That plant is showing septoria
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and a mildew
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Tossing it is not going to stop the other plants from getting it because even now they have it although you might not be seeing it manifest yet
Recommend you treat them all with one level tablespoon of baking soda per gallon of water asap
but recommend using liquid copper fungicide for a better and more lasting remedy; apply as recommended on bottle for tomatoes
Edit: also strongly recommend increasing air flow; I use three fans in every tent