Merulius tremellosus

Merulius tremellosus, often referred to as the jelly rot fungus, is a species of fungus in the family Meruliaceae. While primarily acting as a saprotroph by breaking down dead organic matter, it can also act as an opportunistic pathogen in weakened living trees. This fungus is responsible for causing white rot, a form of decay that compromises the structural integrity of woody plants by targeting both cellulose and lignin components of the cell wall.

The disease is caused by the fungal mycelium colonizing the heartwood or sapwood of deciduous trees. Tree species commonly susceptible to this infection include birch, aspen, alder, and occasionally beech. The infection process is essentially a wood-rotting mechanism that leads to the degradation of the vascular system of the tree, significantly reducing the commercial and structural value of the timber.

Key symptoms include the emergence of gelatinous, fan-shaped, or shelf-like fruiting bodies on the trunk or exposed wood. These structures are often colored in shades of white to pinkish-orange. Internally, infected wood exhibits a pale appearance, becomes brittle, and develops a fibrous texture. In advanced stages, the wood may exhibit irregular dark lines and a soft, spongy consistency, often accompanied by a distinct fungal odor.

The development and spread of Merulius tremellosus are highly dependent on environmental conditions. High humidity levels and moderate temperatures (between 15°C and 25°C) favor rapid mycelial growth. Spores are primarily dispersed through the air and colonize trees via wounds caused by frost, pruning, or physical trauma. Proper aeration and low moisture content in wood are critical factors that naturally inhibit the fungus's colonization.

Control and prevention methods emphasize maintaining tree vigor and strict forest sanitation. Removing deadwood, fallen branches, and infected stumps is essential to reduce the overall spore load in the environment. Preventive measures also include sealing pruning wounds with antifungal sealants to prevent spore entry. Effective management of wood storage areas, ensuring proper stacking and dryness, is also crucial to prevent the spread of the fungus in harvested timber.