Maybe spray with some thyme oil? I have bat guano fertiliser. I try to stay organic with anything even potentially fruit-bearing but options there are limited

Don't know how thyme oil goes with plant leaves but it is supposed to have antibiotic qualities. The bat guano should be good.

The bottle *says* it can be used on leaves. I'm suspicious because of what it did to my orchids, but I can always grow another mango pit if it turns out badly, I guess.

All mangos coming into arizona from out of state are treated with neonicotinoids. It causes a chemical burn . Nothing to worry about if that is the case. Does look like it needs some food though. Potted plants need to be fed often. Mango does not like a lot of nitrogen so be careful and use something mild to fertilize like alfalfa meal and kelp meal.

Oils can be bad if used when it's sunny and/or hot. Seeing it's in a pot
you can always move it to where it's cooler and shadier for the
treatment. The problem with mangos and nitrogen is that it makes them grow fast, leggy and weak.

True what was said above. Oils will likely burn in the sun here. Also yes for the nitrogen comment which is true for most trees.

Thanks, guys.

I'm not in Arizona. This hasn't been treated by anything because it is seed-grown from a local tree.

I'll try to find some kelp.

Well, still could be burn from something such as fertilizer, herbicide, pesticide, sun etc. Still definitely needs feeding. Eastern sun exposure would be best for it or in filtered light (shade cloth) until it gets going.

Maybe too much sun. I'm in the tropics & it sits in full sun. I'll try to find a somewhat shadier spot for it. :)

Yes, I checked where Puerto Vallarta is. One advantage you have there is that being close to the coast the relative humidity should be fairly high. That means plants are less prone to sunburn than they would be if humidity was a lot lower. But the down side is that with higher humidity plants are more at risk of fungal infections. Mangos like drier conditions and are more prone to fungus in humid conditions than many other tropical trees.

You say your plant in the pot sits in full sun. That might be too hot for it. Tree roots under ground are normally protected from a lot of heat by the insulating effect of the soil. In a pot there's very little soil to provide any protection and the roots end up too hot. If you can set it up so that the pot/soil is shaded the plant should do better.

It'll be a work in progress.