Answer: I'd check for insect populations on the undersides of the leaves. Whitefly feeding can turn leaves yellow, as can feeding by aphids. If you find no pests, the next most common cause of leaf yellowing is high humidity. Eggplant likes moist but not soggy soil so you may want to adjust your watering schedule as a first step, then look for insect pests. There's not much you can do about humidity except to space your plants well so they get adequate air circulation all around.
Best wishes with your garden!
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