Sounds like chlorosis, a disease not caused by any bug or fungus. This is an iron deficiency in our plant, the plant version of human anemia. The most common cause of iron chlorosis in azaleas is a soil that's too alkaline. Even if you've been using iron-rich fertilizer, the iron is unavailable to the plant if the soil pH is too high. This problem shows up most commonly with relatively new plantings, by the way. Big established azaleas that have survived the first few years can actually influence soil conditions favorably for themselves.
Overly wet soil can also be a cause of chlorosis in azaleas, which have shallow root systems that rot easily in heavy clay soils with inadequate drainage. Middle Tennessee has a wide variety of soils, but clay soils that are relatively alkaline are quite common, thanks to the limestone bedrock not far below us.
WHAT TO DO:
Azalea chlorosis is cured by lowering the pH of the soil using sulfur and improving drainage by adding organic material to open up tight clay soil.
Get your soil pH tested by your county Extension Agent first, so you can figure out how much sulfur to add to lower your pH to the appropriate level.
Don't use aluminum sulfate to lower soil pH for azaleas and rhododendrons as it's easy to put on toxic amounts.
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