If the lower leaves on your tomato plants are dying, it's likely that your plant is suffering from wilt, a disease caused by a type of fungus. Fusarium wilt is indicated by yellowing and drooping of the lower leaves, which will eventually wither and die. In some cases, the entire plant may die. If your plant has verticillium wilt, the lower leaves will wither, but may rebound at night. The plant's growth will be stunted, and the tomatoes will be small. While there is no treatment for fusarium or verticillium wilt, maintenance and prevention may save your tomatoes.
Water your tomatoes correctly; both too much water and too little water can make your plants susceptible to wilt. Water deeply every two to three days during hot weather. Allow the soil to dry slightly between waterings, but never allow the soil to become completely dry. Water the plants slowly at the base of the plant. Avoid watering your tomatoes with overhead methods, such as with a sprinkler.
Spread 2.5 to 5 cm (1 to 2 inches) of organic mulch around your tomato plants to prevent soil evaporation, but keep the mulch 7.5 to 10 cm (3 to 4 inches) away from the trunk of the plant. Use a mulch such as chopped bark or dried grass clippings.
Fertilise tomato plants lightly, but regularly. Use a liquid or granular fertiliser formulated specifically for tomatoes, but use the fertiliser at a rate of half that suggested on the container. Always water the soil before applying granular fertiliser.
Maintain a clean area around the tomato plant. Pull or hoe weeds regularly, as weeds can prevent air circulation and foster humid conditions that can cause wilt. Destroy any infected plant parts immediately to keep the bacteria from spreading to the soil and to other plants.
Clean any tools that you use to tend affected tomato plants, including hoes, shovels or pruners. Wipe the tools with rubbing alcohol or a solution of one part bleach and ten parts water. Repeat before and after each use.
Look for plants that are bred to be resistant to wilt. Varieties resistant to fusarium wilt will be marked "F"; varieties resistant to verticillim wilt will be marked "V." "VF" indicates that the plant is resistant to both types of wilt. Heirloom varieties are more susceptible to wilt, as they don't carry genes that have become resistant.
If your tomato plants have been infected with verticillium or fusarium wilt, avoid planting eggplant, peppers or potatoes in the same location for at least two to three years. These plants belong to the same plant family as the tomato, and can be infected by bacteria that remain in the soil.