The leaves and stems of a healthy tomato plant are bright green, and the leaves splay outward. An unhealthy tomato plant, on the other hand, exhibits yellow, curling leaves with stems that grow soft and weak. In some instances, you may be able to save the plant, but if the case is severe, the plant will likely die.
Fusarium wilt is a disease found within the vascular system of the tomato plant. The fungus is soil-borne and invades the plant through the root system, causing the leaves to turn yellow and droop downward. The stem is weakened, causing the plant to wilt.
Make a small cut at the base of the plant. If you see brown or brackish colored rings or streaks on the interior tissue, your tomato plant may have fusarium wilt. No treatment is available. Destroy and remove the plant, and allow the soil to remain fallow in the sun to kill off the fungus before replanting. Don't plant tomatoes in that area for one or two seasons following the discovery of fusarium wilt.
Cucumber Mosaic Virus
A tomato plant attacked by aphids may become infected with cucumber mosaic virus. Aphids carry this virus, and infect the plant when they sink their teeth into the stem to suck the juices.
The leaves on an infected plant may appear mottled at first, with brown and yellow spotting. They will begin to yellow, and although they continue to grow, they will be distorted and appear elongated and curled. The virus weakens the plant causing it to wilt. Though the tomato plant may not die from the virus, it produces little if any fruit and will continue to deteriorate.
Monitor your tomato plants for signs of aphid infestation. Aphids are small insects, but travel in swarms. Large groupings of aphids are easily seen. Use an appropriate pest control, such as introducing ladybugs into the garden, to dispatch the insects. If you catch the infestation early, you may be able to save the plant.
Tomato plants require consistent watering, but do not like wet feet. Overwatering may cause the plant to "bloat," causing the stems to bow over. The leaves, however, should remain green. Water less often, keeping the soil slightly moist, rather than wet.
Inconsistent watering and lack of water may cause your tomato plant to wilt due to a lack of nutrients, which may also result in the edges of the leaves yellowing and curling. Maintain a consistent watering schedule and the plant should show signs of recovery.
If you water consistently, and the leaves on your plant are a healthy green color, but your plant wilts in the afternoon, do not panic. As long as the plant revives in the evening, it's likely the plant is healthy and just reacting to the heat of the day.