Hi everyone, I've noticed that some strange, brownish stains have appeared on a few of the lower leaves on my mango tree. The best way to describe them would be watercolor paint staining a piece of paper if that makes sense. Anyway, I just noticed them today, although I'm not sure how long they've been there as the spot color coupled with the dark color of the leaves, makes it hard to tell without examining each leaf closely. I've attached some pictures to illustrate:

Some other information that might be useful

-Current pot size is 6 inches wide/6 inches tall
-The tree is a little over 10 months old
-Currently growing in Fox Farms Ocean Forest Potting Mix
-Added a tiny bit of Osmocote Plus Time Release Fertilizer Pellets in September
-Water once or twice a week depending on how hot it is/dryness of mix
-Live in Southern California, zone 10a

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A full view of the plant taken about a month ago, the 3 immature leaves at the top are now fully grown.

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As always, thanks for the help! :)