The rhododendron is a flowering shrub that is native to North America as well as many parts of Europe and Asia. There are both evergreen and deciduous varieties. Azaleas are a species of rhododendron. Rhododendrons can be infected with diseases that are associated with azaleas, and the other way around.
Rotting Roots and Crown
The "water mold" fungus, phytophthora, strikes rhododendron roots growing in wet, poorly drained soil. The leaves droop and curl inward. Infected roots are soggy and blackened; their exterior can be easily slipped off.
Rhododendrons planted below the soil line or in poorly drained areas can develop crown rot, a brown discoloration of the stem near the soil.
Once root or crown rot symptoms appear, apply fungicides containing metalaxyl or mefenoxam to suppress fungal rot. These fungicides will not kill the disease.
Plant cultivars that resist Phytophthora rot. Do not buy plants that are not normally green, that have discolored, dark roots or that look wilted in the morning.
Since the fungus can't be eradicated from infected soil, do not plant rhododendrons in places where root rot has struck. Plant them in raised beds or make sure they are not planted below the original level of the soil.
Abnormal plant growths are called galls. Exobasidium, a fungus, causes rhododendron buds to become distorted in the early spring. The leaves and sometimes the stems curl and become fleshy and thickened, turning pale green to white. In time, the galls become covered with a white, powdery substance; they eventually harden and turn brown.
Clemson University horticulturists recommend growers pick and destroy the galls on infected plants; if necessary apply copper salts of fatty acids, mancozeb, or fungicide sprays that contain the active ingredient triadimefon.
The fungus botryosphaeria dothidea can cause dieback in hybrid rhododendrons. Another fungus, phomopsis, can cause similar symptoms in azaleas. The branches on an otherwise healthy plant start dying. The leaves also die, but they can stay on the plant until late summer.
To prevent branch dieback, plant rhododendrons in partial shade and water them during dry weather. Remove infected branches by pruning them below discolored wood.
Apply fungicide sprays containing the active ingredients Bordeaux 4-4-100, chlorothallonil, copper hydroxide or copper salts of fatty acids. Use sprays containing mancozeb or thiophanate-methjyl on azelas.
Spots on Leaves
Fungi including botryis, cercospora, pestalotia, phyllostricta and septoria can cause spots of various colors on rhododendron leaves. These fungi often appear after drought, winter freezes, sunburn or windburn.
Water rhododendrons at their roots, not on their leaves. Remove fallen leaves. During periods of high humidity, spray with fungicides including the active ingredients chlorothalonil, copper hydroxide, copper salts of fatty acids or thiophanate-methyl.
Spots on Flowers
Hot humid weather in the southern U.S. can cause infections of ovulina azalea, a fungus that winters in the soil and is common among azaleas as well as some rhododendrons. Tiny, irregularly-shaped spots or "freckles" appear on flowers petals. Clemson University horticulturalists recommends treating with a product containing the active ingredient of captan, chlorothalonil, maneb, PCNB, triforine, vinclozolin or triadimefon.