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Extension > Garden > Yard and Garden > Vegetables > Diseases of cucurbits > Powdery mildew

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Powdery mildew

Michelle Grabowski, University of Minnesota, Extension Educator

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.

Identification

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M. Grabowski, University of Minnesota

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.

Important biology

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.

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M. Grabowski, University of Minnesota

Densely planted vines, plants crowded by weeds, plants in shaded sites, and over fertilized plants are more likely to be infected with powdery mildew.

Management

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