On the back of an old leaf, this is what I've discovered. There are also black spots on the surface of the other old leaves as well. Most of the new leaves look ok, but it seems like the black spots are spreading to the new ones. Any idea what is bothering it?

Hi banana, I've had those and some of them still have them :o(
I have NO idea what it is either but it doesn't seem to be anything too serious? At least if its the same as what you have...that's been my experience...I don't think its anthracnose I've had that too and it looks different. Maybe someone else can diagnose it for us...

Puglvr,

I'm sorry yours also got the same problem. I really hope it's nothing serious ... and yes definitely hope we all can find out what's bothering them. Perhaps those who have mangoes here might have the same problem? Any idea what it is?

I don't know if some variety of mango is more susceptible to diseases. This one happens to be the Nam Doc Mai which had a lot of white spots before I cleaned it all up and pruned it.