A standard permaculture, organic, natural way to deal with fungus is to use a natural fungal predator! If you had a problem with mice or rats in your house what would you do? Maybe get a cat!
A natural predator of fungus happens to be bacteria! Bacteria is a good thing unlike many in the more "developed" countries of the world seem to think! A sort of "war on terror" , "war on drugs" , "war on bacteria" fear mongering mindset is in place to trick us into buying cheap and environmentally irresponsible chemical alternatives to the all natural little tiny critters who live in tribes of millions and billions all around us! These microbial creatures or bacteria can be utilized in so many helpful ways, from pickling your cucumbers to spraying plants with mold or fungus in a concentrated bacterial form known as "AACT" or Actively Aerated Compost Tea, this stuff is the stuff of legends and deserves the term "miracle grow" more than the actual corporate entity.
AACTs can be made with many different ingredients but typically they include sea kelp, worm castings, fish poo(emulsion/hydrolysate), and maybe bat guano or some type of meal. Alternatively nearly anything can be added in addition, such as root stock from a previously healthy crop, a freshly harvested, or dry amount of comfrey or nettles. You name it: cow manure, alfalfa, dandelions, rosemary, cloves, neem, garlic!
Unfortunately I'm not sure what type of ingredients would be best for the particular type(s) of fungus on your precious mangos Gabriella, but I'm sure that any bacteria rich AACT will feast upon your fungus immediately. Just some may be more effective than others. One last hint for AACTs, adding more, sugar, molasses, etc increases the bacterial content of your micro herd !
Best of luck save those trees!