The proper placement and care of rhododendrons reduces the chance of diseases. Plant them in dappled or light shade, such as that created by tall trees, and protected from the hot afternoon sun. Select well-drained soil high in organic matter with a pH between 4.5 and 6.0. Rhododendrons have shallow roots and need regular irrigation. Avoid overhead irrigation, which wets the foliage and promotes fungal diseases.
Cankers caused by the fungus Botryosphaeria dothidea create rough sunken areas of dead bark that expand along the length of infected branches. On diseased branches, the leaves wilt and die and the wood under the bark turns reddish brown. Botryosphaeria is difficult to control, although a fungicide for rhododendrons applied at the rate recommended by the manufacturer may help. Prune and destroy infected plant material to prevent the spread of the disease.
Crown and Root Rot
One of the most common diseases on rhododendrons is rot caused Phytophthora fungi, a soil-borne pathogen that thrives in wet areas. The first indication of crown or root rot is wilting, drooping or curling leaves. The lower branches will be discolored at the soil line in crown rot. The roots become soggy or slimy and blackened in root rot. No effective chemical preventive or control exists for crown and root rot. Remove and destroy infected plants to avoid the spread of the disease.
Numerous fungi cause leaf spots, which can affect the appearance of rhododendrons but are not serious. Circular, oval or irregular spots may be tan, brown, purplish-brown, reddish-purple or black. Some leaf spots have yellow or dark margins or halos. Although infected leaves may drop prematurely, leaf spots normally do not require treatment.
The fungus Ovulinia azaleae causes petal blight on the flowers of rhododendrons. Spread easily by insects, wind and water, it survives in the soil. The initial symptoms are tiny freckle-like brown spots on white flowers or white spots on colored flowers. As the spots spread, the petals become soft and watery, rot and stick to the foliage. Remove and destroy infected flowers and old mulch around the affected plant. Use a fungicide for petal blight on rhododendrons according to the manufacturer's directions.