Answer: Your tree is still in the process of becoming established and rooting for the first few years, so it may be showing signs of heat and/or moisture stress. Heat stress can cause foliage damage, as can excess sun exposure particularly in the establishment phase. This tree does best in morning sun or in bright but dappled light all day long. If it is in full sun or in afternoon only sun you may want to consider moving it to a cooler location in September and providing it some noontime shade until then -- a patio type sun umbrella for example can provide this.
To promote rooting, make sure the soil is evenly moist like a wrung out sponge, not sopping wet and not dry. Use your finger to dig into the soil. If it is still damp, do not water yet. (Overwatering can cause root damage which then shows as foliage symptoms.) When you water, water thoroughly and slowly so it soaks in deep and encourages deeper rooting. This is better than a daily light sprinkling. After watering, wait a few hours and then dig down to see how far the water went, it can be surprising.
Also make sure there is a layer of organic mulch about three inches deep in a flat layer over the entire root area. Keep it flat and do not let it touch the bark of the tree. This helps keep the soil cool and moist and keeps down weeds, it also helps feed the soil slowly over time as it breaks down.
This tree is a not a heavy feeder; over fertilizing can cause root damage or excessively fast growth that may not be supported in the heat of summer. It is not necessary to fertilize it beyond a springtime top dressing of compost and/or a springtime application of a slow release or general purpose granular fertilizer such as 10-10-10 per the label instructions. This combined with the organic mulch should be sufficient.
In case you think none of the above apply, I will mention that another possible reason for foliage discoloration is accidental herbicide or other chemical contact, in case this might have happened. There is also a possibility of insect or disease, if you suspect that you may want to consult with your county extension and/or your retailer. Good luck with your tree!
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