Master Gardener: Powdery mildew – it’s everywhere

By Donna Tatting, Chisago County Master Gardener

We’ve recently been getting calls about plant decline from a white powdery film on the leaves of curcubit plants like squash and cucumber. The culprit here is a fungus called Podosphaera xanthii or more commonly known as Powdery Mildew.

Powdery mildew 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 also a common disease in ornamental perennial gardens and can be found on many shrubs and trees.

The first signs of the disease are 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.

There are some controls home gardeners can take to help prevent or reduce the occurrence of Powdery Mildew.  Plant varieties of plants that have complete or partial resistance to the fungus.  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).  Apply fungicides when a single spot of powdery mildew is first found.  Of course it is essential that you read the label of any fungicide product and follow the directions as given.

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