Answer: If all your other plants are doing well, I'd check the leaves of the yellow tomato plant for insect problems, and dig down around the roots to check for soil moisture. It may be in a high spot or a low spot where it isn't getting the right amount of moisture. It may not be too late to dig the plant, amend the soil with organic matter and then replant. Make sure the soil is moist but not soggy wet around the root system.
Best wishes with your garden!
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