Early blight, Verticillium wilt and gray leaf spot are three conditions that can cause tomato leaves to turn yellow. These three fungi work their way into the root system and damage the stalk, leaves and fruit of the tomato plant.
Early blight causes dark spots and rings to form on the older and bigger leaves of the plant. Gray leaf spot attacks the oldest leaves, causing small black spots that eventually turn gray or brown. Verticillium wilt interferes with the plant's ability to hydrate, turning the plant's leaves yellow during the day. Copper- and sulfur-based sprays can eradicate these fungi if applied promptly.