Apple tree- looks like skin cancer
Hello. This intriguing growth is called burr knot, and is a form of adventitious root growth. (Adventitious means root tissue growing from plant tissue that doesn't normally develop roots.) Nothing can be done about this growth and it may just have to run its course; because it is not a disease, no treatments will be effective.
Some sources say this afflicted wood can become weak and more prone to breakage in stormy weather, but we suggest not pruning this off as it will remove too much of the tree's growth and may recur anyway since it can be triggered by the rootstock.
Here is more information about burr knots, https://ipm.missouri.edu/MEG/2015/12/Getting-to-the-Root-of-Burr-Knots-on-Apple-Trees/