That is an interesting photograph. This doesn’t look like the damage caused by insects or fungi. That said, it does look like there was a leaf injury and perhaps secondary infection set in. 

I would remove the leaf and put it in your yard compost. We have had a very wet spring and early summer so many things that would not normally be an issue, are exacerbated by all the wet. Be sure to continue to clean up all fallen or dying leaves. 

Rhubarb is pretty hardy in Minnesota but it can suffer from root rot if not in a well drained area. 

https://extension.umn.edu/vegetables/growing-rhubarb