Northern honey fungus

The northern honey fungus (Armillaria borealis) is a destructive basidiomycete fungus responsible for root and butt rot in a wide range of tree species. It is a key pathogen within the Armillaria complex, playing a major role in forest ecosystem dynamics by causing progressive decay of the root systems.

This pathogen primary attacks conifer species such as spruce, pine, and fir, although it can also affect various broadleaved trees. By invading the roots, the fungus disrupts the uptake of water and nutrients, which eventually leads to the decline and death of the host tree, regardless of its age or initial health status.

Key symptoms of an Armillaria borealis infection include the thinning of the tree crown, chlorosis (yellowing) of the needles, and excessive resin exudation at the base of the trunk. A definitive diagnostic feature is the presence of white, fan-shaped mycelial mats under the bark of the root collar and the detection of dark, thread-like structures in the soil known as rhizomorphs.

The development of the fungus is favored by moist soil conditions and the presence of dead wood, such as stumps from previous logging operations, which serve as primary reservoirs of the infection. The pathogen spreads underground through root-to-root contact and via rhizomorphs that navigate through the soil searching for new susceptible hosts.

The impact of this disease is significant in terms of forest mortality and timber value loss. Effective management and protection strategies focus on silvicultural practices, such as stump removal after harvesting, careful thinning to reduce tree stress, and the application of biological control agents, including beneficial fungi that compete with Armillaria for space and resources.

• Stump removal to reduce inoculum sources.
• Avoidance of root damage during forestry operations.
• Use of biological fungicides for stump treatment.
• Monitoring of stands for early symptoms of decline.