Q: This canker has been on my oak for many years. Why has such a large canker not killed the tree?

A: Galls and cankers don’t necessarily always kill their host. Many are symbiotic, like cedar apple rust galls, that require living hosts of apple and cedar trees in order to reproduce.

Other galls are caused by insects (like witchhazel gall) which do no obvious harm to their host.

Your gall (canker) appears to be crown gall, caused by the bacteria Agrobacterium tumefaciens. This bacteria causes tree cells to reproduce wildly. If the infection is in the outer layer of bark and doesn’t disrupt the transport of nutrients up and down the trunk, the tree could live for many years.

It will be somewhat weakened at the gall site, and may be more likely to fall in a storm, but the bacteria and the tree may live together happily for many years.

Read more here.