Managing Powdery Mildew

As sure as there will be the usual enjoyable elements of an summer, such as cook outs, vacations, swimming, and sunshine, there are parts of summer for the home gardener that are not enjoyable. Just about every year, there are pests and diseases we have to keep in check.

One inevitable disease, regardless of what is in your garden, is powdery mildew. While the kind of powdery mildew is host specific, meaning you may have it on your tomatoes and your roses, but they are different kinds of the disease. There are two unique aspects of powdery mildew that make it difficult to keep away. One, while other diseases fester on injured or stressed plants, powdery mildew grows on healthy plants. And, powdery mildew does not need free water to grow; other fungi need water already on the plant to grow and spread. Fairly high humidity and moderately warm temperatures are all powdery mildew needs to grow. This makes the disease prevalent in your yard.

Visual inspection is the easiest way to determine if powdery mildew exists. The leaves are covered in a talc-like substance, making it look like powder was shaken all over them. New growth tends to be smothered in it, resulting in a stunted, curled look. Older leaves affected will have the typical white spores and will tend to turn yellow, a problem called chlorosis. In ideal conditions, the disease can be in full force in three to five days.

There are cultural controls a homeowner can use to help manage the disease. What this means is you can take some steps without using chemicals first. The disease is hampered when there is adequate light and ventilation. If your plants are crowded, it’s best to thin them out. Keeping the air moving in and around your plants will inhibit the fungi. Woody perennials can be pruned to maintain light and ventilation. There are many kinds of ornamentals which are susceptible, the disease is not limited to annuals and vegetables, so thin out peonies, viburnum, euonymus, and others. While some other landscape plants may not be affected by powdery mildew, if they are overgrown and shade out the other landscape elements, they can contribute to the disease.

There are options if chemical control is needed, but they should be applied as soon as the symptoms first appear. It is very important to read the label first, as some treatments when applied to the wrong plant, may severely damage it. Using sulfur on viburnum or applied on hot days can kill a plant. Most often, fungicides have to be applied regularly to be effective. Please purchase a fungicide suited for powdery mildew, and it should state it clearly on the label. Chlorothalonil is a popular choice as is a copper based fungicide. Caution must be exercised as all chemicals have some risk to the applicator, the plant, and beneficial insects.

For more information on powdery mildew on ornamentals, the Penn State Master Gardeners in Lackawanna County have a factsheet to help you manage this disease. For a copy, please contact us at 570-963-6842 or email: LackawannaMG@psu.edu

Steve Ward
Master Gardener Coordinator
Penn State Extension
Lackawanna County