Answer: Unfortunately, based on your description it is difficult to make a diagnosis. However, a typical reason for a newly planted tree to defoliate is dry soil, especially if it is combined with excessive summer heat. This tree will need an evenly moist soil while it is becoming established, the first three years or so. Once established, it becomes more tolerant of average and dry soil.
Drought or dry soil while it is building its root system would cause stress and consequently cause it to drop leaves. You might want to check the soil and see if it is moist or if you need to water. Dig down a few inches with your finger, checking both the original root area and the surrounding soil. When you water, water deeply and slowly, wait a few hours, then dig down and check to see how effective your watering was. It can be surprising. Also use several inches of organic mulch over the root area but not allowing it to touch the trunk.
There are also some possible pest and disease problems that could affect this kind of tree, although they are not so common. Examine the leaves carefully to see if there are any spots or other irregularities besides the yellowing. If so, you may want to consult with your county extension and or professionally trained nursery personnel to see if you can get a precise identification of the problem.
Finally, yellowing could also be a response to overfertilizing, accidental chemical overspray, nearby use of lawn chemicals or herbicides, or some other outside factor. You might want to review these possibilities as well since it has happened more than once.
Good luck with your tree.
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