I learned about black knot when I was doing home consultations on growing vegetables. This customer had an ornamental plum that had black knot. I was clueless about it so I had them call in an arborist. He said the best thing to cure the disease was a product that contained T-Methyl (Thiophanate methyl). I remember this because the disease on this tree was so striking that I wrote everything down And when I saw the picture you posted it reminded me and I checked my journal. What he used on the tree was called Clearys 3336. It is only used on ornamentals because it is a systemic. If your tree is fruit bearing don't use it. I drove by the tree a couple of years later and it was looking much much better than the last time I saw it, so I guess the stuff works.
