...another Dave.. Welcome 

A mature tree should be able to handle this bacterial infection.  Do disinfect your pruning equipment after working with that tree, or any trees…  Hitting your tools with Lysol spray disinfectant is a method some use.

I’d been told it (Burr Knots or Ariel crown gall) could be removed, and followed a fellow’s instructions of ‘chiseling out’ the burrs, until I could ‘no longer see the purple vanes,’ then spraying the wound with liquid bleach…  Twas a waste of time & effort, providing an entry point for other infections.

A bacterium, it appears nothing can remove it.  Some suggest removing the ‘entire tree, along with it’s soil.’  It had spread throughout my orchard, some trees being more susceptible than others ...all apple trees, I believe..