If you spot black knot galls, find a pair of gardening shears or scissors and sanitize them with 70% rubbing alcohol. In such a way, you will prevent reinfection.
Using a pruning tool of your choice, remove at least 6-7 inches of the branch around the gall. If possible, trim off up to 10-12 inches. If there are numerous large knots on the stem, remove as much of the branch until there is only healthy tissue left.
Aggressive pruning out of the unhealthy segments ensures that you remove mycelium - the microscopic vegetative part of the fungus. Mycelium will continue growing if left on the plant.
If the tree is infected severely, remove it altogether.
After the black knots have been removed from the plant, they can still produce spores. Therefore, you should destroy the infected parts by throwing them in the trash, burying, or burning them.
After pruning, keep monitoring the tree in the following months and years.