Not a disease.

Instead, it's environmental stress -- more specifically, heat and/or drought. Did you have a recent hot and/or dry spell? Or did the soil get too dry?

I had the same problem this year with my violets which are grown in pots on my NYC deck. I cut them down completely a few times and they kept re-emerging with those same markings. I saw tiny red dots, ants and other bugs in the soil. I finally took some rootings, rinsed them in soapy water, re-potted them in a clean pot with Peat and so far no sign of the blight. I lost most of my beautiful violets. I noticed the same blight in violets planted in a yard in my neighborhood.