Zinnias are prone to two leaf diseases in the Northeast: powdery mildew and leaf spot fungus. They are difficult to impossible to prevent, but here are some ideas for you.
1. Do you grow your zinnias mainly for cutting flowers? If so, just ignore the mildew; it doesn’t kill the plant and we strip the leaves off when we use zinnias in bouquets anyway.
2. If you want zinnias for color in the landscape, try the Profusion series. These come in assorted colors, are lower growing and deadheading is optional. They don’t have long stems for cutting, but are perfect in annual plantings or mixed in with perennials.
3. Be sure to water your zinnias deeply less often. These are plants that came from Mexico and Central America, so they are used to hot and dry. The worst way to water is frequently, by hand, as this doesn’t soak down deeply enough to create strong root systems, and tends to dampen the leaves too often. Once a week with a sprinkler going at least an hour and a half is perfect in Northeastern gardens.
4. If you want to discourage the mildew start spraying with the organic fungicide of your choice EARLY in the season before you see any signs of mildew. Pick a product at your garden center that is labeled for powdery mildew and use it frequently according to directions. This will only suppress the mildew, however, not get rid of it completely. (Which is why many gardeners choose to ignore this problem…like parents of two year olds, we learn to pick our battles since we can’t win them all!)
Many of us agree that zinnias are must-grow flowers for the summer garden, so we hope this has been helpful.