Blueberry leaf blister

Exobasidium vaccinii is a species of parasitic fungus within the order Exobasidiales. It is best known as the causal agent of leaf gall or "red leaf" disease, which affects a wide range of plants belonging to the Ericaceae family, most notably blueberries, cranberries, and rhododendrons.

The disease primarily targets leaves, flower buds, and young shoots. Infection results in the development of fleshy, discolored galls. Initially, these galls are light green or yellowish, but they often turn a bright, conspicuous reddish-pink color. As the disease progresses, a velvety white coating forms on the underside of the leaves, representing the sporulation of the fungus.

Regarding its biology, Exobasidium vaccinii is an obligate parasite. The fungus survives the winter as mycelium within the tissues of the host plant. With the return of warm, humid spring conditions, the mycelium becomes active, colonizing newly developing buds and expanding leaves. The spores are then released into the environment, spreading via water splashes or wind to initiate secondary infections.

Environmental conditions are the main driver of outbreaks. The pathogen thrives in cool, wet spring weather and dense plant canopies where air circulation is restricted. Poorly drained soils that hold excess humidity further exacerbate the development of the fungus, allowing it to complete its cycle more effectively across the plantation.

The impact of this pathogen is primarily economic, as severe infections can lead to premature leaf drop, stunted shoot growth, and reduced fruit yield. To effectively manage this fungus, growers should prioritize cultural practices, including the removal and destruction of symptomatic tissue during the early season. Additionally, ensuring proper plant spacing and applying protective fungicides during bud break can significantly suppress disease pressure.