I just wanted to add my 2 cents. I have many established rhodies in the garden but have acid soil. What I have seen is that rhodies naturally drop some leaves in the late summer / fall. Not sure if you are fertilizing but use only fertilizer for acid loving plants. Garden centers have a special one that says for rhododendrons and azaleas. I usually fertilize only in early spring, that way they have a chance to grow then slow down as we approach winter. Ok I'll go away.
All my Rhodies have some yellow leaves.
