Know this is very late and won't help the original poster but maybe it will help another. I had the same issue with one of my mango trees and was able to bring it back fully.

Immediatly spray the tree with copper sulfide according to the directions and mix strenght listed on it and then redo it again every few weeks. Also cut away all the blackened branch tips so as to promote a healthy regrowth.

One time only...dilute a gallon of the copper mixture with at least double normal water and use that mixture to water around your sick tree's base. You should see a big impovement within a fairly short time. This did wonders for my 10 foot tall Valencia Pride Mango tree.