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Spokane, Washington  Est. May 19, 1883
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Gardening: Cooler weather brings humidity, risk of powdery mildew

The weather appears to be cooling a bit. Temperatures are dropping from the 90s into the 80s and 70s this weekend with nights in the lower 50s. Throw in a bit of humidity and you have perfect weather for the formation of powdery mildew.

Powdery mildew in a group of fungal diseases that cover susceptible plant leaves with a talcum powderlike whitish film that can’t be rubbed off. The mildew reproduces by spores which are produced by pinhead-sized, spherical fruiting structures that age from white, to yellow-brown and finally black. They are present singly or in a group. Susceptible plants include squash, cucumbers, bee balm, zinnias, zucchini, delphinium, phlox, roses, maples and hydrangeas.

Once the powdery mildew has taken hold, the leaves can’t efficiently carry on photosynthesis which reduces the plant’s vitality. Production of fruit and flowers falls off and in severe cases the leaves become disfigured. Temperatures above 90 degrees will kill the spores but once they drop into the 60- to 80-degree range it will flourish. We will be getting into that range in the next few weeks.

There is no cure for powdery mildew once it appears on a plant, so timely prevention is important. The first step in prevention is to buy mildew resistant varieties of plants to start with. Finding these will take a bit of research but more new varieties are appearing on the market every year.

Resist fertilizing with high nitrogen fertilizer late in the summer. The tender new growth stimulated by the fertilizer is much more susceptible to the disease. Avoid or reduce overhead watering as the temperatures cool. Evaporation slows with cooler temperatures which results in higher humidity levels that can stimulate the development of the mildew. If you can, water in the morning so the plants have the heat of the day to dry off. Thin out crowded plantings to create better air flow between plants which will allow them to dry more quickly. Don’t compost infected material as the composting process won’t kill the spores. Throw infected plants in the trash.

Applications of fungicides prior to the appearance of powdery mildew will also help control it. Look for commercial sprays containing sulfur, neem oil, (Rose Defense, Shield-All, Triact), potassium bicarbonate (Kaligreen, First Step) and triforine (Ortho Funginex). The triforine can be used only on ornamental plants. Read and follow the label directions.

You can make a homemade spray like the commercial potassium bicarbonate brands mentioned above by combining baking soda with a lightweight horticultural or dormant oil. Mix one tablespoon baking soda and 2.5 tablespoons of horticultural oil into one gallon of water and apply every couple of weeks to susceptible plants through the growing season. Some recipes like this call for using vegetable oil but horticultural oils are better because they contain a fixative agent that helps the spray stick to the plant.

A side note on the impact of our recent smoky days: While the thick smoke reduces sunlight, it doesn’t have a lasting effect on our plants. They and us will be glad when the rains come.

Pat Munts has gardened in the Spokane Valley for over 40 years. She is co-author of “Northwest Gardener’s Handbook” with Susan Mulvihill. She can be reached at pat@

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