Please help: what is causing this?

[Leafer]

And, should I be worried about it? A little background: this is a container-planted plant, a Pruden's Purple. Its container-mate is a Beefmaster that is starting to show signs of the same yellowing. Also, I have two more of the same plants planted a few feet away in bags, and they are starting the same thing. It is all bottom leaves so far, and they appear to be dying after yellowing. Oh yeah, they are about 36 days after transplant, a week or so into fruitset. Thanks in advance.