Tomatoes are dying! What is it and can they be saved?

These are from 3 of 4 tomatoes in containers. One is a bush cherry, the other a bush goliath, and the small ones are little yellow cherries. I water "as needed," which is about 2 or 3 times a week. I am using TomatoTone, but only about two times all season.



At first I thought it was just too much water (we had a wet early June), but then after drying out the problem seemed get a little better, then get worse, then get better, etc. This is the worst it's been. Recently I thought it wasn't enough water, but two watering sessions a day hasn't helped.

As you can see, there are some SERIOUSLY dead stems near the bottom.



I cut open a stem the inside doesn't look like the fusarium wilt stems I see online (photo attached) - but man the leaves and sure look like what I see in photos online.



Is this some other disease? Is it some sort of watering issue? Should I fertilize more often?

Yellow Cherries:


I have a roma plant that has seemed to dodge the problem - until a few days ago, when I saw yellowing leaves.

I fear my plants are dying! What can I do to diagnose the problem?



(Also, I have seen small gnats on top of the mulch in the containers. Is this a problem? I heard it was only a worry if the plants were young.)