What I thought was winter damage appears to be more than that. Leaves from top of plant down lost their color and dry up and fall off.
What to Do About Rhododendron Die Back
Certified GKH Gardening Expert
Without knowing exactly how the disease looks on the leaves, it would be hard for me to diagnose it. This article may be helpful though in helping you match what is causing the plant's problems: http://www.ppath.cas.psu.edu/EXTENSION/PLANT_DISEASE/pdf%20Flowers/Azalea_RhododendronDiseases.pdf