The trees were derived from seed (mango tree next door). I picked up rotten mangos, opened the husks and planted them into potting mix.

All of them took up, but about 8 months after, most have died after developing these black spots (large on the leaves) and smaller spots under the leaves.

As I only have a few trees left, which I would like to keep, Im trying to get some diagnoses as my own research has led to little substantial.

I have attached an image of the current "infected" plants.

Are the few I have left salvageable somehow?

Do the small dark things move? Can you brushes them off? If so, do so.

Where are they growing -- indoors or out?
In pots or the ground?

They are growing in direct sunlight for the most part. I water them every day. They are growing in small pots (4inch diameter) outdoors in potting mix. I tried feeding them some dynamic lifter to no avail.

Diagnoses now for large splotches are sun-damage, inadequate water or fungal infection?

Looking at the small little round black spots. There seems to be two categories:
1) Dry and can be brushed off.
2) Wet, and can be spread.

Diagnosis for black spots now are insect related?

Thanks for your help