Take a photo of the part of the branch that joined to the plant.
Then take knife to the branch, start whittling away slowly, look for discolorations after each cut. Keep cutting until you get through the center. Take photos when you come to something.
Often this can be induced from a physical injury (like birds roosting) or abuse by someone hitting the plant early on, splintering the joint of the branch and trunk.
If you think this really is canker, send a sample to a lab.
Then take knife to the branch, start whittling away slowly, look for discolorations after each cut. Keep cutting until you get through the center. Take photos when you come to something.
Often this can be induced from a physical injury (like birds roosting) or abuse by someone hitting the plant early on, splintering the joint of the branch and trunk.
If you think this really is canker, send a sample to a lab.