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It sounds like the nutrients in the soil have finally become depleted. Get some fertilizer for Azaleas and follow the directions. There is always the possibility that the soil Ph has been radically altered- dog pee- cars wash chemicals- masonry work cleanup-a soil test kit can rule this out. The fact that the Azaleas have been fine all along points to these possibilities.
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OK- here's a logic story- to show how your logic is corrupt:
A man falls asleep with his shoes on and wakes up with a terrible headache. In fact EVERY time he falls asleep with his shoes on he wakes up with a terrible headache. Naturally he blames the shoes. There must be something about them that is not meant for sleeping in them.
You put old horse manure on your plants. You have done this for several years and never had a problem. Now you have a problem. It can't be using the manure because it always worked before.
You ruled out the manure, the man ruled in the shoes. Both of you used failed logic of association.
The man was drunk. Everytime he got drunk he fell asleep with his shoes on.
Your soil has been steadily depleted of a nutrient Azaelas need but manure does not contain.
The man ruled out drinking as a cause
You ruled out deficient nutrition as a cause
The mans problem was caused by drinking
Your problem is probably caused by a missing nutrient.
General C&ontractor
35 years of troubleshooting construction, remodel, component & material failures. What to do next