What is slowly killing my shade azaleas?

Question

I live in So. CA, near the coast, but not affected by fog or cold. Our area is inland and sheltered by a very large hill area. Our weather is similar to that of mid-Los Angeles. My front yard is shaded, getting only filtered morning sun and is planted with azaleas, camellias, and ferns. After no problems for over 30+ years, my gorgeous shade azaleas (both older and newer plants) are suffering and dying out.
First there is a browning of just the leaf tips, then a withering of the leaves. This happens on only one branch of the shrub at a time, and gradually spreads to the next branch until the entire bush is affected. One large bush (many years old) still has leaves but the plant looks severely withered, in spite of a moist soil. This condition spread across my property, one shrub at a time, over a 2-3 year period. It stopped completely when it reached my oldest plants, and they had their most magnificent bloom this Spring.
I can’t determine the cause and, after taking samples to various nurseries and the local botanical garden, I received a variety of diagnoses (Too much water, not enough / Needs iron, too much iron / thrips, or some other pest /etc.). I cannot find insects on the plants and, although the soil here is clay-like, my garden is highly amended with good quality planter mix.
We’ve had 4 years of drought, but my plants were irrigated. Our drought was recently cancelled by late winter deluges. Can you help?

Answer 1

I will have to agree that all of the information you received from the nursery certainly are possible causes of browning leaves and plant die back.

Your plants could also have a fungal disease or be infected by rhododendron borer or rhododendron stem borer.
Continue the process of elimination.
These links have more information.
https://www.gardeningknowhow.com/ornamental/shrubs/azalea/azalea-branches-dying.htm

https://www.gardeningknowhow.com/ornamental/shrubs/azalea/common-azalea-problems.htm
https://www.gardeningknowhow.com/ornamental/shrubs/azalea/phytophthora-root-rot-azaleas.htm
https://www.gardeningknowhow.com/ornamental/shrubs/azalea/azaleas-noteworthy-shrubs-for-any-garden.htm