Beech bark fungus

Biscogniauxia nummularia is a destructive ascomycete fungus responsible for bark necrosis and canker diseases in various hardwood species. As a wood-decay pathogen, it primarily targets the vascular system of trees, leading to significant structural damage and, in many cases, the eventual death of the host plant.

The primary host for this fungus is the European beech (Fagus sylvatica), though it can also infest oaks and other broad-leaved trees. It is essentially an opportunistic pathogen, meaning it thrives on trees that are already suffering from environmental stressors such as drought, nutrient deficiency, or prior damage from wood-boring insects.

Symptoms are easily identified by the presence of carbonaceous, black, disc-shaped stromata that erupt through the bark surface. As the fungus colonizes the inner bark and cambium, the bark begins to crack, flake off, and die. Infected trees show signs of decline, including branch dieback, sparse canopy coverage, and stunted growth compared to healthy neighbors.

The development of the disease is highly dependent on environmental conditions. Fungal spores are dispersed by wind and rain splash, infecting trees through wounds or natural bark fissures. Periods of prolonged drought are known to exacerbate the disease, as trees become unable to produce sufficient sap to defend against the invading mycelium, allowing the pathogen to spread internally with greater speed.

Effective management relies heavily on preventative cultural practices rather than direct chemical intervention. The most critical step is the removal and destruction of heavily infected trees to reduce the inoculum load in the forest. Additionally, avoiding site damage, ensuring proper soil moisture, and maintaining stand vigor are essential to preventing outbreaks in susceptible woodlands and urban tree populations.