Hi Johannian,
I used to have a coworker that worked in an apple orchard and said that botryosphaeria canker was a very common problem. Goes after trees that have been stressed by things such as freeze, drought, poor management, etc. Not something you can spray for. Can only improve the general health of the tree to help it fight the infection. It looks like the main stem of your tree is dead for a large part of the circumference---I'm not sure if it is saveable at this point.
A link to some info:
https://ipm.illinois.edu/disea...
