See:
Azalea (Rhododendron spp.) - Leaf and Flower Gall
Cause The fungi Exobasidium vaccinii and E. vaccinii-uliginosi have been reported from the Pacific Northwest. The cause in azalea is unknown. Brooms are not common in garden plantings but are often seen on native plants throughout the Cascade and Klamath mountain ranges of Oregon.
Symptoms The disease is characterized by several woody shoots arising from the bud near the terminal part of the plant. Brooms may become very large and numerous on some plants. Brooms can be found on otherwise healthy bushes. Bushes may have healthy branches on the one side and a broom on the other. Leaves on the witches' broom are smaller, chlorotic and may have yellowed-to-cream-colored thickenings.
Cultural control Remove branches several inches below the base of the broom.
Reference Booth, C. 1983. Exobasidium vaccinii. CMI Descriptions of Pathogenic Fungi and Bacteria, No. 778. Surrey, England: Commonwealth Mycological Institute.