This may be apple scab, the most common problem affecting Minnesota crabapple trees.  The U of Mn has prepared a detailed worksheet, including photos, that explains this problem and how to deal with it.  The U recommends cleaning up the leaves in the fall to help prevent spread of the disease.  It also talks about using fungicides to fight the problem.  Here it is:
https://extension.umn.edu/plant-diseases/apple-scab