The problem with black knot disease is that it's hard to detect. My tree was infected with this fungus two years before I saw any sign of the problem. The fungus grows under the bark and is invisible until the branches begin to swell. Rain spreads the spores.
What to Do to Treat Black Knot Disease
Check your tree each spring.
If you find rough swollen branches, cut them off.
If you find black gulls growing, you are in big trouble. These must be cut back at least six inches beyond the healthy part of the branch.
Every single branch with these gulls must be cut—you can show no mercy.
The cut branches must be removed and disposed of in such a way the spores cannot escape into the air or be exposed to rain. If you leave them for curb pick up, you might want to label the bag. It is probably not a good idea to burn them, but you could cut them up and bury them.