Yes, so you too think it's salt burn. Is there a way to remedy this situation

Flush with water ..fill,and let it drain a few's times or until you get bored-lol.

Another cause- I've seen this- is they HATE to sit in a saucer of water. Very sensitive roots and it translates to fast leaf off- coloration. Remove the sauce asap and they recover.

Sorry, I'm a little slow, when you say remove the sauce do you mean to remove water under the roots? The tree is planted like on a mound so to say few cm above ground level. I put a lot of fresh, just chipped uncomposted wood chips around it, about 30 cm. When I saw the leaves in bad shape,as pic suggests, I removed some mulch. The mulch was quite warm, and composting while i removed it.

i will flush with water as long as I could

maybe too much watering? or transplant shock? how long has it been in the ground?

I think too much nitrogen

Could this be a fungal issue? I have an indoor Manila Mango I am growing from a seed (it is now 1 year old) and some of the original leaves have this EXACT same coloring/damage. Anthracanose?

It should be noted that I go out of my way to use either snow melt in winter or rainwater when available. I kept the plant on the dry side during winter. It is in gritty mix with Oscomote CRF.

Osmocote is ok but gritty mix is useless.

Aka1976....do you have a better soil suggestion for indoor Mangos? My understanding is Mangos prefers to be on the drier side and need fast drainage. The gritty mix accomplishes both of these....

Also....do you think the leaf discoloration we both have is a nutrient deficiency or fungal?

Photos from today:

Check: the making of our custom soil, with shamus O'Leary on YouTube. Mine is too much nutrients not deficiency, yours looks also too much nitrogen or very little sun. How often do you water? You might consider watering according to plant needs.

I doubt it is too little sun. The plant is in front of a large SW window with no obstructions outside. It gets 4-5 hours of direct sun, even in winter. I have a Calomondin Orange that was right next to it all winter in the same window and that tree did just fine and is now growing vigorously.

The Mango will be going outside once the nighttime temps are consistently over 60 F.

I checked my notes (I have been keeping a log on each of my plants since Dec 2016) and it looks like I was barely watering it in winter (would thoroughly water every 2-3 weeks). Maybe it was not getting enough water?

If it is Nitrogen, what would cause the plant to have TOO much N?

Stop watering. It is because of over watering.

Use 4gm copper oxychloride/sulphur 80% with one litter water and pour on the soil.after 15 days give same treatment until it becomes fresh. Don't give excess water

It could have been the peaty mix potting soils. They hate that. Even with plenty of perlite for drainage Mango seeds hate to deal with the high acidity.

Try a shredded bark mix or best of all, sandy loam.