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Powdery mildew, caused primarily 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.
Powdery mildew is first evident as pale yellow leaf spots. White powdery spots can form on both upper and lower leaf surfaces, and quickly expand into large blotches which ultimately can cover entire leaf, petiole, and stem surfaces. When the majority of the foliage is infected, the plant is weakened and the fruit ripens prematurely.
Powdery mildew infections are favored by humid conditions with temperatures around 68-81F. In warm, dry conditions, new spores are produced and easily spread the disease. Symptoms of powdery mildew are often first noticed mid to late summer in Minnesota. The older mature leaves are more susceptible and will be infected first. Spores produced in leaf spots are blown by the wind to infect other leaves. Under favorable conditions, powdery mildew can spread very rapidly, often resulting in complete leaf coverage.
Although powdery mildew primarily infects leaves and vines, infections occasionally occur on cucumber or melon fruit. Squash fruit are not directly infected. Regardless of direct infection of the fruit, fewer and smaller fruit are produced on infected plants. Reduced fruit quality occurs due to increased sunscald, incomplete ripening, poor storability, and poor flavor.
Densely planted vines, plants crowded by weeds, plants in shaded sites, and over fertilized plants are more likely to be infected with powdery mildew.
- 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.
- If susceptible varieties are being grown in an area where powdery mildew has resulted in yield loss in the past, fungicide may be necessary.
- Once a week examine 5 mature leaves for powdery mildew infection (in large plantings, repeat at 10 different locations in the field).
- Apply fungicides when a single spot of powdery mildew is first found.
- 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.