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Gardening: Conditions right for powdery mildew to strike

Powdery mildew coats the leaves of a domestic, hybrid lupen. (FILE The Spokesman-Review)
Powdery mildew coats the leaves of a domestic, hybrid lupen. (FILE The Spokesman-Review)
Pat Munts

As we move into Labor Day, we will see a major cooldown in temperatures, and that means it is the beginning of powdery mildew season.

The evenings are cooling and because we are still watering gardens, the evening humidity is rising. It’s the perfect recipe for the formation of powdery mildew on vegetables like lettuce, cucumbers, squash and tomatoes, as well as grapes, ornamentals, fruits and berries, apples and a few dozen more.

Powdery mildew is a generalized reference to dozens of different fungi that colonize leaf surfaces when the humidity is between 50 to 90 percent in the twilight hours and temperatures are between 60 and 80 degrees. The fungi need living tissue to germinate on and then reproduce by sending spores into the wind, spreading the infection from plant to plant. Sometimes the cycle from initial infection to the production of spores can take as little as 48 hours.

In general, symptoms initially appear as circular white spots that quickly enlarge as thread-like mycelia begin to grow in an irregular pattern across the leaf surface. Between the mycelia and the spores, the surface looks like it is covered with a white-to-yellow powder. They also block sunlight from the leaf surface. The mycelia send a root-like structure into the green leaf tissue to draw nutrients. As a result, the plant is stressed and starts a slow decline.

Smaller plants like vegetables and fruits can be killed by it. When it occurs on larger plants, like maples, or vines, like grapes or honeysuckle, the plant can usually withstand the effects because it has more leaf surface to draw energy from than a smaller vegetable plant. As a side note, zucchini is susceptible to powdery mildew, but don’t confuse its appearance with the white pattern naturally found on the leaves.

You can pre-treat readily susceptible plants before the appearance of the spots by spraying a sulfur-based fungicide (example: Safer Garden Fungicide), a potassium bicarbonate (Kaligreen) or a homemade baking soda spray. The sulfur-based fungicide is applied as a wettable powder and can damage plants if applied at over 90 degrees. The potassium carbonate spray will kill the fungi as well as prevent it if applied prior to infection. Other controls include neem oil, milk (one part milk to two parts water) and household vinegar (two to three tablespoons to a gallon of water).

A simple recipe for a baking soda spray is to mix one tablespoon of baking soda, one teaspoon of oil (horticultural or cooking oil) and one teaspoon of insecticidal or dish soap into a gallon of water. Apply the mix every one to two weeks.

While sprays can prevent infections as they occur, consider buying plants that are resistant to powdery mildew to start with. Read the label when you buy plants or ask the nursery staff about their experience. Don’t overfertilize plants, and make sure they get enough sun. Improve air circulation around plants by trimming them to remove excess moisture.

Pat Munts is co-author, with Susan Mulvihill, of the “Northwest Gardener’s Handbook.” Munts can be reached at pat@inlandnw

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