Answer: Like all broadleaf evergreens, rhododendrons shed their oldest leaves as part of their renewal cycle. If the yellowing leaves are the oldest (toward the center of the plant), rather than the newest leaves, what you're seeing is probably normal.
Rhododendron leaves sometimes develop variously-sized and -shaped spots. These are usually dark purple in color. The exact cause is unknown, but cultural or environmental stress may result in physiological leaf spot. This problem does not usually cause any permanent harm to the plant.
If the entire shrub exhibits yellowing leaves or black spots on most of the leaves, there might be a fungal infection. Leaf spots can be caused by powdery mildew or rust, or a number of other common pathogens. Powdery mildew on rhododendrons is a fungal infection of the leaves. On rhododendrons, the disease often fails to develop the distinctive powdery white growth normally associated with powdery mildews. Leaves may display yellowish-green or purplish-brown spots or blotches on the upper surface. Lower leaf surfaces may show brown or purple areas of various shapes and sizes. This disease may cause extensive leaf drop and even death of very susceptible plants. Powdery mildew is more severe on shaded plants and is favored by the high humidity found in crowded plantings and damp locations. Yellowish spots on the upper leaf surface and dark brown or purple spot on the lower leaf surface can indicate the presence of a rust infection. Later, the underside develops blisters which break open to reveal characteristic yellow to orange or reddish spores. Rusts can also affect azaleas. The disease overwinters on the host plant. Phytophthora blight is a fungal disease whose symptoms are often confused with chemical or mechanical injury or leaf scorch. Irregular dead spots on the tips or margins of leaves, shiny black cankers on shoots, and leaf loss and tip dieback on affected branches are among the common symptoms. The disease is spread by wind and splashing water.
I hope these descriptions of problems common to rhododendrons will help you determine the cause of the spots on the leaves.
Best wishes with your landscape!
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