Sorry for the bad photos of this I could not get a really good one but this is the best.
I am located in zone 11, around the 1000' elevation of the east side of the Big Island of Hawaii, and we get over 120" of rain per year. As you can see there are white spots and yellow spots that are only on the lower branches thus far.
Ooo, that looks like a fungus. I'd put a ;eaf in a ziplock and take it to a nursery and ask. Ziplock to not be taking them a problem. Are there other mangoes in your neighborhood? Go look at them. Mangoes do like rain, but not tooo much rain.
The bad thing is that the people that work in the nurserys around here do not know anything about plants it took me over an hour of talking to people to find out what hardiness zone I was in here, and some of the people did not even know what I was talking about...lol.
1,000 ft. elev. is a bit high in this area to raise Mangos, because it is so wet here. Mangos are very susceptible to anthracnose disease. Be sure to keep up on the nutrition of the tree and give it plenty of calcium carbonate and dolomite.
At 1,000 ft. you are in Zone 10 b; Zone 11 is down at the coast.
Thanks man for all the information Dave...I will keep my eye on the trees but I am not sure if they will turn black because some of the trees have it in a more advanced stage and the very very center of the "fungus" is the only black area.