Answer: Your plant might have a wilt disease, which can be fairly common on tomato plants. Fusarium wilt causes the leaves to turn yellow, then brown, then die. Lower leaves are affected first. Fusarium is a soil inhabitant, and it infects
plants through the roots. Fusarium overwinters in the soil or plant debris. Once established, this pathogen can remain indefinitely, although its populations may vary. Cleaning up crop residue at season's end can help. The best solution is to plant resistant varieties. The fruit is safe to eat. It's also possible that your tomato plant is just showing signs of "age" at the end of the season and the leaves are dying off naturally. In your case, I'd recommend throwing the soil away just to be on the safe side, as once these diseases get into the soil it's hard to eradicate them.
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