"Your cherry tree has a fungal disease called black knot canker. Although the growth looks cancerous, it's not truly a cancer.
The canker fungus can completely encircle a stem or branch and kill the branch from that point outward. If the tree is highly infected, it can weaken it to the point of setting the tree into decline from which it may not recover.
The source of the fungus is usually wild chokecherry trees. The fungus spreads through the air, so it is difficult to control. It only affects stone fruits (cherries, plums, chokecherries, and to a lesser extent peaches, nectarines, and apricots), so those are the only trees that you need to worry about contracting the fungus.
You may be able to keep the disease under control by removing wild chokecherries nearby that harbor the disease and by pruning out and destroying the swollen branches on your affected cherry. Fungicide sprays of captan or thiophanate-methyl in spring just before the buds open can help prevent reinfection, but won't eliminate the cankers that already exist."