Q: A large oak tree on my intown property has a ‘sunken-in’ area at the base of the trunk, approximately 10″ high by 10″ wide. The center of the sunken-in area has turned black. I do not see any signs of insect infestation. Is the tree diseased and can it be saved?
A: It sounds to me like the sunken area is the result of damage to the tree several years ago. It could have been as commonplace as a child hitting it with a hammer. It could have been as unavoidable as an errant Buick giving the trunk a hard bump. Whatever the situation, the tree responded by “walling off” the injury. It pumped chemicals and resins to the area behind the wound so the damage could not spread. You can’t see this, of course. All you can observe is the missing bark and blackened interior.
Putting a poultice over the spot would only make matters worse by providing a convenient hiding place for insects and a moist spot for fungi. Though I have not seen the contusion, I think your best course of action is to keep the tree healthy so it can fight its own battles. You might occasionally spray an insecticide labeled for borer control on the dark area but nothing else. Water your tree regularly and deeply during the summer and it will attempt to cover the wound with callus. If the damage really worries you, hire an arborist to examine the tree and offer his or her opinion.