Royal red cherry tree leaking sap...canker sores?

[boxturtle]

It's been in ground for one season.  I noticed it leaking sap couple months ago.  Today I went a removed one of 3 sap balls and this is what I found.   Area effected is about 2 feet off the ground. I did buy the tree from Armstrong nursery  but would hate to replace if I don't have too.  What is it and how can I treat it. 
  Like always your input is always appreciated