The main form of mildew that forms on azaleas is powdery mildew. Powdery mildew most often causes the leaves and other parts of the azalea to become covered in a white growth that may have small black specks, but it can also appear as a dark-colored mildew. It generally forms on the upper side of the leaves, but can be anywhere on the above-ground parts of the plant. The formation of powdery mildew on azaleas has several causes and, once formed, is difficult to control.
Humidity and moisture increase the development of powdery mildew, and these conditions do not necessarily have to be weather-related. Too much humidity and moisture can be created when azaleas are planted closely together or when they are overwatered.
Lack of Sunlight
Powdery mildew is more prevalent on plants that are in the shade and on the bottom side of the azalea plant. Once the mildew forms on the leaves, it shades the leaves more and inhibits the photosynthesis process, keeping the plant from thriving.
Overwatering and over-fertilization of azaleas provide ideal conditions for mildew to grow. Mildew is more likely to form on the new growth of the azalea plant. This makes young azaleas the most susceptible to powdery mildew growth.