Where are you located? That looks like textbook fusarium.

Maryland, near DC.

Contact your county extension office and get a diagnosis. I suspect that you have fusarium in your soil, though. That bright "yellow flag" symptom on one leaf/one side of the plant/one side of a leaf is pretty classic for fusarium.

If so, you'll have to grow your tomatoes elsewhere for a while or stick to resistant cultivars or plants grafted onto resistant rootstock.

In the meantime, adding obscure soil amendments without a soil test is shooting in the dark. Better to use a balanced fertilizer for tomatoes or fruiting plants.

Well, fusarium isn't an emergency, as it spreads pretty slowly in soil, and it's not contagious through air. There's also nothing you can really do for your plants if they have it.

As it looks like it struck late and your plants have fruit on them, I would give the plants a good feeding with a liquid tomato/fruiting veg fertilizer, just in case your issue is related to nutrient deficiency/nutrient leaching due to overwatering. Cut back on watering and only do so when the soil a couple inches down is dry.

If/when your plants go downhill to the point where you want to pull them, examine the roots for the galls of root-knot nematodes. Take pics of the roots and post them here if you're not sure what to look for.

Also, before you throw your plants away, cut the stem both cross-sectionally and lengthwise near the base and look for browning vascular tissue. It will look similar to this and this.

If you find the brown vascular tissue, that's a pretty definite diagnosis of fusarium in addition to the "yellow flag" and wilting symptoms. Root-knot nematodes often provide the damage that lets fusarium and other soilborne pathogens into the roots, which is why it's good to check for them, too.

If you find fusarium, you will have to plant solanaceous plants (tomato, pepper, eggplant, etc.) in a different location for the next few years, grow in containers, stick to F resistant hybrids, or graft non-resistant varieties onto F resistant rootstock.

If you find RKN, that's a slightly more annoying problem, as they can hurt thousands of species of plants. For many veg, tomato included, there are RKN resistant varieties, though. Many resistant tomato varieties are both RKN and F resistant.

Thank you for your words of encouragement! It sickens me to see the plants dying after doing everything I could this year that was right - I appreciate it!! Would you suggest I cut the infected branches off the plants and would they show the symptoms like the stems would? And...when you say a liquid tomato fruiting veg fertilizer - what specifically would you suggest?

I would remove any yellow or dying foliage because it's not contributing to the plant at all. If there is a stem (not a leaf rachis, but an actual stalk of the plant) that is mostly dead and you want to sacrifice it, you can cut that and check for browning tissue inside. But sometimes it takes a while for the internal browning to make its way up the plant, so the surest diagnosis will come from cutting the main stalk after you pull the plant.

For liquid fertilizer, I'd just get whatever's easy to find locally. Dr. Earth Home grown liquid is a common organic option. Miracle Gro Tomato formula (not the regular general plant formula) is a cheap and readily available option in many stores. You maynot want to use it regularly all season in beds (though it's good for containers), but it's probably one of the more quickly bioavailable liquid formulas for a "quick rescue" for nutrient-deprived plants and good to have around for that reason. It's in powder form, so shelf life isn't too much of an issue.

Just make sure to properly clean your pruners between plants, and don't move soil around in your beds. If you have to dig in beds with suspected fusarium or soilborne disease, then wash your tools well afterward before using them to dig in other locations in your garden.

i clipped off a few larger stems and am attaching a photo here of the inside - which doesn't look brown in the core, a little on one side but not the middle...?

As I said, to really know, you'd have to examine the/a main stalk. This doesn't rule out fusarium.

Also, the vascular browning wouldn't necessarily be in the core--you want to look for a browning in the layer that contains the plant's circulatory system, which would be a layer between the outer layer and the pith/core, although the pith can also be affected.