Answer: Once a plant has developed the fungal disease called powdery mildew, there's nothing you can spray to stop the progress of the disease. However, it is not usually fatal to a plant. After all the leaves have fallen from the plant, rake them up and remove them from the garden. Powdery mildew most often develops when nights are cool, days are warm and air circulation around the plant is poor, so it doesn't always develop on the same plants year after year. However, if your azalea develops the disease again next year, you may want to spray with a fungicide the following spring to protect the newly emerging leaves.
Best wishes with your azalea!
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