Rhododendron's older leaves can turn yellow for a couple of reasons.

Older leaves can turn yellow and then are replaced with new leaves every one to three years. When the leaves are yellow but the region along the veins remains dark green, the plant likely suffers from chlorosis, which is caused by a magnesium, nitrogen or iron deficiency. This deficiency can be tested out by contacting your state Cooperative Extension Service.

For more on rhododendron care see our guide Azalea and Rhododendron Maintenance.

More: When Rhododendrons are mature they usually need renovation. Plants need proper growing conditions to do well in the landscape of course. Here is more help on the how to and specifically problems of Rhododendron here: http://www.rhododendron.org/disease.htm  and the complete list here: http://www.rhododendron.org/plantcare.htm

Courtesy of the NYBG Plant Information Service