Squash 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.

Causes

  • 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


Management

  • 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.



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