I can't identify the exact culprit, but I will say that I've had a fair number of young trees defoliate in late summer only to come back happy and healthy the next year
When you reach your lowest point, you are open to the greatest change.
-Avatar Aang
The tree looks thirsty! If you give it a heavy soaking and the browner leaves fall off while the marginal leaves recover to green, that will be confirmed.
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