From the photos, it is not nematodes and it is not TSWV. It is clearly systemic. The indications are typical of verticillium or fusarium. The way to diagnose it is to slice one of the major stems at an angle and look to see if the inside of the stem is either tan (verticillium) or brownish (fusarium). For comparison, you can break off a healthy tomato side stem and slice it open where you should see nothing but pale green with some white vascular tissue.
Here are some questions to ask.
Have tomatoes been grown in this soil in the last 2 or 3 years?
Were they affected? or were they hybrids with known tolerance?
Is the soil a sandy loam?
If one of more of these is true, then V or F is likely.
These plants appear to be in need of some nitrogen. I would suggest an immediate foliar feed of all your plants using seaweed emulsion or something similar to miracle grow for tomatoes.
Another possible issue would be if the plants have been in standing water for any length of time, then the roots could have been killed by lack of oxygen. The symptoms would be similar to what you show but the stems would not exhibit discoloration as for V or F above.
One last possible problem would be exposure to 2-4-d herbicide. Is there any chance someone in the area used 2-4-d recently? It can drift several hundred yards in some conditions. Did you purchase compost to apply to your garden? There have been several recent cases where composted cow manure was contaminated with herbicides that were used on the hay fields. The contaminated compost then acted on the tomato plants.
One of the plants shows signs of flea beetle feeding. You might consider spraying with pyrethrum to get them under control.
DarJones
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