My small Glenn is fruiting now, about 5 dime size mango's still growing and haven't fallen off (hopefully it stays that way)

I noticed on a few leaves on one of its branches are starting to turn yellow starting from the center of the leaves and look like its spreading outward.. is this a sign of an infection or are these leaves just turning old and dieing back?

its only been in the ground for a year now, so its not an old tree by any means. (it was 3 or 5gal grafted when i purchased)