Azalea bushes are hardy shrubs that bloom with bright flowers in a variety of colors. Because azaleas are so low-maintenance, their health issues are often overlooked until the problem has become fairly significant. In some cases, it may take some time before you notice fungal infections, which tend to look slightly different on azalea leaves than they do on other plants. As a result, some common infections may actually appear to be a blue fungus or have a purplish hue, which can misguide diagnosis.
Powdery mildew is usually described as being black, gray, white or pink. However, on azaleas' dark green leaves, it can appear blueish white and may be speckled. It will retain its characteristic "chalk-dust" appearance, however, which may help you identify it. Remove all affected foliage using sterile pruning and rake away any plant debris from beneath the plant. Dispose of all the plant material in a sealed bag or by burning to prevent reinfection, and water azaleas if necessary with a drip hose to avoid getting moisture on the leaves.
Cercospora Leaf Spot
Cercospora leaf spot can develop on azalea leaves and create purplish or dark blue lesions that develop yellow rings if left untreated. This can lead to cosmetically unattractive bushes and early leaf fall, as well as fewer flowers and less growth. Remove all spotted leaves as soon as you see them, making sure not to drop any of the diseased material on the ground.
Gray blight, not surprisingly, gives the leaves of the azalea a gray-blue cast, according to the University of Massachusetts Extension. However, before everything gets a blue hue, the blight starts out with brownish lesions on the leaves. As soon as you spot the infection, remove all of the impacted foliage and dispose of it completely. Do not drop it or compost it. In most cases, this will be sufficient to kill this blue or gray fungal infection, but if not, you can treat the bush with a fungicide.