I have found that one of my mint have little brown spots in the reverse of the leaves (you can notice them in the other side too).
I don't know if you can see it well in the photo. At first sight, I thought they were eggs from some insects but I tried to remove them with my hands and I couldn't so I don't know if it a fungi, eggs or...
To make a comparision they are like the spores of ferns.
Do you have any idea?