Drought can cause entire branches or entire plants to die. We have had several years of drought here and we observe that if rhododendrons and azaleas are not watered during a drought some plants will die, but others will just have one section of the plant die. It seems to be the plants way to conserve what little moisture it has. Prolonged drought weakens plants and often results in the appearance of fungal cankers on the branches of older azaleas. Look for branches that wilt in hot, dry weather in late summer and be sure to water azaleas if drought drags on more than a few weeks. Prune out the affected branches to stop the spread of fungal canker diseases.

Another possibility is borers.

If a Rhododendron Stem Borer, Oberea myops, or Rhododendron Borer, Synanthedon rhododendri, is in a branch, the entire portions of a plant beyond (away from the roots) will wilt and die. Borers only affect the portion of the plant away from the roots from the borer. If the borer is in the main trunk, then the entire plant will wilt and die. The plant can be save by cutting off the area with the borer and letting the plant regenerate from the roots. There are no conventional insecticides that will kill stem borer larvae once they are inside the branches. The best control option for homeowners with only a few plants is to prune out and destroy wilting branches in early spring or late summer.

I sent a diseased plant to Texas A and M University for diagnosis and was told it was a soil borne fungus, Armallaria, which attacks older plants due to weakened immune system due to age.

Armallaria is a naturally occuring fungus and occurs most often in oak forested areas.

As far as I know,there is nothing to kill armallaria. In addition, when cutting out the dead sections, you have to sterilze with bleach your cutters each time a cut is made.

This year, I will replace my infested azaleas with newer, younger ones and see if they are able to survive. I have researched the internet and found very few plants which are immune to armallaria.

Good luck

Azalea Dieback is caused by the fungus Phomopsis rhododendri. Typically, dying branches (stem dieback) begin to appear on an otherwise healthy plant. The leaves die and can remain attached to the plant until late summer. Usually a single branch on an established plant is affected. Scraping away the bark with a knife reveals discolored wood under the bark that appears chocolate brown. Dieback is difficult to control on azaleas in the landscape. The azalea varieties that are the least susceptible include: 'Delaware Valley White,' 'Hershey Red,' 'Pink Gumpo' and 'Snow.' Reduce stress to the plants by planting in partial shade and watering during dry periods. Avoid wounding the plant. Prune infected branches well below all discolored wood and dispose of dead plant material. Clean pruning tools between cuts with a dilute solution of household bleach (1 part bleach to 9 parts water).