Answer: There are three possibilities that come to mind. First, if there is a black walnut tree nearby, the toxin produced by its roots, which extend beyond its branches, can cause such symptoms.
Another possibility is that you have a strain of a wilt fungus in your soil that is not affected by your plant's resistance. You may notice tomato varieties with VF1F2N after their name. This indicates resistance to two different strains or races of the Fusarium fungus.
Next year, you should try planting the tomatoes in another location or in containers filled with a good clean growing mix. Another option is to try varieties that boast multiple strains of a wilt fungus.
A final possibility is that the plants were not the variety that they were supposed to be. You indicated the nursery was reputable. However, not all such mix-ups are intentional. There are likely several places up the line from you and your nursery where an innocent mistake could have been made.
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