Lemon tree is died inspite of regular watering . Please provide the reason and remedy

No, sir. Mal Secco starts infecting a twig and then to another but here total healthy tree is decaying. Mostly soil born pathogens may causing

Please drench with Trichoderma & Psuedomonas for effective control of soil borne pathogens like Fusarium wilt.

Put bliatex chemical near the roots in soil the worms will dead

Then give water