It is always on new growth. New azalea leaves will be distorted, thickened, and turn a whitish color. Both of these symptoms, called galls, are caused by a fungus which attacks new, tender growth in the spring when temperatures are mild and leaves remain wet for prolonged periods of time. While the galls do no serious damage to the plants, they appear unsightly. Most times, by the time symptoms appear, it is too late to apply fungicides.
Affected leaves can be hand-picked and discarded.
Don't just throw them on the ground where the fungal spores would mature there, ready to infect new leaves when conducive conditions return next year. To prevent the fungus, the plants would need to be sprayed with a fungicide just prior to buds opening, and perhaps again 10 days later. Oak Leaf Blister. This is another leaf fungus that appears nearly every spring. As the name suggests, oaks are the host for this fungus.