Answer: Unfortunately, based on your description I am not able to diagnose the cause of the defoliation. It might be a pest or disease problem or it could be related to root damage or a change in drainage patterns or accidental herbicide exposure, or some other cultural problem. I would strongly suggest you consult with your local Cornell cooperative extension and/or a professionally trained and certified arborist to get a specific identification of the problem(s). Based on knowing that, you can decide what to do. An arborist would have the necessary equipment to spray a tree of that size, should a spray treatment be necessary. But, it is really important to get a firm diagnosis before proceeding. Also, it is a good idea to rake up and dispose of all the fallen leaves in the trash to try to limit sources of reinfection in case it is a disease or pest problem. I hope it is nothing too serious.
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