WSU Extension


Cucumber, Pumpkin, Squash
Angular leaf spot 
Cucumber mosaic virus 
Curly top (Beet curly top virus) 
Pollination failure 
Powdery mildew 
Root rot 
Verticillium wilt 
White mold 
Brown marmorated stink bug 
Spider mites 
Squash bug 
Western spotted cucumber beetle 

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Caption: Powdery mildew on cucumber
Photo by: R.S. Byther
Cucumber, Pumpkin, Squash : Powdery mildew
(revision date: 4/4/2022)

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

Non-Chemical Management
  • 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.
Select non-chemical management options as your first choice!

Chemical Management

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.

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 reg no: 54705-10
  • Monterey Horticultural Oil [Organic]
    Active ingredient: mineral oil/pet distillate light  |  EPA reg no: 48813-1-54705
  • Safer Brand Garden Defense Multi-Purpose Spray Conc [Organic]
    Active ingredient: clarified hydrophobic extract of neem oil  |  EPA reg no: 70051-2-42697
  • This list may not include all products registered for this use.

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Caption: Powdery mildew on cucumber
Photo by: R.S. Byther
Caption: Powdery mildew on cucumber
Photo by: R.S. Byther
Caption: Powdery mildew on squash
Photo by: R.S. Byther