Most rose growers have grappled with powdery mildew, a fungus that coats leaves, stems, buds and fruit with a dingy white growth that disfigures and debilitates them. But this pervasive disease affects plants ranging from sycamores and crape myrtles to grapes and squash, dahlias and zinnias. The fungus thrives in the moderate temperatures and humid air prevalent during June gloom. But oddly enough, it needs a dry plant surface to become established. When the powdery white fungal growth first appears, spray affected leaves with a strong blast of water that destroys and dislodges spores. This is best done early in the day so plant surfaces dry before night. Frequent rinses will destroy mildew spores. Horticultural oils and fungicides appropriate for the affected plant also can be used following package directions. If mildew is an on-going problem, replace afflicted plants with resistant varieties and plant them where they will have plenty of air circulation and sunshine. For downloadable information on powdery mildew and other plant diseases, visit www.ipm.ucdavis.edu.