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If the soil isn't already high in Ca, B, and Fe, you will also need to add those.
Fungus. Spray with Copper, repeatedly. In cool time of year, use spray-oil.
Kocide (Copper Hydroxide) fungicide/bactericide works mainly as a preventive, when sprayed before symptoms appear. It has been used on mango treesin conventional agriculture since at least the 1970's, usually tank-mixed with mancozeb/Manzate. Mancozeb now gets a lot of bad press from California.
Julie,Spot spray the visible problem and close by. In weather conducive to the spay's staying wet for half an hour or so, use either Neem Oil or Spray-Oil, per label directions.
I have not found any clear answers about mangos' tolerance of Chloride or Sodium, except that tolerance varies tremendously from one root-stock to another, as well as per scion grafted on those different root-stocks. If you can get some 13-1 seeds(or 13/1), that might be of help.