There are a number of possible reasons for the splits in your plum tree bark.  Dry weather, which we've experienced the last couple of years, followed by wet conditons can cause significant tree growth, leading to splits in the bark.  Suncald, especially in winter months, can also cause bark injury to thin-barked or young trees, such as plums.  Borers aren't often seen affecting the bark on plum trees in Minnesota.  You might trim the wounds with a sharp knife.  This helps the tree compartmentalize the exposed area.  Consider too wrapping the damaged area with a light-colored tree wrap.  This can help the healing process.   

For more information about plum tree problem s, see the following links:

https://extension.umaine.edu/fruit/growing-fruit-trees-in-maine/diseases/
https://minnetonkaorchards.com/plum-tree-diseases/