Your trees are most likely suffering from a soil moisture problem. Dry conditions, especially on newly planted trees, can cause needle browing and drop. Other possble causes include needle blights (fungal diseases that are often brought on by frequent wetting of the foliage and which can be managed with fungicide sprays), or possibly an attack of mites of other such pests on the foliage.
I suggest that you take a sample from the plant along with a photo of the plant in its growing location (if possible) to your County Extension Office to see if they can help diagnose the problem and recommend an appropriate course of action.
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