Answer: It sounds like your tomatoes have been struck with fusarium wilt, a fungal disease that causes plants to yellow and wilt. To confirm this diagnosis, split an affected stem lengthwise and look for a brown streak between the green "outer bark" and the cream colored stem interior. There's no curing it, unfortunately, but you can try to keep plants alive til the fruit ripens. The good news is that next year you'll know to plant a tomato variety that is labelled "VFN" (resistant to verticillium and fusarium wilts, and nematodes). Best of luck!
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