First attempt with pictures failed to post. Here are the pix.

How long have you grown tomatoes in that bed?

I think this is the 4th summer.

I think you can't really strategize for next year until you figure out what happened this year. The photos don't appear to show a fungal foliage disease or foliage pest, but I can't really see that much detail in the photos. Based on what's visible, I would suspect that it's either too much watering rotting the roots or a soilborne disease.

When you pull the plants, you should probably do various tests to see whether you have a soilborne fungal or bacterial problem, i.e., putting a cut stem in water to see whether you get milky discharge (bacterial wilt), doing a longitudinal cross-section to see whether the vascular tissue right under the skin is brown (verticillium/fusarium), etc. You should examine the roots as well for nematodes, if you're in a problem area for them, and for root rot.

A professional soil test done at a lab is probably a good idea, too, mostly because it's always a good idea to have one done if you have any serious garden issues. And consulting your local county extension office is probably a good idea, too. Maybe document with photos and take in samples of the main stem and roots to see whether someone can give you a diagnosis, if the local office performs that service?

Thanks for the advice. It looks like I've got a mystery to solve. The county extension plant diseases office is open next Tuesday, so I guess I'll pay them a visit with some samples.

Take the pictures with you.

I'm pulling a few up and bringing them with me. They really helped me with what turned out to be cane borers in my Marionberries.