Rhododendrons: azaleas dying mysteriously

I am slowly losing some long-established, consistently healthy azalea bushes, and I am baffled as to the cause / what to do.
Suddenly, major branch that had been healthy days before, will turn brown and die. I cut out the dead section, and in a few weeks, another branch will suffer the same fate. This started in August and is slowly destroying a beautiful, very large bush. There is now noticeable damage to the stem (as you might see with freeze damage) and there is no observable fungus, insect problem, etc.
Any ideas?
Thanks.
Kingwood

 

Don't know about "suddenly" but otherwise sounds like root or stem rot due to water molds (phytophythora and friends), a typical problem for rhododendrons and azaleas. Take samples of affected portions to local state Extension office.