Blue stain fungus

The disease is caused by the microscopic ascomycete fungus Endoconidiophora coerulescens (previously Ceratocystis coerulescens). It is a well-known pathogen responsible for the phenomenon called blue stain, which causes characteristic dark-blue or greyish-black discoloration in the sapwood of logs and lumber.

This pathogen primarily targets coniferous species such as pine, spruce, and fir, but it can also affect hardwoods like maple and beech. It typically colonizes freshly felled timber or weakened standing trees, where the physiological barriers of the host are lowered, allowing the fungus to invade the nutrient-rich sapwood.

The primary symptom is the appearance of blue to black streaks and wedge-shaped patterns in the wood cross-section. Unlike surface molds, the mycelium of Endoconidiophora coerulescens penetrates deep into the wood structure, colonizing the parenchyma cells and utilizing starch and sugars stored within, which is why the discoloration persists deep inside the timber.

The development of the fungus is favored by high wood moisture content and moderate temperatures. The spread is primarily facilitated by wood-boring insects, particularly bark beetles. As these insects bore into the trees, they carry the fungal spores on their bodies, directly inoculating the inner tissues and providing the fungus with a gateway into the host.

The economic impact of this disease is significant due to the degradation of timber quality. While the structural integrity of the wood remains largely intact compared to decay fungi, the aesthetic degradation renders the lumber unsuitable for clear-finished applications, construction requiring aesthetic quality, and the furniture industry, leading to substantial commercial losses.

• Rapid transport and processing of logs after harvesting to minimize exposure.
• Forced drying of lumber to reduce moisture content below 20% as quickly as possible.
• Application of chemical fungicides to the ends of logs and freshly cut surfaces.
• Integrated pest management to reduce populations of insect vectors in storage areas.