New leafs are getting damaged when they start growing. Look like they are decaying at initial stages.

Hi Das Damage is due to mango leaf cutting weevil.

Das Spraying of insecticides such as Deltamethrin (1 mL/L) or Cypermethrin (0.5 mL/L) or Carbaryl (4 g/L) during new leaf emergence can effectively prevent the weevil damage.  Close monitoring has to be carried out during spraying as these weevils can fly away and stay on the neighbouring plants (orchards) and can return later to cause enormous damage.  

Sorry Das Sreekanth M I have a similar problem in my mango trees and I was thinking it is a die back disease and was applying blue copper. Kindly advise.