Well, if you're fertilizing, stop.

But, when it comes to overwatering versus underwatering, you need to check. So, stick a finger into the rootball -- most likely it's dried out. That can be true in spite of watering and/or rainfall.

Another possibility is a soggy rootball. As roots die, the top dries out.

Thanks so much for the quick response. I usually water once a week or every two weeks. Do I need to loosen/mix soil so that more water gets down to the roots?

If you try to loosen soil you will damage any new roots which have developed.

Don't know how much water you apply, and by which method, but perhaps it's not enough? Or too much? (I would bet on the first.)

I agree with Jean ... likely not enough water ...a tree that size nust be watched carefully the first year especially ...unless you got alot of summer rain a good every two week soaking would NOT be enough and maybe even a one a week soaking if it was super dry ...My first instict was not enough water and too much fertilizer... thus the tourched leaves...But I am no expert If it survives the winter I would bet it will be fine next year...If it made it this far it is probably ok many trees around here are dropping "dead" leaves from the dry hot spell we are having they are NOT turning color as much as just drying up ...Of course these are big oak trees with large root systems so it doesn't look like yours but some of my smaller maples have some leaf drying going on ...I think it's just too dry ...but as jean says it could be too wet also I don't know where you live or your summer rainfall ...but I doubt it. Remember there are alot of factors that give fall color adequate rainfall and nighttime temps beteen 50 and 36 degrees being the most important.neither of which are being met HERE at least..... most folks arn't expecting much color around here this fall. That all being said I have seldom seen newly planted trees that young have much or any color til well established... many look like yours by fall if it's dry. David