Psyllids

• Psyllids are insects that feed on fruit and foliage of tomato plants. Their feeding leaves a toxic salival residue on leaves and other parts of the plant's anatomy that turns leaves yellow. To confirm a psyllid infestation on your tomato plants, check the underside of the leaves for yellow or green psyllids in nymph form. Psyllids can be effectively controlled with a spray of sulfur on the topside and the underside of leaves.
  
  

Early Blight

• Another possible diagnosis for yellow tomato leaves is early blight, a disease caused by the fungus Alternaria solani. The first symptoms of early blight are brown and black spots on leaves, but as the weather gets warmer leaves turn yellow, dry and brittle and defoliate prematurely. To control the disease, remove all infected parts of the plant and dispose of them immediately. Allow proper air circulation to avoid spreading the disease to other tomato plants. Severe infections can be treated with sulfur dust.
  
  

Fusarium Wilt and Crown Rot

• Fusarium oxysporum is the fungal agent of fusarium wilt and fusarium crown rot, two of the most common diseases to affect tomato plants and cause yellowing tomato leaves. Depending on the strain of the disease that is infecting your plant, yellow leaves either turn downward and droop or turn brown and black, wilt and eventually defoliate. No fungicides are currently rated for control of fusarium wilt or fusarium crown rot, so the best way to protect your plants is to buy one of the many tomato cultivars that are naturally resistant to these diseases.
  
  

Other Possible Diagnoses

• A great difficulty in diagnosing tomato sicknesses is that symptoms such as dry, yellow and defoliating leaves can often be the result of simple causes, such as drought stress and underwatering. To see whether this is the case, move to a more aggressive watering schedule and see whether your tomato leaves return to their normal color. Precise watering schedules depend entirely on individual climate conditions and on the tomato cultivar you selected; consult a gardening expert or university extension office for accurate watering schedule recommendations.