See:
Azalea (Rhododendron spp.) - Leaf and Flower Gall
Cause Exobasidium vaccinii, a fungus that is believed to overwinter as spores adhering to bark and bud scales and to cause infection with the opening of buds in spring. Older leaves are resistant to infection. Infection is dependent on high humidity and moisture during bud break. Gall growth is due to plant cell division resulting from the stimulus of hyphal growth within host tissue. When galls are still soft, the fungus grows between epidermal cells, breaks the cuticle, and sporulates. Basidiospores are formed on the underside of the leaf blisters and on the surface of the infected fruits in a thin layer of fungal tissue. They are ejected during wet weather. Hosts of this fungus include azalea, Camellia, huckleberry (Vaccinium spp.), Kalmia, kinnikinnick, Leucothoe, madrone, Pieris, and salal.
Symptoms Pinkish blister-like galls that make the leaves distorted and twisted. It attacks the fruit, making it turn red and swell to several times its natural size.
Cultural control
- Remove and destroy galls when noticed.
Reference Elliott, M., 1999. Diseases of Pacific madrone. The decline of the Pacific madrone, Chapter 7, pp.42-49. https://depts.washington.edu/hortlib/collections/madrone/ch07_el.pdf