Prunes/Plum

I have cankers on my prune tree this looks like a black cancer looking
growth on my tree.  I have pruned off these growths cleaning my pruners
after every clip but they still come back.  Is there any treatment?

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OhioBeeFarmer
Getting kids involved in Beekeeping
http://www.homestead.com/BeeKeepers/BeesRUs.html
http://www.homestead.com/BeeKeepers/Opening.html