Whatever is causing these kinds of symptoms seems to be a mystery on here (I don't recall a definite answer ever coming up on what would be causing it). I remember FadeToBlack had the same sort of thing happening too not so long ago. One thing that stood out to me was that both what he and I had, it appeared to 'spread' and didn't affect certain varieties.... if that is of any help.
Gas I remember that well, tried to find it earlier but no luck.
I decided to sacrifice a plant for the cause, cause I want to know what the heck is going on. Looks like root rot but it isn't real bad. Guess that's why the plant is still alive. I did the hydrogen peroxide thing just to see if it will make any difference. Too late in the growing year to get too crazy with trying to save it.
Here's the pics. This is the plant that is pictured above on the right.
Was the plant wilted for a really long period of time with sun and or lots of wind? I want to say windburn but it could be some kind of pest damage also. The roots look perfect, it's not root rot or the entire plant would be wilted with foliage dropping.