Powdery mildew is a windblown fungus that begins in small patches and forms a white powdery coating over leaves, stems, flowers, or fruit. The fungus is most common during spells of high humidity but little rain. Powdery mildew most commonly attacks phlox, zinnias, various houseplants, lilacs, alpine currants, flowering crabapples, and roses. Each species of powdery mildew affects a limited number of plants. The one that attacks roses, for example, doesn’t spread to lilacs.
Powdery mildew usually doesn’t cause plant death, but it can distort leaves, shoots, and flowers and slow the plant’s growth. It’s also unsightly.
• Plant disease-resistant cultivars.
• Make sure plants get at least six hours of sun daily.
• Put susceptible plants (like phlox and zinnias) in areas with good air circulation. Don’t crowd them. Make sure there’s plenty of room between plants.
• Destroy diseased leaves in the fall.
—photo courtesy of the Kemper Center for Home Gardening at the Missouri Botanical Garden.