IDENTIFICATION — The white granular patches look like dust on the stems, the tops and bottoms of leaves and occasionally on flowers and fruit of phlox, lilac, bee balm, aster, rose, lungwort, zinnia (in the photo), crabapple and many others.
DAMAGE — Spores of powdery mildew move by air. Powdery mildew won’t kill a plant; it only affects the appearance. You’ll find it any time during the growing season wherever susceptible plants are grown.
PREVENTION — Plant resistant varieties. Keep plants healthy and well watered to avoid stress. Always provide good air circulation around susceptible plants. Apply potassium bicarbonate fungicides and horticultural oils before you find mildew or spray them to keep the problem from spreading to more leaves or other plants.