Powdery mildew is at the top of my list of ugly garden diseases. It can turn a vibrantly hued garden into a dusty white mess in a matter of days. I have found that the best strategy for dealing with this fungal disease that attacks leaves is to squelch it as much as possible, but forget worrying about the small patches that continue to thrive despite eradication efforts. Full sun and good air circulation are key to avoiding powdery mildew. Continue to thin plants, beginning in late spring. Whenever possible, avoid getting the plants' foliage wet-especially in the afternoon, evening, and nighttime.
When selecting plants that are especially susceptible to powdery mildew, such as phlox, zinnias, and roses, look for varieties that are resistant to the disease. For example, David garden phlox is reputed to have excellent resistance to powdery mildew. You can control a thriving case of powdery mildew with a garden fungicide formulated for the disease. You will find several excellent fungicides at your local garden center. Begin treatment as soon as you see yellow spots form-just before the powdery white spots appear on the leaves.