A few yellow leaves at the base of a tomato plant are often of no concern. However, when the plant has a significant number of yellowing leaves and the problem starts with one branch or side of the plant and then spreading, it may be infected with fusarium or verticillium wilt. These fungal diseases impact water-conducting tissues, gradually spreading until it kills the plant. There is no cure.
Identification: cut affected plants at the base of stems and examine them in cross section to see the browning of the water-conducting tissue compared to the healthy ivory of uninfected plants. Refer to the Pest Notes for example photos.
Planting resistant varieties is usually the best strategy. When purchasing plants or seeds, look for labels listing F, FF, or V. If growing susceptible varieties, it's a good idea to avoid planting tomatoes in one location every year. If planting in an area that has had wilt disease, problems can sometimes be minimized by removing all residue, including roots, which may be susceptible, and using
soil solarization before you plant.