Identifying Burls and Crown Galls on Oak Trees

What Are Those Knobby Growths on Your Oak?

A homeowner noticed their entire oak tree covered in unusual bark deformities and wondered if it was an infection. The growths shown in the photo are burls (also called crown galls) – essentially benign tree tumors.

Burls vs. Crown Galls

Burls are irregular, woody swellings caused by genetic mutation, virus, or injury. They produce beautifully figured wood prized by woodworkers for furniture and veneer. Burls are generally harmless to the tree's health.

Crown galls are specifically caused by the soil bacterium Agrobacterium tumefaciens. The bacterium enters through wounds, transfers gall-forming genes into the plant's DNA, and triggers uncontrolled cell division in the cambium layer. Galls often appear near the soil line but can form higher on trunks and branches.

Disease Cycle and Spread

• The bacteria persist in soil for years
• Entry occurs through small wounds (mechanical damage, insect feeding, etc.)
• Airborne soil particles may carry bacteria to new wounds
• Once inside, the bacterium multiplies as the gall expands

Management

There is no cure for crown gall. Prevention focuses on avoiding trunk and root injuries. For existing galls:

• Do not attempt to remove large galls – this creates new entry points
• Maintain tree vigor with proper watering, mulching, and fertilization
• Monitor for secondary decay or structural weakness

If galls are extensive or the tree shows decline, consult a certified arborist or your state forestry agency for an on-site evaluation.