Do you have a photo to post? The black edges sounds like a temperature problem. Is the foliage very fussy about temperature?

When I was a teenager in Indiana where catalpas are native, I would gather the huge caterpillars found only on catalpas for fishing. Catfish went crazy for them. These insect larvae were black and yellow-striped, I recall. They were numerous and could strip a tree of leaves in a few days. The trees did fine, regardless of the onslaught.

I do not know if these caterpillars are also found in MN. Still, I suspect some insect pest.