Large Knot on Tree: Crown Gall or Callous Tissue?

What That Knot Likely Is

A large, woody knot on a mature tree—especially near the ground—usually indicates one of two things:

• Crown gall: Caused by the bacterium Agrobacterium tumefaciens, this creates tumor-like growths. There's no cure, but trees often live for decades with it.
• Callous tissue: The tree's natural response to injury (mower damage, frost crack, pruning wound). The wood attempts to seal the wound, forming a swollen ridge.

How to Tell the Difference

Crown galls typically appear as rough, irregular, cauliflower-like masses at the soil line or on roots. Callous tissue forms more organized, rolled edges around a visible wound center. If bark is loose but mostly solid, callous tissue is more likely.

Management Advice

Do nothing aggressive. Cutting into callous tissue reopens the wound and invites decay fungi. Crown galls cannot be eliminated without removing the tree. In either case:

• Monitor for structural weakness (cracks, leaning).
• Avoid damaging the knot with equipment.
• Consult a certified arborist if the knot compromises stability or exceeds 1/3 of trunk circumference.

Most trees coexist with these knots for decades. Intervention is only needed if safety becomes a concern.