What you should do is contact the appropriate local agency in Ontario. Here in the US we have two resources: state university ag extension, and state forestry services. But in the meantime I can tell you about my experiences with this black knot fungus. I am a timberland owner and my most valuable product is high quality black cherry logs, especially veneer grade logs. These are very valuable and can bring $6 per or more per board foot in the log Black knot fungus is common in my woods and I have done little or nothing to try to control it. Usually it is not a problem. Occasionally it will get into the trunk of a tree and cause a serious canker. This normally does not kill the tree, but in a rare case it will weaken the trunk and the tree may break. Usually, however, it just serves to degrade the log, but most times I can buck the log at so I cut out the canker and the loss is minimal. These cankers are not the source of any rot that progresses through the tree. Most of the time the fungus causes small cankers on twigs, this kills the twigs and they die and fall off. In a few cases I have seen an individual tree with hundreds of these twig cankers on them, and then a few years later they are all gone and the tree in unaffected. Many trees never seem to get any infections at all. So, what I am tellling you is, you probably do not have a serious problem. If you can reach and prune off the cankers, and dispose of them so no spores are released, I would do that. If you have a tree growing with a canker on the trunk, you may want to cut that tree down if it will make more room for a better tree to grow. But a tree with a black knot canker on its trunk can live a long time and produce much valuable wood and/or provide the other kinds of benefits a good black cherry tree can provide. But you need to check with whatever agency you have that can tell you about the problem in your area and what, if anything, you should do. In the meantime, you can look in the woods there and see how much the black cherry in your area is affected. --Spruce

They can be burned or buried. I'd do this in the winter and definitely before ascospores discharge in spring. Waiting until the tree shows decline will only allow the disease to spread.

relatively speaking ... cherry is a short lived ornamental tree ... 20 years is doing pretty good ... it might be time to start thinking about replacement as in my experience.. once a tree gets BK .. it will always have BKnot ... and there isnt much to do about it other than keep hacking away at it.. until you arent happy with what is left ... a pic would help us.. help you .. of the tree in situ ... if you have space.. it might be time to plant something else.. for the future ... outside the BK family of hosts.. so that when you are ready to give up on this one... you already have a well established tree ... [unless of course.. if its jammed right up against the house on a small suburban lot] here's some info.. note i converted to the latin name .. for better links:: https://duckduckgo.com/?q=Prunus+padus+black+knot&t=ffcm&ia=web ken

the best idea.. would be to plant a new tree .. and mess around with the old tree .. but then.. odds are.. the new tree will get it too ..... sooner or later ... and that is why i got rid of all my plums .. and various other susceptible trees ... they just irritated the beegeebees out of me .. and it didnt matter they were gifts form dad.. GRHS ... ken