Exobasidium

Exobasidium is a fungal plant disease caused by members of the Exobasidiaceae family. These pathogens are obligate parasites that induce hypertrophy and hyperplasia in host plant tissues, resulting in distinct and characteristic morphological changes known as leaf or stem galls.

This disease primarily targets plants in the Ericaceae family, such as rhododendrons, blueberries, and cranberries, though it can occasionally affect other species. The pathogen disrupts cellular development, causing the plant to redirect nutrients towards the growing gall instead of normal growth and reproduction.

The visual symptoms of an Exobasidium infection include thickened, fleshy, or blister-like swellings on leaves, buds, and young shoots. These galls often turn from pale green to bright shades of pink, red, or white. Eventually, the surface of the gall develops a waxy or powdery white texture, which is the fungal layer producing spores.

Environmental conditions that favor the disease include prolonged periods of high humidity, frequent rainfall, and cool temperatures during the spring growth phase. The spores are highly mobile, spreading efficiently via wind currents and water splashes, which facilitates rapid transmission within dense plant populations.

The impact of Exobasidium is significant, causing both aesthetic damage to ornamental garden plants and substantial yield losses in fruit-bearing crops. Chronic infections lead to reduced plant vigor, increased susceptibility to secondary environmental stresses, and potential dieback of affected branches if the disease remains uncontrolled.

• Sanitary pruning to remove and dispose of all infected plant tissues.
• Thinning out dense plant clusters to improve air circulation and reduce humidity.
• Application of preventative fungicides during bud break in the early spring.
• Avoiding overhead irrigation to reduce leaf wetness duration.
• Monitoring plants regularly to detect and manage the first signs of gall formation.