Armillaria root rot

Pathogen and disease type. The disease is caused by the basidiomycete fungus Armillaria mellea. It is responsible for white root rot, which affects the vascular tissues of trees and woody shrubs. This pathogen is considered one of the most destructive tree diseases worldwide due to its ability to survive for decades in infected root systems.

Susceptible plants. Armillaria has an exceptionally broad host range. It affects almost all species of conifers and broad-leaved trees, including high-value orchard crops like apple, pear, and cherry. The fungus is especially aggressive in environments where host plants are already stressed by drought, flooding, or mechanical damage.

Symptoms and signs. Typical symptoms include crown thinning, leaf yellowing, and stunted growth. The most reliable diagnostic sign is the presence of white, fan-shaped mycelial mats under the bark at the base of the trunk. Additionally, dark, shoelace-like structures known as rhizomorphs can be found on the roots and in the surrounding soil.

Conditions for development. The fungus thrives in moist conditions and spreads primarily through root contact or via rhizomorphs that navigate through the soil. Old stumps and decaying root systems serve as the main reservoir for the pathogen, allowing it to remain dormant until it can infect the roots of neighboring healthy plants.

Harmfulness and management. Armillaria leads to severe decay of root systems and the lower trunk, eventually causing the tree to collapse. Control strategies include:

• Stump removal and complete extraction of root debris from the site.
• Improving soil drainage to minimize plant stress and inhibit fungal growth.
• Isolating infected areas with physical root barriers or trenches.
• Monitoring the orchard and removing symptomatic trees early to prevent spread.