Answer: I'm not sure whether you are describing the symptoms of petal blight or of powdery mildew. Both are caused by fungi. Ovulinia petal blight (caused by the fungus Ovulinia azaleae) can cause considerable damage to the flowers, especially on azaleas. The occurrence and severity of the disease is highly dependent on wet, cool weather conditions during the flowering period. The symptoms are tiny, pale or whitish spots on colored petals and rust-colored spots on white petals. The spots enlarge rapidly and infected tissue becomes soft and watery, such that the whole blossom collapses. The petal blight fungus only attacks flowers. Petal blight occurs more frequently on mid- to late-season cultivars than early ones. The disease also occurs on rhododendrons.
If petal blight has been a problem in your azalea planting in previous years, apply a fungicide just as flower buds begin to show color. A second application may be made if favorable weather conditions persist during the blooming period.
The damage from powdery mildew (Oidium sp.) is generally slight, but the disease can cause damage on highly susceptible cultivars such as Hinodegiri as well as some deciduous azalea cultivars.
The disease appears as a diffuse white powdery growth on the leaves. It is more frequently seen in the fall months particularly after a dry summer. Diseased leaves may drop prematurely.
If the infection becomes serious consider spraying the plants two or three times with an appropriate fungicide.
If, after reading the descriptions of the diseases, you are uncertain which is attacking your azalea, you might want to take a sample of the problem to your Cooperative Extension Office or Master Gardener Clinic. Trained volunteers can help determine exactly what is causing the white spots on your azalea.
Best wishes with your garden!
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