Rhododendrons are popular landscaping shrubs because they have glossy, evergreen leaves (in mild climates) and showy flowers. Few pests and diseases bother them, although they are particular about soil and growing conditions. Gardeners should consult local nursery experts to make sure rhododendrons grow well in their area.
Curled, browned leaves are the result of sun scorch or wind damage. The appearance of brown, wilted leaves and stems is most likely "stem die back." Stem die back affects only one portion of the plant, while remaining leaves and stems are healthy.
Sun scorch occurs in the winter and is caused by harsh winter sunlight and cold temperatures, combined with inadequate watering. Heavy winds also cause brown leaves. Stem die back is caused by two fungi, Phytophthora and Botryosphaeria, according to the Ohio State University Extension.
Sun scorch can be prevented by providing late season irrigation before heavy frosts occur. Additionally, planting rhododendrons near buildings or evergreen trees provides shelter from wind, harsh sun and cold temperatures. Rhododendrons are more vulnerable to stem die back when they've experienced water stress. While rhododendrons don't tolerate waterlogged soils, they need evenly moist soil. Allowing them to dry out makes them more prone to disease. Curled leaves are the first sign that rhododendrons aren't getting enough water.
Infected stems and leaves of rhododendron are cut back and discarded, disinfecting the pruning tools between cuts. Additionally, plants with stem die back can be treated with a fungicide.
Rhododendrons prefer mild, moist climates. They must have acidic soil with a pH range of 5.0 to 5.5. These shrubs need plenty of water and some protection from extreme weather.