Answer: Tomato plants naturally lose their oldest (lowest) leaves as they become unproductive. As long as the rest of the plant looks healthy and is putting out new growth, it's nothing to be concerned about. Just trim off the yellowing leaves.
However, tomatoes are subject to a number of diseases, including late blight. This will also cause small, dark, "watersoaked" patches on the leaves and the tomatoes will have greenish grayish spots on them which then turn brown and hard.
If there are no watersoaked spots on the leaves, it's probably nothing to worry about.
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