Exobasidium

Exobasidium is a genus of biotrophic fungi within the order Exobasidiales. These pathogens are known for inducing complex morphological changes in their hosts, primarily by infecting leaves, flowers, and stems, leading to the formation of galls or thickened, discolored tissues commonly referred to as exobasidiosis.

The primary hosts of this pathogen include various members of the Ericaceae family, such as rhododendrons, azaleas, and vaccinium species (blueberries and cranberries). The disease manifests as localized swelling and hypertrophy, often covered with a white or pinkish powdery layer of basidiospores once the fungus reaches the reproductive stage.

The life cycle of Exobasidium is highly dependent on humidity. In the spring, spores are released and dispersed by wind and rain onto susceptible plant tissues. Upon penetration, the fungus colonizes the intercellular spaces of the host, triggering the plant to redirect nutrients towards the infected area, which causes the characteristic deformation.

Development and spread are favored by cool, wet weather conditions, particularly during the early growing season. Constant moisture, fog, and poor air circulation in landscapes or greenhouses provide an ideal environment for the spores to germinate and spread to healthy plant parts, potentially infecting the entire plant canopy.

Effective management strategies require an integrated approach. Pruning and immediate removal of all galls and distorted foliage is crucial to reduce the inoculum load. Combining sanitation with preventive fungicide applications, particularly during early bud break, helps mitigate the disease. Improving horticultural practices, such as increasing plant spacing to optimize airflow, is equally important for long-term control.