Tomato plants turn yellow for several reasons, including insect infestation, viral infections, too little water and too much sun. Most of these problems affect only the plant's leaves, but the fruit can also shows signs of distress.
The psyllid is a common insect that affects tomatoes; it leaves behind distorted stem growth and yellow leaves that later turn purple. Aphids and whiteflies also cause tomato leaves to yellow, although the damage to the plant is usually minimal. Fusarium wilt is the result of a fungus that causes the leaves to discolor and turn downward. The curly top virus is spread by the beet leafhopper, and it turns the leaves yellow and stunts the plant's growth.