The white spot on your mango tree look like scale insect (Rastrococcus iceyoides). Both nymphs and adult female suck the sap from tender leaves, shoots and inflorescence. The severe infestation leads to reduced growth, wilting and affects fruit setting.

Scale insects secretes honeydew (a sugary substance) which causes sooty mold. The infected leaves and inflorescence become shiny black and sticky because several species of fungus which grows on this honeydew. Though sooty mold don't infect plants directly, they may reduce photosynthetic activity in leaves. So the infected plant may show stunting and leaves may die and fall off prematurely.

Also the scale insects are always associated with ants (ants feeds on honeydew and in return helps scale insect in spreading to different parts of the plant).

Management of sooty mold:
You can spray starch to control sooty mold.

Management of scale insect:
1. Collect and burn the fallen leaves and twigs.
2. To control insects you can spray basic soap spray (add 2 tablespoons of liquid dish detergent or baby shampoo to 1 gallon of water) or fish oil rosin soap or neem oil.