The appearance of yellow leaves on your tomato plant is usually not a death sentence. However, it does mean that your plant has a problem in need of attention and correction.
Tomatoes need nitrogen; their leaves will turn yellow without it. Mix some compost or used coffee grounds in with the soil to determine if a lack of nitrogen is the problem.
If the bottom leaves of your tomato plant turn yellow, it's probably lacking in important nutrients. It may not be getting adequate sunlight. Move the plant to a sunnier spot, if possible, and mix a little tomato plant food into the soil.
Common fungi that may turn your tomato leaves yellow include early blight. In these cases, remove plant debris from the surrounding area. Apply sulphur dust to protect new growth.
Curly Top Virus
This virus--which is carried by the beet leafhopper--causes leaves to turn yellow. It also causes them to crinkle or curl before the plant's growth becomes stunted, or the plant may simply die. There is no known treatment to cure this disease.
Whiteflies, psllids, worms, aphids and caterpillars create pest problems that will turn the leaves yellow. They will also harm the plant. Application of organic pesticide is the best and safest solution.
Too much or too little water can cause tomato plant leaves to yellow and wilt. Tomatoes require a good deal of water, but adequate soil drainage is important for the health of the plant and its fruit.