Crab apple stem
Can you tell me what has attacked my tree. If I prune below will it regrow or is it time to dig it out.
That is a nasty attack of canker. You can try pruning below any sign of damage, give it a couple of weeks to go fully dormant first. If there are still signs of canker in your pruning cut keep going lower until it is clean. Clean your pruning saw after each cut to make sure you aren't speading disease. Also clear away all fallen leaves and clear any grass around the base to a radius of a couple of feet.
I would be inclined to dispose of the tree. Unfortunately it's going to look rather pitiful if you decide to keep it and prune out the canker.
A gardener's work is never at an end - (John Evelyn 1620-1706)