It's probably Septoria fungus but it might be better to show a sharp close up of the infected circles from above and undersides of leaves. Not that it will make a huge difference though it would be nice to see.

Daconil is your best friend as a reasonable anti-fungal though mainly used as a preventative, if you can get ahead of it and ride out the season before you lose the leaves. There is another thread going on right now which covers what to do well about pruning & disposing of waste. I'd follow it but use Daconil.

how to treat leaf spot (other current thread)

also, you can get further info here:

Rutgers, great page on Septoria

Best luck

PC

PS Don't know what your climate is like in OK, but if the heat sets in soon, it probably will help you burn it off. It is important to get air circulation on the lower part of the plant so the fungus doesn't get an oasis to live in. Its enemies are dryness and heat as you probably know! I think above 82 F Septoria gets burnt off.