Answer: If you've fed your plants and are watering regularly, there may be a disease working its way through your plant. I can't tell from your description whether or not a disease is present, but tomato plants can fall victim to early blight, late blight, and a whole host of other problems. Fusarium wilt is a fungal disease that causes plants to yellow and wilt. If your plants are yellowing and not wilting, it may be that they just need to be fed. If they are wilting, it may be too late to save them. 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 labeled "VFN" (resistant to verticillium and fusarium wilts, and nematodes). Best of luck!
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