Charlie,
I am not an arborist, but I believe the tree you have there is a variety of Crabapple. Many older varieties are prone to a variety of diseases. The black 'galls' look like Black Knot disease, but I suspect that it may have scab and fire blight as well. If it were me, I would remove the tree entirely, burning as much of it as possible to kill the pathogens and I would definitely NOT replant any type of plant that is prone to any of those diseases.
Jennifer
