Answer: If the Glenn Dale hybrids were not evergreen, I'd say the leaf drop was normal for this time of year. However, these are evergreen azaleas. Evergreens do eventually drop their oldest leaves (those near the center of the plant, on the inner parts of the branches). If your plants have healthy leaves on the ends of the branches but the oldest leaves are falling, this is normal and nothing to worry about. New foliage will grow next spring.
If, however, the dropping leaves include new growth as well as old, there's a root problem. Poorly draining soils and/overwatering can cause root rot. Very alkaline soils can also cause problems with azaleas. They prefer acidic soils, which you can achieve by feeding with an acidified fertilizer and mulching with peat moss.
I'd dig one of the plants and inspect the root system; healthy roots will be creamy white inside - dead roots will be brown or rusty colored. If the root mass is mushy or there's an odor associated with it, you're dealing with root rot. If everything appears healthy, replant the azalea and wait for it to recover from whatever stress caused the dropping leaves. Most landscape plants require one inch of water per week, applied deeply so the roots grow deep.
Hope the above information helps you determine just what is causing the leaf drop!
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