Cut the wounds out conservatively and spray the wounds with chlorathalinil. In the future, cut out the BK as soon as it appears. You are probably wasting your time with other fungicides judging from Cornell recs.

thanks hman!

I just cut them out and that worked every time but once when a little came back by one edge; I cut that little bit out and it was all gone. Spraying would help more but for me it was not needed.

I don't consider black knot a major problem, just keep alert and you can nail it. Make sure to look for the green knots as well, thats the color when they first form.

Scott

How bad a problem it is depends a lot on the site and region, as you know Scott. Sometimes it is a major hassle to keep up with all the cutting necessary at a couple of sites I manage. I've even had to cut down Methely plums on a couple of occasions because it is a typhoid Mary of black knot. On large trees it can get out of hand on that variety.

A site with high dew, eastern shade and nearby infected cherries is a perfect storm, but sometimes it has been a problem at sites where I couldn't figure the cause. On my property the issue is driven mostly by eastern shade and the plum part of my nursery is now in the best eastern exposure on my site.

For the first ten years I grew plums here it was never a problem, now I have to cut out all the black knot every month during the growing season in my nursery and some of the orchards I manage.