It looks like your mangoes are suffering from a common bacterial infection called bacterial black spot. This is caused by damaged fruit skin usually from wind which provides an entry for the bacteria that thrives in damp, warm conditions with poor air circulation. Try not to wet the leaves or fruit, improve air circulation with proper pruning to open up the tree and perhaps a wind break to keep sand and debris from blowing about and damaging the skin of the fruit. An anti-bacterial spray containing copper is the recommended prescription for serious cases but your tree will most likely survive. The Master Gardeners or university in your area that helps farmers grow this fruit as a commercial crop will have the latest information on which tree varieties are more resistant and what the most effective timing and type of spray to use on fruit trees. Keep growing, Marianne Binetti ry ht pleaf loo