Many plants are susceptible to different kinds of galls. Galls are abnormal lumps that develop on a wide range of plant parts. Various types of insects-especially wasps (some are scarcely visible to the naked eye), aphids, mites, and midges-can cause galls. Insects feed on or lay their eggs in the leaves, stems, roots, or flowers of a plant, stimulating abnormal growth and causing the plant to grow around the eggs.
Sometimes fungi, bacteria, or nematodes infect plants and cause galls. Azalea leaf and flower gall is caused by the fungus Exobasidium vaccinii. Galls generally aren't a severe problem, though a few can girdle stems and cause dieback. Once the gall has formed, protective insecticide or fungicide sprays will be ineffective. If the problem is minor, you can trim out the affected part of the plant. In most cases, no treatment is necessary.