Later on in the spring or early summer, old galls turn brown, shrivel, and fall to the ground. On some susceptible rhododendron cultivars, infected leaves do not become galled but show yellow or creamy white spots that cover the leaf surface. Later, a white spore layer develops on lower leaf surfaces. Persistence And Transmission. The white or pinkish spores produced on galled tissues or on lower surfaces of some rhododendron varieties are spread by wind and splashing rain to nearby healthy leaves, shoots, or buds.
Initial infections occur when the spores contact susceptible tissues.
Galls form the following spring. Cool, moist weather favors spore development, dispersal, and infection. The disease is often more severe in shaded areas with high humidity. Hand remove and discard all galled leaves and yellow or white spotted leaves on rhododendrons before they become white or pink with spores. You can add galled leaves to a hot, properly maintained compost pile.