Azalea gall, caused by the fungus Exobasidium vaccinii, is a common and often alarming disease affecting plants in the Ericaceae family, primarily azaleas and rhododendrons. From a botanical perspective, the disease represents a fascinating, albeit detrimental, interaction between a pathogenic fungus and its host plant, resulting in a significant disruption of normal growth patterns and resource allocation.
The infection process begins in spring when the fungal spores (basidiospores) are released from previously infected plant material and are dispersed by wind and splashing water. These spores require a film of water on the plant's surface to germinate and penetrate the young, succulent new tissues of the azalea. The fungus does not kill the cells outright; instead, it secretes specific compounds, including the plant hormone indole-3-acetic acid (IAA), a type of auxin. This hormonal hijacking disrupts the plant's normal regulatory systems, leading to uncontrolled cell division and enlargement (hyperplasia and hypertrophy). The result is the formation of the characteristic gall, which is essentially a tumor-like growth providing the fungus with a protected, nutrient-rich environment in which to thrive and reproduce.
The symptoms are a direct manifestation of the plant's physiological disruption. Initially, affected leaves or flowers become thickened, fleshy, and pale green. As the galls develop, they expand dramatically, often becoming several times the size of healthy tissue. Their color transitions to a conspicuous white or pinkish hue as the fungal hymenium (spore-producing layer) develops on the surface. This white coating is a mass of fungal spores ready to be released to initiate new infections. From the plant's viewpoint, these galls are a massive sink for photosynthates and nutrients, diverting energy away from critical processes like root growth, flower production, and overall vigor, thereby weakening the plant over time.
Effective control strategies focus on breaking the disease cycle and reducing stress on the plant to bolster its natural defenses.
Cultural Control: The most effective method is the physical removal of galls. This must be done as soon as they are noticed, before the white spore layer appears. Prompt removal robs the fungus of its reproductive structures and drastically reduces the inoculum source for the following season. Improving air circulation through selective pruning reduces leaf wetness duration, creating a less favorable environment for spore germination. Furthermore, maintaining overall plant health with appropriate watering (avoiding overhead irrigation), mulching, and a balanced, slow-release fertilizer ensures the plant is not predisposed to infection due to stress.
Chemical Control: Fungicides are generally considered a last resort. If necessary, a protective fungicide containing mancozeb or chlorothalonil can be applied as a foliar spray in early spring, just as new leaves emerge. This creates a protective barrier on the plant's surface, preventing spore germination and infection. It is a preventative, not curative, measure and must be timed correctly with bud break to be effective.
