Inside the branch where you cut it, was the wood clean creamy white? Or were there dark stains?
If the dark stains go on down to the roots, the tree is probably a gonner, and the ground contaminated; however, you might be able to save the tree by drenching the ground with a conventional systemic fungicide, such as Alliette or Flanker, which often bring plants back from the edge of death.
If the wood looked clean where you cut it, you still might want to drench the soil with a Potassium Phosphite product, such as Organocide Plant Doctor (NOT Organocide 3-in-1). You may have to follow the directions for drenching avocadoes, as most labels don't manage to mention drenching mangoes, as established trees in groves are unlikely to need it.