Gardening Tips- Powdery Mildew

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Over the past ten days I’ve had a number of calls about a white, fluffy substance growing on the leaves of various vegetable and ornamental plants. This is due to a common fungal disease known as powdery mildew, which affects a wide variety of plants. It is most commonly found on squash, crape myrtle, dogwood, rose, and phlox.

Like most of the fungal diseases we encounter in the garden and landscape, powdery mildew development is largely tied to weather conditions. Powdery mildew thrives in cool, moist, and shady conditions. As such, we most often see it in our gardens and landscapes during cool periods in the spring and fall. However, we’ve had several periods already this summer where the temperatures have been stuck in the mid 80’s for several days, with rain and cloud cover. This has caused a number of people to see powdery mildew developing on their plants right in the middle of summer.

What’s the best way to treat powdery mildew? It depends a lot on the plant it is found on. In the case of most trees, powdery mildew usually does not necessitate treatment. Exceptions may be made for severe cases or infected trees in high profile locations. For vegetables, roses, or perennials, treatment might be more prudent. Treatment may include use of fungicides, pruning to remove damaged leaves, or both. For some plant species, resistant varieties are also available.

The white material seen on the leaves is the asexual growth of the fungus, and the spores on this growth can be spread by wind and water. Different strains of the fungus affect each plant species, so an infected crape myrtle wouldn’t necessarily infect a nearby squash plant. It can easily pass from one crape myrtle to another or from infected leaves to healthy ones on the same plant, however. Because the disease moves this way, if infected leaves are removed early enough, spread of the disease can be slowed or halted. In some cases, the fungus will begin to reproduce sexually in the fall, leading to brown spots on infected leaves. These leaves should absolutely be removed, as failure to remove them will provide an avenue for the disease to return the following year.

Humidity is likely to be an issue throughout the summer, but if we have a run of hot weather the temperature should slow down the spread of powdery mildew in most cases. You can lessen the humidity immediately around affected plants by removing excess leaves or stems to improve air flow, and water with drip or traditional hoses rather than sprinklers. Removing any limbs from nearby trees that may be shading damaged plants will also help create a less favorable environment for the disease.

If fungicides are necessary, a number of different products are available. Look for products containing the active ingredients chlorothalonil, myclobutanil, or propiconazole. Sulfur based fungicides also work well. Make sure the product you choose is labeled for the type of plant you intend to use it on and follow all label directions when spraying. Many plants will react negatively if sprayed with fungicide if the air temperature is over 90 degrees, particularly when sulfur fungicides are used, so spray early in the morning on days when it is not expected to be terribly hot.

Matthew Stevens is the horticulture extension agent for Halifax County Cooperative Extension. If you have any questions about this article or other aspects of your home gardening, please contact Matthew at 583-5161 or matt_stevens@ncsu.edu.