Azalea leaf and flower gall is a disease that causes concern to many home gardeners each year. It is caused by the fungus Exobasidium vaccinii. In home landscape plantings, the disease is more alarming than damaging, but in greenhouse plants grown under very humid conditions, galls may become so abundant that they cause considerable harm if control measures are not implemented. Closely related species of Exobasidium cause similar galls on other plants, including species of Arbutus, blueberry, Camellia, Ledum, Leucothoe and rhododendron.

Symptoms

Exobasidium vaccinii causes leaves and flowers to become swollen, curled, waxy and fleshy (Fig. 1). The swollen plant tissues or "galls" are made up of abnormal plant tissue. Infected leaf tissue is usually pale green in color during the early stages of the disease; infected flowers are usually pinkish. Later in the season, a white spore layer covers the infected plant parts. Galls eventually turn brown and harden as the season progresses. Lower leaves on plants are usually the most seriously damaged, but under humid conditions and in shaded locations galls may occur at the ends of upper branches.

Disease Cycle

The occurrence and intensity of the disease depends on weather conditions and on the source of the causal fungus. Spores produced in the whitish mold on the surface of galls in late spring to early summer are blown and washed to leaf and flower buds where they cause new infections. Galls form the following spring. Cool, wet weather favors dispersal of the spores.

Control

Cultural Control

When only a few plants are involved, as in a home planting or a small greenhouse area, the disease is easily controlled by hand picking the galls and burning or burying them. To prevent new infections, it is important to pick the galls before the white spore layer appears. Fungicide control is generally not warranted in home landscapes.

Chemical Control

In commercial operations, a combination of hand picking of the galls and application of a fungicide may be warranted. Two applications of a fungicide containing mancozeb (e.g. Dithane), one made just before leaves unfurl in spring and one 10 days later, will help prevent new infections. Follow label rates or refer to the current Virginia Pest Management Guide for Horticultural and Forest Crops (VCE Publication 456-017), http://www.ext.vt.edu/pubs/pmg/, for details on rates and timing of application. For information on the proper use of pesticides and fungicides, refer to any current VCE pest management guide.

Resistance

Some azalea cultivars with resistance to leaf and flower gall have been reported. Resistant and susceptible cultivars of azalea are listed in Table 1. The Purple Splendor and Roseum cultivars of rhododendron are also highly susceptible to this disease.

Author: Mary Ann Hansen, Extension Plant Pathologist, Department of Plant Pathology, Physiology and Weed Science, Virginia Tech