant cucumber varieties include ‘Calypso’, ‘Cherokee’, ‘Cross Country’, ‘Dasher II’, ‘Flurry’, ‘Marketmore’, ‘Pioneer’, ‘Quest’, ‘Regal’, ‘Royal’, and ‘Turbo’, among others.
• Rotate crops. Do not plant cucumbers and related species in the same locations for at least two years.
• Space plantings to provide good air circulation.
• Remove plant debris from the garden. Destroy or discard (do not compost) diseased materials.

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides.

• Usually, the disease does not warrant fungicide applications.
• Do not use spray oils if temperature is below 50° F, above 90° F or when plants are under heat or moisture stress.
• Do not apply when foliage is wet. Good coverage is essential.

Approved Pesticides

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.

• Bi-Carb Old-Fashioned Fungicide [Organic]
  Active ingredient: potassium bicarbonate [EPA registration number: 54705-10]
• Monterey Horticultural Oil [Organic]
  Active ingredient: mineral oil/pet distillate light [EPA registration number: 48813-1-54705]
• Safer Brand Garden Defense Multi-Purpose Spray Concentrate [Organic]
  Active ingredient: clarified hydrophobic extract of neem oil [EPA registration number: 70051-2-42697]
• This list may not include all products registered for this use.

Additional Images