Stewrtia Japonica dying

Our Stewartia is in its third year outside of Boston. It had a few blooms in July and looked health. It has been watered well but not to excess. Suddenly last week all the leaves started turning brown. There might be a borer entry near the ground and we applied some Sevin-5 to the hole and on the ground around the trunk.
Can anyone guess what the problem might be?
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