Lavender colored phlox grows in mostly clay soil develops mildew each year. It is in a sunny area not over watered. How can I prevent this?
Hello! Thank you for your inquiry to the Toronto Master Gardeners. Without a photograph, I cannot verify my hunch that the problem you are seeing is powdery mildew, a very common fungal disease seen in phlox. Powdery mildew is an air-borne fungi, and is one of a large group of the most common fungi that cause harm to garden plants. Powdery mildew appears as opaque and powdery grayish-white spots. According to the book, What’s Wrong With My Plant (And How Do I Fix It?)by plant pathologist David Deardorff, PhD, and naturalist Kathryn Wadsworth, the best approach is prevention by removing infected plant material (and putting it in the garbage, not the compost), mulching to prevent water from splashing around and spreading the fungal spores, providing air movement around the individual phlox plants (so thinning them out to prevent overly dense clumps), and choosing resistant cultivars. When you water your garden make sure to water only the roots of the plant, keeping the foliage dry. The authors recommend a number of organic and inorganic remedies for air-borne fungal infections. These include the use of neem and other remedies, but when it comes to fungicides, caution must be exercised.