Annosus root rot

Pathogen and disease type. The fungus Heterobasidion irregulare is a destructive basidiomycete pathogen responsible for annosus root rot. This disease is characterized by the progressive decay of root systems and the lower trunk area of woody plants. It functions as both a saprophyte, decomposing dead organic matter, and a necrotrophic parasite that invades living tissues, eventually leading to the death of the host.

Susceptible hosts. This pathogen primarily affects conifers, with high susceptibility observed in pine and spruce species. It is a major concern in commercial forestry, plantations, and parks where large numbers of susceptible hosts are densely planted. The presence of stump sites following forest management activities significantly increases the risk of host infection.

Symptoms and signs. Early symptoms include thinning foliage, reduced terminal growth, and needle discoloration. As the disease advances, structural stability is lost, leading to windthrow. Characteristic signs include the formation of fungal sporocarps (fruiting bodies) at the base of the tree or on root flares, often appearing as woody, perennial shelf-like structures. Internally, the wood undergoes a transformation into a reddish-brown decay, often showing white mycelial mats.

Development conditions. Spread occurs mainly via airborne basidiospores that colonize fresh wounds, stumps, or broken roots. Once a tree is colonized, the fungus spreads to adjacent healthy trees through root-to-root contact. Humid soil conditions, mild temperatures, and the presence of fresh wood surfaces promote the rapid establishment and expansion of the fungal colony.

Harmfulness and protection. The disease causes significant economic loss due to tree mortality, timber degradation, and increased vulnerability to wind damage. Effective disease management strategies include:

• Applying biological control agents, such as Phlebiopsis gigantea, to stump surfaces immediately after logging.
• Executing harvesting operations during winter months when airborne spore dispersal is minimized.
• Avoiding site disturbance that may damage root systems.
• Implementing regular forest health inspections to identify and remove infected centers early.