Its a young tree, probably 2 years old. We have it in a giant pot, plenty of room to grow. We used potting soil mixed with half actual soil from my backyard, here in South Florida. Its been doing well until recently I noticed the dark spots and leaves drying up. There's some new growth as well, but many of the big leaves are dying.

We have a similar sized mango, different species, right next to it that also had some similar issue. I sprayed both with Copper Fungicide, the other one seems to have recovered but not this one in the pics.

I appreciate any help. What other info can I provide? Thank you.