Dear sir, the farmer facing this problem in tomato.
Damping off disease of tomato crop
drench metalaxyl 35 % 1 gm/L dissolved pesticide mixture about 100 ml per plant which are still alive.
Krilaxyl power or Ridomet 1 gm/L of water
https://www.bighaat.com/products/krilaxyl-35-ws-power-fungicide OR https://www.bighaat.com/products/ridomet-fungicide