Keep zinnias free of powdery mildew

QUESTION: I love zinnias and plant them every year. Sometimes they do well and look great all summer, but many years the leaves are covered in powdery mildew. How do you get rid of this problem?

Powdery mildew is a fungus that appears as gray or white splotches on leaves, stems and flowers of zinnias and other ornamentals and some vegetable plants. It travels by airborne spores, and thrives when nights are moderately cool and foliage stays damp. A mild covering of powdery mildew is merely unattractive, but a severe case can cause distorted shoots and leaves, misshapen flowers, or can prevent flowering altogether.

The best defense is to grow zinnias in full sun, since hot temperatures inhibit the growth of mildew. Don’t allow them to get too crowded; you want air to circulate freely around the plants. Water only in the morning so foliage has time to dry before nightfall.

I always try to suggest the good-cultural-practices methods first for problem-solving, but there are also fungicides available that can deter powdery mildew. If you choose that route, apply fungicide as soon as you begin to spot the mildew. The University of Tennessee Extension has a short list of products in a publication about powdery mildew here. Be sure to read and follow directions on the product label.

Zinnias are not the only garden favorites plagued by powdery mildew. Lilacs, roses, crepe myrtles and other woody ornamentals, and many herbaceous ornamentals and bedding plants are also targets when weather conditions favor the fungus.