Azalea Leaf and Flower Gall
R. H. Gruenhagen - Extension Plant Pathologist
Virginia Polytechnic Institute, Blacksburg, Va.
Reprinted with permission from "Nurseryman's Notebook,"
a publication of the Extension Service, Virginia Polytechnic Institute.
The azalea leaf and flower gall disease is a widespread and very common problem on cultivated azaleas, as well as other species such as camellia and rhododendron. The occurrence and intensity of the disease is dependent upon weather conditions and upon a source of the fungus which causes the disease.
The symptoms are typical and easy to recognize. On azalea the disease causes the leaves to become thickened, curled, and fleshy. They turn pale green to white or pink in color during the early stages of the disease, and turn brown and hard as the season progresses. Infection of the flowers appears as fleshy, waxy, galls. These galls are made up of abnormal petal tissue. Lower leaves are usually the most seriously damaged, but under humid conditions and in shaded locations. infections may occur out to the ends of top branches. Flower parts of other plants are also frequently affected, especially the petals of evergreen species of rhododendron. The galls which are formed from the swollen, distorted tissue frequently are covered with a whitish mold-like growth during periods of high humidity.
This disease is caused by the fungus Exobasidium vaccinii. Closely related species in the genus Exobasidium cause the same type of gall formation on a wide variety of other plants, such as Arbutus, Blueberry, Ledum, Leucothoe, and many others.
For the average home owner, this disease is more alarming from the point of view of disfigurement of the plants than it is from the actual damage produced. However, under very humid conditions and without adequate control measures the infection may become so widespread as to considerably harm the plants. Where only a few plants are involved, such as in a home planting or small greenhouse area, the disease is rather easily controlled by picking and burning the galls as soon as they appear. When plantings are more numerous and when the disease becomes more widespread, a spray program is necessary.
Any of the following materials may be used. Material Amount bordeaux mixture 3-1-50 per gallon per 100 gallons captan, 80% wettable powder 1½ tablespoons 2 lbs. ferbam, 76% wettable powder 2 tablespoons 1½ lbs. zineb, 65%. wettable powder 1/3 tablespoons 2 lbs.
The 3-1-50 designation following bordeaux mixture means 3 lbs. of copper sulfate plus 1 lb. of spray lime in 50 gallons of water. Dry preparations may be available and the amount of active ingredients is stated on the label. Although bordeaux mixture is effective, it tends to leave a rather unsightly spray residue on the blooms and foliage.
Two spray applications are usually sufficient to give satisfactory disease control. The first spray should be put on when galls are first evident. The second should follow in 10 to 14 days. If the weather is particularly humid or the disease unusually severe, a third spray may be necessary.