Verticillium wilt? Fusarium? Could be some nasty soil fungus like that.
At this stage, it has probably infected the whole plant, so there is not a whole lot of hope for this crop. The usual suggestions are to plant another crop next year that is a different variety with some resistance.
You may, and this is a longshot, you may get some help if you douse it with an aerated
compost tea that can outcompete the fungal infection. This is a longshot because the compost tea application will be foliar, while the infection may be in the stems and the
roots.
I am a strong believer in the power of cover cropping mustard to reduce fungal soil pathogens. There is controversy in the literature whether this works or it doesn't, but I tried it and it worked for me. Or
should I say it was one of the things I did and now I have much fewer problems (I do have one tomato seedling that is off to a suspiciously poor start this year).
Long term, you should keep adding rotted organic matter to your soil to build up the good soil fungi. With
enough good soil fungi present, the pathogens can't compete and they die off.