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Powdery Mildew

Powdery Mildew

What you’ll see…a dusty white to gray coating over the leaves or other plant surfaces. Powdery mildew will begin as discrete circular, powdery white spots. As these spots begin to expand they will coalesce, producing a continuous matt of mildew (similar to dirt or dust). Symptoms usually appear late in the growing season. Injury due to powdery mildews includes stunting and distorting of leaves, buds, growing tips, and fruit. Yellowing of leaves and death of tissue may result in premature leaf drop. Nutrients are removed from the plant by the fungus during infection and may result in a general decline in growth and vigor of the plant.

The reason for the problem…is many different fungi. These fungi which cause powdery mildew are spread by spores produced in the white patches. These spores are blown in the wind to other parts of the plant or on to other plants during the growing season. During the winter these fungus survive on infected plant parts and in debris such as fallen leaves. It may produce a resting structure known as cleistothecia, which resist harsh winter conditions. These will appear as small black dots within the white powdery patches. The next spring spores within the cleistothecia are shot up into the air and carried by air currents to leaves of plants. This fungus is favored by high relative humidity periods or site conditions that promote this type of environment, such as close spacing plants, densely growing plants, or shade.

You should…keep the tree growing with proper watering and maintenance. Sanitary measures will also help slow the spread of the spores.

  • Destroying plant material and debris.
  • Collecting and destroying leaves or composting them under several inches of soil.
  • Improve sunlight and air circulation by thinning and pruning of surrounding trees and shrubs.
  • Plant resistant varieties.

We can help by…applying the appropriate disease control for the specific type of plant. These should be continued during the growing season.


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