slit pear tree
Not much. Make the wound as clean as possible. Research on paints and wound dressings in trees is inconsistent but general show not benefit. The tree will try to wall off the wound. A fungicide spray would not hurt and neither will the would dressings sold in upper end garden stores. If you have some interior grape latex paint on hand you can dilute it about 6 to one and cover the wound with paint.
White is best but any light color will do.