Chlorosis

• Chlorosis, or the absence of chlorophyll, is a condition that causes leaves on a hydroponic tomato plant to turn yellow. There are many different causes: poor drainage, injured or compressed roots and nutrient imbalance. The solution depends on which problem is causing leaves to yellow.
  
  

Nutrient Toxicity/Deficiency

• Nutrient imbalance in hydroponic tomato plant food causes leaves to turn yellow. Boron toxicity causes leaf tips to yellow. Calcium deficiency causes the upper leaves to yellow while the lower ones remain green. Magnesium and nitrogen deficiency also lead to yellowing of the leaves.
  
  

Late Blight

• Late blight is a mold disease that causes leaves on a hydroponic tomato plant to turn yellow. This disease spreads by mold spores moving from an infected plant to a healthy one. In the beginning of the infection, either the leaf edges or the center turns yellow. It is best treated by a dry and sunny climate or copper fungicides.