Exobasidiosis

Exobasidiosis is a plant disease caused by parasitic fungi belonging to the order Exobasidiales. These fungi are biotrophic pathogens, meaning they depend on living plant tissues to grow. They are particularly well-known for causing distinct hypertrophic growth and deformation in their hosts, often leading to the formation of fleshy, abnormal structures known as galls.

The most common hosts for these fungi are members of the Ericaceae family, particularly rhododendrons, azaleas, blueberries, and cranberries. The infection usually occurs when basidiospores land on susceptible young leaves or flower buds. Once infected, the plant cells begin to divide and enlarge abnormally, resulting in the characteristic thickened, pale, or discolored appearance of the affected plant parts.

Symptoms include leaves that become swollen, succulent, and discolored (often turning white, pink, or light yellow due to the fungal spore layer). As the disease progresses, these tissues turn dark brown or black and dry out. This can cause significant defoliation, stunt the plant's growth, and in agricultural settings, drastically reduce fruit yield.

The development of exobasidiosis is highly dependent on environmental conditions. It thrives in cool, wet weather, particularly during the spring when new growth is tender. High relative humidity and prolonged leaf wetness allow the spores to germinate effectively. Poor air circulation and overcrowding in nurseries or gardens are major factors that accelerate the spread of the pathogen.

To control and prevent the spread of exobasidiosis, it is crucial to remove and destroy infected plant parts as soon as symptoms are noticed. Cultural practices, such as ensuring proper plant spacing for better airflow and selecting well-draining soil, are essential. Preventive fungicide applications, especially during early spring before the buds open, can effectively protect susceptible plants from infection.