Answer: This plant is winter hardy into zone 6, so if it is in a windy, cold, exposed location it may be due to the winter. It also will not tolerate a wet location where the soil is not perfectly drained. In a soggy or low spot or in heavy clay soil or if overwatered this can be a problem and kill it, especially over the winter. There are also some pest and disease problems that could affect it. If it has defoliated I think sadly it is probably dead but you should consult with a local professional nurseryman either on site or show them photos and samples to get another opinon. I'm sorry about your tree.
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