Mildew

Two popular types are downy mildew and powdery mildew. Although unrelated, both leave a white coating on the leaves. It's affect on plants can be anywhere from mild to devastating. Common to crepe myrtle, zinnias, grapes, and roses - even bee balms. It shows up in cool, wet weather. Drip irrigation can prevent some occurrences. Plant disease resistant varieties (if available) where mildew is a problem.