Certified GKH Gardening Expert
This is a sucker that was let to get very mature. They steal valuable nutrients from your grafted tree, and need to be removed when young. You will find, if you follow the branch down, that it originates from below the graft point.
Removing it now will be the best option, but keep in mind that once they get this mature they can suffer from infection when pruned.
Make sure that you shears are sterile, and you wipe the cut with peroxide or a 10 percent bleach/water solution after pruning.
These articles will help: