It could be a mango malformation disease caused by the fungal pathogen, Fusarium mangiferae. To confirm the pathogen it is better to send the sample to the phytopatological lab like as suggested by Mario.

Also, it looks like the leaves are infected with the red rust disease (algal disease) where the infected leaves exhibit circular greenish gray elevated spots which later turn to reddish brown. These spots may join together to form large irregular patches.

For more information
https://www.daf.qld.gov.au/plants/hea...
http://vikaspedia.in/agriculture/crop...