We have a non fruit bearing tree that blooms in May (white flowers) that...
We have a non fruit bearing tree that blooms in May (white flowers) that dropped its leaves mid-summer last year and its leaves are yellowing and starting to drop again this year. I suspect apple scab, but am not sure.
Ramsey County Minnesota apple scab
It's possible that your tree is a non-bearing crabapple. they are suseptible to apple scab.
Here is a link that will help you identify this disease, and offer some suggestions for control: