The white waxy layer (image 1 and 2) on your mango tree is mealybug (Drosicha sp.). Mealybug is an major insect pest in mango. Both nymphs and adult female suck the sap from tender leaves, shoots and inflorescence. The severe infestation leads to wilting and affects fruit setting.

Mealybugs secretes honeydew (a sugary substance) which causes sooty mold (image 3, 4, 5 and 6). 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.

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

Management of mealybugs:
1. Collect and burn fallen leaves and twigs.
2. Mealybugs lay eggs in soil near tree trunk. So flooding orchard with water during October kills egg. Also deep ploughing in November exposes egg to sunlight.
3. After hatching the nymphs start climbing tree and suck sap. To avoid this band the tree trunks with polythene sheet (400 gauge, 30 cm wide) at a height of about 30 cm from the ground level and apply grease at the lower edge of band. Or you can use Funnel Type Slippery Trap (image attached).
4. To control insects already on tree you can spray fish oil rosin soap or azadirachtin (neem products).
5. Also soil application of the spores of the fungus, Beauveria bassiana helps in reducing mealybug population.
6. If infestation is severe you can spray carbosulfan (0.05%, 25 EC, @ 2 ml/liter of water) or dimethoate (0.06%, 30 EC @ 2 ml/liter of water).