Small bright yellow spots is a symptom of azalea rust, Puccininastrum vaccinii, a fungal disease. Orange-red pustules containing spores form on the lower surface of the leaf. This rust is only sporadically severe and typically infects deciduous azaleas. Control by avoiding those hybrids and species which are very susceptible to rust. Good air circulation is helpful. Triadimefon (Bayleton) seems to help, but may not be registered for use on rhododendrons and azaleas. Registered fungicides include sulfur and Bordeaux mixture.

Rust first appears on rhododendron and azalea in late summer as small, yellow, circular spots on the upper leaf surface. By early fall, the fungus produces an abundance of bright, orange-colored pustules (uredia) on the lower surface of affected leaves. These spores are airborne and spread to hemlock, where the fungus overwinters and causes a rust disease of this host the following spring. Rust spores produced on hemlock are blown back to Azalea and Rhododendron in mid-summer and the disease cycle begins anew.

Disease management: To manage this disease, avoid planting azaleas near hemlocks, maintain plant vigor, employ good sanitation practices (i.e., remove infected leaves), reduce humidity, improve aeration, avoid excessive leaf moisture, and utilize resistant cultivars when possible. Mancozeb may be applied (per manufacturer's recommendations) at 10- to 14-day intervals from mid-August through September in areas where rust is troublesome.