Dear Roger,

       Thank you for your question. I have had problems with black knot myself and have had some experience with dealing with the fungal disease.  It would be helpful to see a picture of the extent of the problem.  Removal of the affected areas is key to dealing with the disease.  That said, if you have a tree completely engulfed, it may be difficult to treat.  Branches should be removed back at least 2-4 inches past the area of infection.  There is good information about dealing with black knot in Bulletin 940, Midwest Home Fruit Production Guide. I find it very helpful in my own home orchard. It is available  to purchase through Ohio State Extension. 

As to which fungicides to use, I would of course check labels to see which ones will help with black knot. If there is an indication they will help, it should state so on the label. Label directions should also include how often to spray and at what saturation.  

I have included some resources to better help deal with black knot.  The University of Minnesota article lists several active ingredients with will help, captan and lime sulfur being some of them.

Black knot | UMN Extension

Interestingly, Cornell does not recommend   the use of fungicides in home gardens but they also provide helpful information here: blackknot.pdf (cornell.edu)

Additionally here is an Ohioline article that will be helpful. 

Black Knot of Plums and Cherries | Ohioline (osu.edu)

If you have additional questions, feel free to send me a followup  question.  I would like to see a picture of the black knot infestation if you could provide one.

Sincerely,