Don’t panic when you find a powdery white substance covering the leaves of some of your flowers, vegetables, and shrubs. This is a good clue that your plants are infected with the fungal disease, powdery mildew. Most otherwise healthy plants can tolerate the damage and perennials will return the following year.
You may see this disease on a variety of plants including zinnia, phlox, bee balm, lilacs, roses, cucumbers, and squash. Powdery mildew is caused by several different fungi with each attacking specific host plants. This means the powdery mildew on one plant won’t necessarily infect its neighbors. But when the weather conditions are right, high humidity at night and low humidity during the day, powdery mildew can develop on a variety of unrelated susceptible plants.