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Powdery mildew of cucurbits

Quick facts

  • White powdery spots can form on both upper and lower leaf surfaces, and quickly expand into large blotches.
  • Symptoms of powdery mildew first appear mid to late summer in Minnesota.
  • Powdery mildew weakens the plant and the fruit ripens prematurely. Infected plants produce fewer and smaller fruit. 
  • In warm, dry conditions, new spores form and easily spread the disease.
  • Provide good air movement around plants through proper spacing, staking of plants and weed control.
  • If susceptible varieties are growing in an area where powdery mildew has resulted in yield loss in the past, fungicide applications may be necessary.

How to tell powdery mildew from other cucurbit issues

Areas of powdery mildew on the underside of a squash leaf.
Powdery mildew on the underside of a squash leaf
  • The first sign of powdery mildew is pale yellow leaf spots.
  • White powdery spots can form on both upper and lower leaf surfaces, and quickly expand into large blotches. The large blotches can cover the entire leaf, petiole and stem surfaces.
  • When powdery mildew infects the majority of the foliage, the plant becomes weak and the fruit ripens prematurely. Fruit can become sunburned.

Biology

  • Powdery mildew, mainly caused by the fungus Podosphaera xanthii, infects all cucurbits, including muskmelons, squash, cucumbers, gourds, watermelons and pumpkins.
  • Powdery mildew infections favor humid conditions with temperatures around 68-81° F.
  • In warm, dry conditions, new spores form and easily spread the disease.
  • Symptoms of powdery mildew first appear mid to late summer in Minnesota. The older leaves are more susceptible and powdery mildew will infect them first.
  • Wind blows spores produced in leaf spots to infect other leaves.
  • Under favorable conditions, powdery mildew can spread very rapidly, often covering all of the leaves. 
  • Although powdery mildew primarily infects leaves and vines, infections occasionally occur on cucumber or melon fruit. Powdery mildew does not directly infect squash fruit.
  • Fewer and smaller fruit grow on infected plants. Reduced fruit quality occurs due to increased sunscald, incomplete ripening, poor storability and poor flavor.
  • Powdery mildew is more likely to infect densely planted vines, plants crowded by weeds, plants in shaded sites and over-fertilized plants.
    Four pumpkin leaves with progressively severe powdery mildew, laying on a surface.
    Differing severities of powdery mildew on pumpkin leaves

Managing powdery mildew in the home garden

Managing downy mildew on farms

Field of squash plants with downy mildew.
Powdery mildew causing leaf dieback in squash
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Authors: Marissa Schuh, horticulture IPM Extension educator, and Michelle Grabowski

Reviewed by Natalie Hoidal, Extension educator

Reviewed in 2022

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