Question
I have aok trees with huge baseball size growths on them-they grow about half way up the tree, which is straight, then the tree grows straight up from them. My carpenter friend cut one to see what was inside, and they are beautiful pieces of wood! What causes these? Some of it looks like a combination of red and white oak- can the oaks be two different kinds in one tree?
Thanks, LL

Answer
It is the same oak not two different oaks. These are called burls or crown galls. Burls are basically benign tree tumors that cause growth with an irregular grain structure.  Burls are treasured for their irregularity and the colors associated with this sporadic growth. Burls are used in furniture making.

Crown galls are growths instigated by Agrobacterium tumefaciens, a soil bacteria. These are the burls or 'tumors' that are appear near the base of tree trunks. Sometimes the galls form higher on the trunk, or on branches. The Agrobacterium stimulates the plant host into making the gall. The uncontrolled cell division in the cambium is instigated by genetic material from the bacterium.

The bacteria can live for long time periods in the soil. It is suspected that the bacteria enter the trees through small wounds. The bacterias, seemingly, can be carried on air-borne dirt in the wind. Once inside a tree they insert the gall forming genes into the host, and then multiply as the gall grows.

Note: Not all burls are caused by Agrobacterium. And not all strains of Agrobacterium tumefaciens stimulate the formation of visible galls.