Trying to diagnose my tomato problem

[lycomania]

Hi All,
I'm having a major yellowing leaves problems with some of my tomato plants. It doesn't look like a foliar disease, and I don't think it's vert or fus wilt, going by the symptoms, but I could be wrong. The last picture is a bisected stem.

I had one plant get one of the wilts, and it would wilt before the leaves started dying. That's not happening with these plants. They mostly yellow first, starting from the bottom. I've been growing tomatoes here for about 8 years or so, and I've never seen it.

It's spreading upwards quite quickly. Help please?!