Tomato plants develop yellow leaves as a result of a variety of infestations and viruses, and the recommended treatments include pesticides, weed control and transplantation. The specific treatment for a given tomato plant varies with the specific condition.
Yellow leaves that have purple, distorted veins are a sign that psyllids have infested the tomato plant. Treatment involves looking on the bottom of leaves for insects, removing them and applying a pesticide to the growing area.
Tomato plants that stop growing and turn yellow all over may have the curly top virus. The beet leafhopper transmits this disease by jumping from plant to plant. Row covers form a shield against the leafhopper, but no known chemical treatments manage this particular pest effectively.
Yellowing that takes place in the summertime, right before the leaves drop off, may come from early blight, caused by a fungal infection. Removing all affected leaves is the best treatment, followed by a treatment with sulfur dust. Planting tomatoes somewhere else the next year is recommended to avoid the problem.
Yellow leaves with a sticky coating may have been infested by aphids and whiteflies. Treatment protocols include an insecticidal soap for the leaves, but this condition does not harm the tomatoes themselves.
One final cause for yellow leaves is Fusarium wilt. Older leaves turn yellow first, dropping at the same time. Fusarium wilt is a fungal disease that takes up residence in the vascular system of the plant, which delivers water to the leaves. Discolored roots, or a brown discoloration of the lower stem, are symptoms of Fusarium wilt. No treatment exists for this affliction, and it is suggested tomatoes should be moved to a different spot for the next year.