I have a similar problem. My tomato plants are already setting fruit, but now the plants are turning yellow and look like they are dying. I have them in raised beds, with a lot of organic compost mixed in. We used that fertilizer recommended here (can't remember the name, something like yummy?). We have put in a thin layer of coffee grounds on top of a thick layer of mulch, which is keeping the squirrels out. Like everyone else, we have had a lot of rain. Even though my beds drain well, is all the rain causing the problem?

As with the other post, the question I have is how much additional water are you giving them right now? Spray bioform (fish/seaweed emulsion) or Garrett juice on the leaves and if the soil is very moist, let it dry out just a bit. The tomatoes don't like to have constant wet feet...IMO.

I have not watered at all since we have had so much rain. My beds have yet to dry out, but at least they are raised and drain perfectly.
I thought since I had used that one fertilizer, it would be bad to add more, so I haven't. I guess, at this point, it would be good to try. Interestingly, 3 of my plants are looking very bad, and one is growing well, all in the same bed.

Every year I have the same problem. I always thought it was early blight (and it might have been in previous years). This year I have faithfully sprayed with Plant Wash and plants were doing fine . . . then all of a sudden, yellow leaves. I saw tiny, rust-colored dots on the undersides of the leaves and I assumed they were spider mites. I took a magnifying glass to get a better look and the "dots" were moving. So, now I am adding liquid seaweed to the Plant Wash. Those mites can do a lot of damage in a short time. I've removed the damaged leaves, sprayed again yesterday, and I hope I can keep them away!