Oxidate 2.0 Broad Spectrum Bactericide / Fungicide does not list black knot as one of the targeted disease on its product label. Bonide Liquid Copper Fungicide Concentrate lists black knot, but the label states it should applied as a dormant spray. I was also able to locate this article that provides really good information about how to treat for black knot disease. I'd recommend taking a look at the article and then maybe contacting your local cooperative extension to see if it is too late to treat for the disease now.
