Your trees are probably suffering from some type of root stress or fertilizer burn. This can show up as tip and margin browning of the leaves. Excess fertilizing (salt buildup) can burn roots and result in burning of leaf margins as well. If the plants were put in the ground with a tightly packed root ball the roots may not be venturing out into the soil and thus are more prone to drought injury during a brief dry spell. This can be fixed by cutting downward through the outer roots with a sharp shovel in 3 places around the outer part of the original root ball. You don't need to dig the plant up to do this.
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