My Rhododendron has yellow leaves. 

What's wrong with the plant?

The yellow leaves on your rhododendron reflect normal leaf drop.  While we think of rhododendron as evergreen, they gradually cast off leaves on older wood as the plant grows and matures.  Note, this year's growth on the branch tips is green.  The leaves color up before dropping because the process involves an alteration in chemical interchange between leaf and stem.  This process is called abscission, which accounts for Autumn foliar color in all plant material.  In any event, leaf drop can happen in the Spring or the Autumn, both seasons, or not at all.  In other words, the leaf drop is not a finite event, as it is with deciduous plants.  Rhododendrons vary in the matter of leaf drop, some hybrids hold their leaves for two seasons or even three, while others drop annually.  The tendency to hold foliage is a highly desirable characteristic because plants with this tendency look full, and hybridizers select "parents" that behave this way and hope the tendency is reflected in their children.