Ornamentals Powdery Mildew Patches of white or grayish powder on garden plants, lawns, shade trees, or shrubs are probably powdery mildew.
Powdery mildew is a fungal disease found throughout North America.
It can cause severe problems in irrigated areas. Powdery mildew can attack a wide variety of turf plants, garden vegetables, and landscape trees and shrubs. The degree of damage varies with the plant and environmental conditions. In some cases, yield is lost or plants might die. In other species of plant, yield is not affected and damage is negligible.