Armillaria Root Rot of Trees: Identification and Management

Understanding Armillaria Root Rot (Shoestring Root Rot)

Armillaria root rot, caused by various Armillaria species, affects most deciduous and coniferous species. Plants under stress from prolonged drought, repeated insect defoliation, root injury, or recent transplanting are most susceptible.

Life History

Armillaria survives well in dead roots, stumps, and soil as long brown shoestring-like rhizomorphs (root-like structures). These can grow several feet through soil from stumps to nearby trees and tree-to-tree—unusual for fungi, which typically remain dormant unless hosts are very near. Rhizomorphs invade trees at buttress or upper root system. Just under bark, obvious white fans of fungal growth form. Honey-brown mushrooms develop in autumn on roots/buttress of infected trees; windblown spores spread to new areas.

Symptoms

• Slowed top growth
• Branch dieback
• Root rot
• Rapid tree death appearance
• White fungal fans under bark at soil line (when bark peeled)
• Abundant resin flow at soil line (conifers)

Management

Encourage good tree vigor by reducing stress:

1. Control defoliating insects.
2. Irrigate to relieve drought.
3. Protect against lower trunk and upper root system injuries.
4. Do not replace Armillaria-killed plants with other woody species.

Integrated Armillaria Management

There are no effective chemical controls for established Armillaria infections. Management focuses entirely on prevention through stress reduction and avoiding planting susceptible species in infested sites. The rhizomorphs' ability to grow long distances through soil makes site history critical—areas with previous Armillaria-killed trees remain infested for years. Tree vigor is the primary defense; healthy trees can compartmentalize and resist invasion, while stressed trees succumb rapidly.