It looks like some kind of fungal disease, like verticillium wilt or fusarium wilt. I know it is awful to do, but given how close the plant is to lots of other plants, you would probably be best to pull and get rid of it (NOT in the compost pile) before it spreads. Disease like this can ruin your whole garden.
I'm in Ohio too (Cincinnati) and it has been VERY wet, all season, which is conducive to all the fungal diseases. In your case it may be compounded a bit by how close you have everything planted. Good air circulation is important in helping prevent fungal diseases.
If you really want to try to save it, remove every leaf showing signs of infection and dispose of them then make a solution of milk or yoghurt and water and spray everything that is left. I recently did this in my garden using 50:50 yoghurt and water. (Helpful Gardener recommends milk, but all the milk is pasteurized, I think you want live Lactobacillus). The yoghurt doesn't smell very good and draws flies (at first), but it worked beautifully on tomatoes that were starting to look a little like yours and on some other things that had powdery mildew. Spray tops and bottoms of leaves, and spray the neighboring plants to the infected one, even if you don't see infection.