How are you watering the plant? How often?

Have you fertilized it, and with what?

My first guess would be build-up of 'salts" from frequent shallow watering and too much fertilizer.

It does look like salt burn and water stress.
Is there any mulch around the base?

Ah, you guys probably nailed it.

Water: along with the lawn sprinklers, 15 minutes daily.
Fertilizer: irregular, about 3 weeks ago I sprinkled some 15-15-15 around it. Prior to that, nothing.

How do you suggest I do it from now on?
Thank you!

15 minutes a day isn't going to keep any tree happy for very long especially in summer.
You need to give it long slow soakings. This will give it a chance to fully hydrate,help leach excess salts in the soil and encourage the tree to put down deep roots. Can you water out at the drip line or flood the planting area? Don't water up next to the trunk, that won't help.
Since your tree is showing signs of salt burn I wouldn't give it any more fertilizer for a while. Fertilizers are salts and might aggravate the situation. Could you put some yummy compost around it instead?
Would it be possible for you post a photo of where the tree is growing? If we could see that we might could come up with more ideas.

Here are the photos, let me know if you need to see more.
Thanks,
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