Armillaria species, commonly known as Honey Fungus, is a highly destructive and persistent root pathogen of woody plants. It attacks and kills the roots of a wide range of trees and shrubs, including both conifers and hardwoods. Honey Fungus is an opportunistic organism that thrives when plants are weakened by drought, poor soil, or other stressors. The main challenge in managing this disease stems from its ability to survive in the soil long after the host plant has died.
Recognizing the Signs of Honey Fungus
Above-ground symptoms often mimic general plant decline, such as dieback in the crown, sparse or yellowing foliage, and a lack of overall vigor. Infected trees may also produce an unusually heavy crop of flowers or cones in a final effort before succumbing to the infection.
Definitive physical signs of an active infection are found at the base of the plant. During the autumn, clusters of honey-colored mushrooms, which give the fungus its common name, can emerge from the base of the trunk or nearby stumps. Peeling back the bark at the root collar of a declining tree may reveal a white or cream-colored, fan-shaped sheet of fungal tissue (mycelium) actively growing between the bark and the wood.
Another sign is the presence of black or reddish-brown, root-like structures called rhizomorphs, often referred to as “bootlaces.” These structures grow in the soil, usually within the top eight inches, and can also be found flattened beneath the bark in advanced stages of infection.
How Honey Fungus Stays in the Soil for Decades
The persistence of Honey Fungus in the soil is primarily due to two distinct biological mechanisms: specialized survival structures and its feeding strategy. The fungus does not typically rely on airborne spores for new infections, which are relatively rare, but instead spreads through direct root contact or by growing through the soil.
The first mechanism of long-term survival is the rhizomorph, a highly organized, cord-like structure that functions as the fungal equivalent of a root. These structures possess a tough, melanized outer layer that protects the internal fungal tissue from desiccation, physical damage, and attack by other soil microbes. The rhizomorphs are the primary means by which the fungus spreads underground, moving from an infected food source to the roots of a healthy neighboring plant.
The second factor is the continued presence of its food source: the dead or decaying wood of infected stumps and roots. Armillaria acts as both a pathogen, killing living tissue, and a saprobe, feeding on the dead wood. As long as a large, buried woody mass remains, the fungus can survive and draw energy from it, acting as a reservoir of infectious material. The fungus often remains active for 10 to 30 years, with some reports suggesting survival for up to 50 years in large stumps.
Strategies for Managing Contaminated Areas
Because chemical treatments are generally ineffective against this fungal root rot, the most successful management strategy focuses on eliminating the food source and preventing the spread of rhizomorphs. This requires the complete removal of the infected host material, including digging out the main stump and as many large, lateral roots as possible, since these residual woody pieces can sustain the fungus for years.
After removing the host material, physical barriers can be installed to contain the spread of any remaining rhizomorphs. Barriers made of thick plastic sheeting or buried vertical metal flashing should be placed in the soil to a depth of about 18 inches to prevent the bootlaces from growing into adjacent healthy planting areas.
For replanting an area known to harbor the fungus, the most practical approach is to select species resistant to Armillaria infection. Less susceptible plants include:
- Boxwood
- Holly
- Yew
- Certain conifers like Dawn Redwood
Maintaining the health of all remaining plants by ensuring proper drainage and avoiding drought stress is also important, as healthy, non-stressed plants are better able to resist initial infection.