Seeing as how your other two plants, growing in the exact same conditions and are completely fine, I would hazard a guess and say some type of mould or fungus, or even both, have decided to use your little caboose as party central. Perhaps a wound caused during defoliation invited them in? Or hung out the "welcome" sign.
However it happened, no one can un-dead plants quite yet, at least not on Earth.
I would have to advise against defoliation though, being one of those things I love to hate....................cue the lecture.....................................
Each healthy green leaf that is removed, reduces your plants capacity to produce energy, resulting in slower/smaller growth........ or to put it another way...............
Maximum healthy green leaves = maximum energy produced = maximum growth. And that is all I'm saying on the matter ...... for today at least......
Good luck with the other girls. Organoman.