Conifer root rot

Pathogen and disease type: The disease is caused by the basidiomycete fungus Coniferiporia weirii (formerly Phellinus weirii). It is a wood-decay fungus that specializes in attacking the root system and the lower bole of trees, inducing white laminated rot. This pathogen is considered a major destructive force in coniferous forest ecosystems worldwide.

Host plants: The fungus primarily affects a wide range of coniferous species. Major hosts include Douglas-fir, hemlock, spruce, and true firs. The disease is particularly problematic in natural forest stands and plantations where trees are closely spaced, facilitating the subterranean spread of the mycelium.

Symptoms and signs: The initial signs of infection are often subtle, including reduced height growth and chlorosis (yellowing) of the needles. As the rot progresses, crown thinning becomes evident. A definitive diagnostic feature is the presence of white, felt-like mycelial mats between the wood and bark of the root collar, along with brittle, laminated, or stringy decay within the roots.

Conditions for development and spread: Coniferiporia weirii spreads primarily through root-to-root contact. When a healthy root touches an infected root or a stump containing the fungus, the mycelium bridges the gap and infects the new host. The fungus can persist in old stumps and root fragments in the soil for several decades, making it a long-term threat to subsequent forest rotations.

Protection and control measures: Effective management relies on identifying and isolating infection centers. Strategies include removing infected trees along with their root systems and clearing stumps from the site before replanting. In some cases, creating deep trenches around infected areas helps sever root connections and stop the movement of the fungus through the soil to neighboring healthy trees.