Removing galled leaves by hand is recommended. On azaleas in heavy shade, prune the surrounding trees and shrubs to increase exposure to sunlight and air. Powdery Mildew Powdery mildew is a fungal disease found on both evergreen and deciduous hybrid azaleas. Disease development is favored by warm days and cool nights in the spring and fall. Patches of white, cottony fungal mycelia appear on the upper and lower surfaces of the youngest leaves.
Heavy defoliation can be a problem on severely infected plants, especially in the fall.
Azalea cultivars resistant to powdery mildew are available. Susceptible cultivars may be sprayed with a fungicide in the spring and fall to prevent disease outbreaks. Leaf Spot Diseases Leaf spot diseases caused by several fungi are found primarily on the old leaves of evergreen and deciduous azaleas. Damage to plants in landscapes is rarely serious. Wet, humid weather encourages disease development.