Rhododendrons are shrubs that belong to the Ericaceae family of plants along with azaleas, heathers, mountain laurels, blueberries and cranberries. Rhododendrons have evergreen leaves and clusters of large flowers that can be pink, white, or purple. Rhododendrons are susceptible to several fungal diseases, some of which can be very serious.
Azalea gall is a big problem in the United States, and attacks most varieties of both rhododendron and azalea. Cankers are dead spots on branches or trunks. Powdery mildew is a disease common not only in rhododendron and other outdoor plants, but in house plants as well. Sooty mold is another fungus that can attack many plant species.
Azalea gall appears as green, pink, white or brown crystalline masses on the leaves, branch tips, flower parts, and/or seedpods in the spring after it was infected. As the disease progresses, they will turn white. Cankers appear as calluses that grow in rolls around the infected area. The cankers and tree grow alternately, giving the area the appearance of a target. Powdery mildew makes the leaves look like they are covered in dust. Sooty mold looks like a black charcoal on the leaves, twigs and branches.
Azalea gall is caused by cool, wet weather when the fungal spores are spread by the wind and splashed by the rain onto the plant. Poorly aerated soils give the fungus a place to winter over so they can grow in the spring. Cankers are caused by fungal or bacterial disease that enters thought a wound in the wood. Powdery mildew is caused by several air born fungal diseases. Sooty mold happens when insects secrete a substance called honeydew to which airborne fungus sticks.
Fungal diseases can be prevented by making sure the soil is well drained and there is sufficient air circulation by planting far enough apart and pruning away overlapping branches. Dead and diseased branches should be pruned off and disposes of to prevent the spread of the disease. Give the plant a good mulching to help keep the soil moist so frequent watering is not necessary. Cankers can be prevented by not mowing close to the brush. Mulch or gravel around the plant will keep grass from growing. Infected branches should be removed immediately. The only way to prevent sooty mold is to control the honeydew producing insects.
Azalea gall can cause defoliation of the bush. Canker diseases can kill off the bark in the infected area and all parts above it or in the worse cases, it it affects the trunk, the whole bush may die. Powdery mildew kills the tissue under the infection causing the leaves to drop and the growth to become stunted. Rhododendron can suffer sever damage or death from sooty mold because they are low-growing shrubs that grow in the shade of other taller ones.