Like most plants, the rhododendron is susceptible to both diseases and weather conditions that may begin to turn vibrant, green leaves a shade of brown that hints of the plants' distress.
Rot, Dieback and Rust
Root rot and stem dieback, two diseases caused by a fungus, affect the overall health of a rhododendron. In serious cases, proper nutrients are denied to the shrub, resulting in a browning, wilting and potential death of the rhododendron leaf.
Another disease, known as rust, may move onto leaves, bringing the formation of pustules with a tint of orange and red that may appear somewhat brown to the eye.
A virus with no known cause, called necrotic ring spot sometimes attacks rhododendron leaves.
"The symptoms are reddish-brown rings or spots on the leaves. It generally occurs only on the two year leaves of a few rhododendron cultivars such as R. 'Unique', or on Kalmia latifolia," according to Harold E. Greer, the author of an article published by the Fraser South Rhododendron Society of Canada.
Leaf Spotting and Weather
The fungus Cercospora handelii plays a role in the condition referred to as leaf spotting. "Small, lopsided to round tan-brown spots with yellow margins appear throughout the year on leaves infected with Cercospora handelii," reports the University of Massachusetts Extension service. Winter weather conditions may also "burn" the leaves of the rhododendron, resulting in a drying and browning of the foliage.