Preventing Powdery Mildew on Phlox

Q. What can I do to prevent powdery mildew from attacking my phlox and zinnias?

A. Powdery mildew is a late-season fungus that favors hot humid days, cooler nights and mornings heavy with dew. Interestingly enough, rain does not promote this mildew as it does other fungal disorders. Symptoms include a white-to-gray powdery appearance on leaves, new shoots, stems, buds and flowers. In some cases, the leaves turn yellow or brown, curl up and drop off the plant. Flower buds might be distorted or stunted. This fungus also attacks vegetables, fruits and woody plants as well as other herbaceous plants. The damage is often more aesthetic than life-threatening. Some of the most susceptible plants to this disease include lilac, sycamore, dogwood, crab apple, rose, zinnia, phlox and beebalm.

When choosing your plants, select varieties that are resistant to powdery mildew. Allow enough space between individual plants since poor air circulation and overly dense plantings encourage mildew. Water your plants early in the morning, and try to put the water directly into the root zones, not on the foliage. Avoid handling plants when they are wet since the fungus can be spread on your clothing, gloves or tools. Make sure plants that require full sun are indeed receiving full sun. If mildew is detected early in the season, approved fungicides can be used to discourage its spread.

Download more information on phlox in Plant Evaluation Notes #13.