Maintain tree vigor, avoid over/under watering, and keep an eye on the areas for insect damage. With respect to that area and surrounding areas that would be the root collar borer and the Columbian timber beetle. The following contains some information on pest insects commonly associated with yellow-poplars.
More importantly, given this may likely be causing or will cause continued damage, canker diseases. Specifically neonectria canker
and possibly fusarium canker as well, in any part of the tree.
Prune the tree at the appropriate time to build a strong scaffold, to eliminate any crossing/rubbing branches, and to remove diseased wood.