It appears your tree was affected by anthracnose, the most important fungal disease of mangos in Florida. Infections on the flower clusters start as small, dark brown or black spots which can grow in size and kill the flowers. On the leaves, the lesions start small but can enlarge to form extensive dead areas. Since the fungus attacks the flowers, leaves, and fruit, it will greatly diminish the number of fruit that reach maturity.
Mango trees should be sprayed with a liquid copper fungicide as soon as the flower buds appear, and then weekly until the fruit has set. Continue spraying monthly until mid-May or June depending on whether your variety is early or late maturing.

Additional Resource

http://edis.ifas.ufl.edu/VH048

http://edis.ifas.ufl.edu/MG216

http://plantpath.ifas.ufl.edu/takextpub/FactSheets/pp0023.pdf