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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.
  • Powdery mildew weakens the plant and the fruit ripens prematurely. Fewer and smaller fruit grow on infected plants.
  • In warm, dry conditions, new spores form and easily spread the disease.
  • Symptoms of powdery mildew first appear mid to late summer in Minnesota.
  • 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 may be necessary.
Leaves with white spots, infected by powdery mildew

Powdery mildew, mainly caused by the fungus Podosphaera xanthii, infects all cucurbits, including muskmelons, squash, cucumbers, gourds, watermelons and pumpkins. In severe cases, powdery mildew can cause premature death of leaves, and reduce yield and fruit quality.

Identifying powdery mildew symptoms

  • 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 entire leaf, petiole and stem surfaces.
  • When powdery mildew infects the majority of the foliage, the plant weakens and the fruit ripens prematurely.

What causes powdery mildew

  • 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
Pumpkin patch infected by powdery mildew, white spots on leaves
Pumpkin patch infected by powdery mildew

Preventing and managing the disease

  • Plant varieties with complete or partial resistance to powdery mildew.
  • Apply fertilizer based on soil test results. Avoid over-applying nitrogen.
  • Provide good air movement around plants through proper spacing, staking of plants and weed control.
  • Once a week, examine five mature leaves for powdery mildew infection. In large plantings, repeat at 10 different locations in the field.
  • If susceptible varieties are growing in an area where powdery mildew has resulted in yield loss in the past, fungicide may be necessary.
    • Apply fungicides after finding the first spot of powdery mildew.
  • Home gardeners can apply sulfur products to both the upper and lower surface of the leaves.
  • Commercial growers should refer to the Midwest Vegetable Production Guide for pesticide recommendations.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Michelle Grabowski

Reviewed in 2018

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