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Now that we have received some welcome rain, all we need is some warm temperatures and high humidity to see some powdery mildew on shrubs. The fungi thrives in shade and causes the most severe infection on young, succulent shoots. In addition, most powdery mildew fungi can only cause disease in one genera or one family of plants. For example, the fungi on lilacs will not spread to the roses. A different species of powdery mildew fungi must be present to infect the rose bush.

Powdery mildew is easy to recognize because it looks just like its name. Powdery white fungal mycelia and spores can be found on the surface of infected leaves, stems, flowers and fruit. If the disease starts on mature leaves, the fungus is often only a few spots or a light coating of white on an otherwise healthy looking leaf. These infections are typically considered minor infections and have little effect on the overall health of the shrub. If the disease comes in on young developing leaves, shoots and flowers, leaves may be severely affected. Sometimes the young leaves are crinkled or cupped and often turn yellow. Young flowers and fruit may be completely coated with the white fungus and often fall off prematurely. Young shoots that are severely infected may even be killed.

One way that the powdery mildew fungi survive Minnesota's harsh winters is by colonizing young tissue within the plant buds. In these cases the fungi starts up new infections as soon as the buds open in the spring. A shrub that has a few very severely infected young shoots next to other, completely healthy shoots, most likely had powdery mildew fungi surviving in its buds all winter long.

Many powdery mildew infections do not need to be controlled. An example is the lilacs that may have mildew year after year and yet produce beautiful blooms every spring. In cases where the leaves and shoots are distorted, yellowed, and stunted, action should be taken. Good air flow around the plant will reduce the humidity. Mulch around the shrub with woodchips or other organic material to keep moisture in the soil. Controlling the weeds also improves the air circulation.

If none of these practices are enough to control the disease, a protective fungicide can be used. Sulfur is a low impact fungicide that is effective, but since it can burn some plants the sulfur spray should be tested on some lower leaves before spraying the whole plant. Sulfur should be applied during cool evening or morning hours. For plants that are sensitive to sulfur or plants with severe infection on young growing shoots, the systemic fungicide thiophanate menthyl can be used. Be sure to read the entire label before using a fungicide.

Parts of this article were information gathered by Michelle Grabowski, Extension Educator.

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