Rhododendrons produce showy blossoms set against glossy foliage, making them a prized ornamental addition to many backyards. They grow quickly and are relatively low-maintenance, but can sometimes be attacked by various bacteria and fungal diseases. Not only do these diseases create unsightly growths or ugly rotting on the plant, but they can also kill your shrub.
Leaf gall (Exobasidium vaccinii) is a fungus that appears most often in early spring. Symptoms include swollen, bulbous growths on the shrub's foliage and twigs. Over time, these growths turn white and burst open. The disease's effects are mostly aesthetic and don't create a serious health problem, according to North Carolina State University. Use pruning shears to cut off branches or leaves that have the swollen growths before the growths turn white. If you wait too long, the growths will send out new spores and cause the disease to spread.
Twig blight (Botryosphaeria dothidiae) is the most common problem for rhododendrons, according to North Carolina State University. In outdoor plants, this is usually caused by the Botryosphaeria dothidiae virus; nursery plants are usually hit with the Phomopsis sp. version. Early symptoms are a slight wilting of the foliage, followed by foliage death and the dying off of entire branches. There are no sprays or chemical treatments that consistently keep this problem at bay, according to Penn State University. Pruning off the wilted and dying shrub parts is your best bet.
Phytophthora Root Rot
Phytophthora root rot (Phytophthora cinnamomi) is a fungal disease that attacks not just rhododendrons but also azaleas and camellias. Symptoms include wilting, curling leaves and death at the plant's top. Roots will appear black and soft. Grow shrubs in well-drained soil and allow the roots to dry out between watering sessions. Applying a preventive fungicide such as metalaxyl can help stop new plants from getting the disease, but may not effectively kill all of the fungus on a plant that's already infected.
Fungal growths on the rhododendron's flowers is known as petal blight (Ovulinia azaleae). When afflicted, the rhododendron's flowers are dotted with white or brownish-red spots. If left untreated, the entire blossom rots and falls off. Spray the shrub with plant spray formulated with triadimefon, following the specific product's labeled guidelines since toxicity varies widely by product.