Could be tomato blight or Fusarium wilt. Maybe Didymella stem rot fungus. I would try to get to the point where your tomatoes are harvestable, and then pull and destroy. The spores are probably in the soil, so you may want to relocate your tomatoes for a few years. That being said, there are resistant cultivars.

The blight has reared its ugly head again on my tomatoes. I tried all new cultivars that are resistant to early blight and/or late blight, all except ONE -- Sweet Million. And that is the one that the blight is showing up on. I picked off some leaves today, if it spreads further up the plant I'm going to have to take the whole thing out. It's been a couple years since I've had good tomatoes because of that bloody blight, I finally have some nice tomatoes setting on these new varieties and want to harvest at least something this year, I'll sacrifice the SM if I have to...

What the poster's plant has is Septoria leaf spot, it's not early blight, late blight, Fusarium wilt or even Verticillium wilt. All these are easy to tell from another.

MXK3, if you just spray your plants with a fungicide every 10 days you can easily save the plants from Septoria leaf spot or early blight if the plants are healthy and not deprived of nutrients. A weak plant is more susceptible to fungal attack.

There is a difference between these 3 and easily identifiable and nothing like below.

Fusarium wilt, mistaken for nutrient deficiencies and water issues because it blocks water uptake to the plant. WILT.

Verticillium wilt, nothing like Septoria or the 2 blights. Also mistaken for watering issues because it also blocks water uptake by the plant. WILT.

At least in the early stage, Septoria leaf spot usually doesn't affect the stem. OP says the problem started with the stem. Seehttps://extension.wvu.edu/lawn-gardening-pests/plant-disease/fruit-vegetable-diseases/septoria-leaf-spot

I can ensure you it's Septoria leaf spot and it does without question root in the stem of the plants. I've been dealing with it for 20+ years including this year and know it well. I saw it two weeks ago and cut off infected bottom branches on my tomatoes and sprayed copper, and was going to spray chlorothalonil today but the massive rain storm last night keep me out of the garden.

I get it every year and usually after wet humid weather and keep an eye on it. The last 3 years were hot and dry and I didn't need to spray, this year is wetter then I can remember so I'll be spraying all season. It has to be stopped before it gets like the OP's plants and I've had that too, but it's to the point the plants aren't worth spraying because with so little vegetation left the tomatoes don't taste good and most tomatoes get sunscald.

People are told it's something else and they must pull it or else, but this isn't true with Septoria leaf spot or early blight. Just treating it with a fungicide will stop it but the plants must be healthy and well fed, so it's worth learning how to tell the differences and how to deal with the diseases.