Black knot on the plums can get very bad quickly. Especially if its a cultivar thats very prone to it. Stanley Prune is the worst, speaking from experience. Almost all of my dozen varieties came down with it last year. Even the ones that were reported to have moderate resistance, like Santa Rosa , and Shiro. They are covered. My Bruce and Methley are gone from it now. I ordered new trees this spring to replace those lost . Supposedly, The President has the best resistance to Black Knot, and I think it was Early Italian, or something like that. Will know in a few years. The best solution is to replant with trees that have the best resistance, and Plum Trees grow fast, so plan on getting them in soon. Not sure what is happening yet, but my Superior and Alderman havent shown any Black Knot yet.