My tomato plant is rotting in ground at the root and plant is dying.
How to Control Tomato Plant Rot in Ground
Certified GKH Gardening Expert
This normally indicated that the soil is not draining well. If you are doing any manual watering, stop for a little while and allow the soil to dry out. The plants only need 2" of water a week so only give them that much.
There may also be a fungus issue in the soil that is causing this. It will not hurt to treat the soil around the plants with a fungicide.