Exobasidium leaf gall

The disease is caused by the fungus Exobasidium sydowianum, a specialized parasite that attacks plants belonging to the Ericaceae family. This pathogen is known for inducing abnormal tissue growth in its hosts, leading to the formation of galls.

Common hosts include various species of Rhododendron and Azalea. While this fungus does not typically kill the host outright, it significantly impacts the aesthetic value and long-term health of the plants in nurseries and landscape plantings.

The key symptom is the appearance of thickened, fleshy, and pale-colored areas on the leaves. As the infection progresses, these areas develop into galls. A white, powdery spore layer often covers the underside of the affected leaf tissue during humid conditions.

Development is heavily favored by cool, moist spring weather. The fungus overwinters in the buds or debris, and spores are spread via wind and water splashing during rainfall. High humidity and poor air circulation are critical factors that facilitate the rapid spread of the pathogen.

The damage caused by the fungus results in reduced photosynthetic efficiency and premature leaf drop. Severe infections may leave the plant weakened, making it more susceptible to environmental stress and secondary invaders, which eventually reduces the plant's overall vigor.

• Prune and destroy infected plant parts immediately upon detection to reduce the inoculum level.
• Apply preventative fungicide sprays during the early spring before bud break.
• Improve air circulation by thinning the plant canopy and spacing plants appropriately.
• Avoid overhead irrigation to keep foliage dry, as moisture promotes fungal germination.