Mike Hogan
| Special to The Columbus Dispatch
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Problem Solved
Powdery mildew is a fungal disease affecting many plants, causing a white or gray coating on leaves and slowing growth.The fungus thrives in warm, dry weather with high humidity, especially in crowded, shaded areas with poor air circulation.Controlling powdery mildew involves early detection, thinning plants, choosing resistant varieties and proper sanitation.
Powdery mildew is a fungal disease which attacks thousands of different species of plants. This common foliar disease seems to be flourishing in Greater Columbus this summer, as we have identified it on bee balm, peony, zinnia, basil, lilac and vine crops such as cucumber and pumpkin recently.
While the damage from powdery mildew is primarily cosmetic, the fungus can slow the growth of plants, and infected leaves can turn yellow and die. Sometimes, infected parts of the plant become stunted and distorted, and flowers may be fewer in number. When the organism is present on fruit trees such as apple, the yield and quality of the fruit can be reduced.
What is powdery mildew?
There are many different species of powdery mildew fungi that infect foliage of susceptible plants when weather conditions are just right for the organism. The growth of powdery mildew is greatest in warm, dry weather.
Unlike many other plant diseases, powdery mildew does not require excess moisture in the air or on foliage but does require high relative humidity around the plant to spread.
Powdery mildew is most severe in crowded plantings, especially in shaded areas with little air circulation. Presence of the fungus causes a gray or white coating on the foliage, which can look like plants have been dusted with baby powder or baking flour. Depending on when the fungus appears, the white coating can also be present on flowers, buds, stems and even fruit.
Controlling the fungus
The key to controlling powdery mildew is early detection and management of cultural practices such as sanitation, planting location for certain plant species and thinning. Like many plant diseases, fungicides will not cure or remove the disease organism. Once you detect the presence of the disease, it is too late to treat with a fungicide.
Fungicides are effective when applied to healthy green plant tissue early in the season before infections occur. However, routine protective applications of fungicides to plants in the home landscape early in the spring as an insurance policy against infections of powdery mildew is rarely justified or recommended.
Because powdery mildew flourishes in crowded plantings in locations with little air movement, thinning plantings affected by the disease is an effective strategy for reducing the susceptibility of certain plants to become infected.
Over time, a patch of bee balm in my home landscape grew much larger than when it was planted, and for several years, the plants would turn white with powdery mildew each summer. Once I thinned out and divided and moved some of these plants to other locations in my landscape, powdery mildew never appeared again on my bee balm.
Choosing the right locations in the home landscape for certain plants is also an excellent strategy for preventing outbreaks of powdery mildew. The patch of bee balm in my landscape was located in a corner bed next to my deck and the house.
With hydrangeas planted on both sides of the planting, this location received very little, if any, air circulation, providing the perfect location for powdery mildew to flourish.
Powdery mildew-resistant varieties of some flowering plants are available, so when planting species of plants most susceptible to the fungus — such as bee balm, peony, lilac and others — look for and select resistant varieties when available.
Sanitation is key after infection
Once you identify powdery mildew on a plant, sanitation is key to controlling the infection and preventing outbreaks of the disease in subsequent years. Powdery mildew survives the winter inside a hard protective structure called a chasmothecia. These small, round structures may be attached to affected foliage or on top of mulch or soil beneath the plant.
Be sure to remove affected foliage in the fall and consider removing the very top layer of mulch or soil beneath affected plants. Dispose of these materials in your household trash and do not add them to the compost pile.
Mike Hogan is Extension Educator, Agriculture and Natural Resources and associate professor with Ohio State University Extension.
hogan.1@osu.edu
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