Answer: It's natural for some of the older rhododendron leaves to turn yellow and fall off, to make way for new leaves. The older leaves are lower down on the stems, and on the inner part of the plant. As long as the plants look otherwise healthy, I wouldn't be overly concerned about a few yellow leaves.
If newer leaves are turning yellow, it might indicate iron chlorosis (especially if the main leaf veins are still dark green), or it could indicate a root rot problem. You can fertilize the rhodies to combat chlorosis, but root rots are difficult to overcome. If the leaves wilt and roll downward yet remain attached to the plant, I'd suspect root rot from poorly draining soils.
Best wishes with your rhodies!
Q&A Library Searching Tips