Rhododendrons and azaleas are closely related and some diseases can attack both of them. If a rhododendron is attacked, check any nearby azaleas for the same symptoms and vice versa. Rhododendrons should be planted where they can get partial shade and in a rich, well drained soil. Do not heavily prune rhododendrons, just prune away any unsightly branches and any that become diseased. Proper care from the beginning can go a long way in making the plant less susceptible to diseases.
Rhododendron root rot is a fungal disease that cannot be controlled once it has attacked the plant. It is composed of thick-walled spores that live in the roots, low stems, fallen debris and the soil; it can last for many years. The first part of the plant to be affected are the fine roots, which turn brown. It then progresses to the rest of the root system and eventually up to the crown of the plant, the part just above the soil line. When it reaches the crown, it can encircle the plant and spread into the stems. The first noticeable symptoms are the leaves that turn yellow and roll up. The best cure is to prevent it from occurring in the first place. Make sure it is planted in well drained soil, preferably with a southern exposure. Do not plant another rhododendron or azalea in the same place where a diseased one had to be removed. There are fungicides available that are considered to be preventative.
Petal blight strikes in the spring and early part of the summer. It usually gets critical about May. The more the weather is muggy and rainy, the more serious it gets. It is prevalent in the Southern, Southeastern and Middle Atlantic States. The flowers will be small and have white- to rust-colored spots on the petals. The spots will grow fast and the buds and flowers will become soft and mushy. It will only take two to three days for the whole flower to rot away. The flowers will fall off, bringing the fungus to the soil where it can survive the winter and come back again the next spring. It is important to get to them and destroy them before they fall off. Pick up and dispose of any debris around the plant. There are fungicides that can be applied to the flower buds as soon as the color starts to appear that can help as a preventative measure. Look for one that specifically says it is for petal blight control.
Twig blight attacks the larger branches of established plants. It can also appear on new plants that have not had time to set their roots firmly in the soil. The first signs are when the twigs wilt and the leaves die off. The bark on the infected twigs will have a red/brown discoloration underneath, which can range from a few inches to several feet long. Prune away and get rid off all of the infected branches. Unfortunately, fungicides will not help prevent or cure twig blight.