Black knot canker on plum trees

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I have 4 plum trees (Stanley prune, seneca, mirabelle, yellow shiro).
The Stanley plum was the very first tree to show the canker. Two years later it appeared slowly on the other 3 plum trees as well.
Now, every one of them is exhibiting black canker on their branches in varying sizes.
The Shiro plum tree even had one large canker area(6 by 10 inch size) on its trunk! (I had applied a paste of ground up corn to the site and held it in place with a wrapping of landscape fabric. The site healed after 4 months).
Over the years I had pruned away every bit of canker I saw, but this year I just have too many of them and would have to cut away more than half the tree to get rid of them.
Is there any cure for this type of canker?

I live in PA, zone 6.