Glossy, evergreen foliage and large, showy flowers coupled with relatively easy maintenance make rhododendrons a popular plant throughout the coastal areas of the United States. They are prone to some diseases, however, that cause unsightly brown spots on foliage or may even kill the plant. Chemical controls are sometimes necessary, but keeping the plant healthy through good cultural practices is usually the best solution. Keep the leaves dry, avoid overfertilizing and provide good air circulation.
Rhododendron Leaf Spot
The leaves of rhododendron infected with the fungus Cercospora handelii may have round or irregular tan to brown spots with yellow margins. The disease infects young plants in early spring, although symptoms may not appear until later in the summer. Leaf spot is rarely severe enough to warrant treatment.
Phytophthora dieback is usually seen in nursery culture rather than home landscapes, according to the North Carolina State University Plant Pathology Extension. The disease causes chocolate brown spots on the leaves, which may drop several days later. Young rhododendrons are most vulnerable, although the disease can kill mature plants as well.
Rusty brown spots containing fungal spores form on the lower sides of rhododendron leaves. These brownish spots are unattractive, but generally harmless. Choose resistant varieties in areas where rust is a problem and ensure good air circulation to keep conditions less conducive to fungal growth.