Disease

Root and butt rot

Armillaria, Ganoderma and other decay fungi

Fungi that rot the roots and lower trunk from the inside. The tree can look healthy until late, and a tree with a rotted base can fall without warning, so a tree near people or buildings needs a risk assessment by an arborist.

Signs

• Mushrooms or shelf-like conks on the trunk, the root flare or the ground over the roots.
• White, fan-shaped fungus under loose bark at the base, or black shoestring-like strands (Armillaria).
• Slow thinning, small leaves and dieback; soft or crumbly wood at the base.

What favors it

Stress, root injury from construction or trenching, buried root flares, and old stumps that feed the fungus.

Prevention

• Keep heavy equipment, grade changes and trenching out of the root zone.
• Remove stumps and large roots of trees lost to Armillaria before replanting.
• Water through drought and keep the root flare exposed.

Crops it shows up on

Shade & ornamental trees; Evergreens & conifers (pine, spruce, juniper); Apple, pear & stone fruit.

Often confused with

Sources

• Armillaria Root and Butt RotUMass Amherst Extension
• Armillaria Root DiseaseUniversity of Wisconsin-Madison Plant Disease Diagnostics Clinic
• Root and Butt Rot caused by Ganoderma sessileUMass Amherst Extension
• Armillaria root rotWashington State University HortSense

Why →