If your tree has invasive rotting at the base of the tree it is called “butt rot”. The trunk of the tree is like the stem on a flower. If the stem gets broken it is not good. This condition may occur without you even noticing. There might be a shelf of fruiting bodies or mushrooms which you didn’t realize can indicate this problem. Unfortunately there is nothing you can once this is discovered as it has probably has been harming the tree for years, wood decay works from the inside out. Cutting the rotten wood out leaves the tree more vulnerable to disease because the tree can’t heal damaged tissue they will try seal them off and continue to grow. Painting the tree with wound dressing does not close wounds. A certified arborist could be called to see how extensive the damage is. If the tree is at risk of falling on a house or on people it may need to be removed.
If the tree is stable it would be good properly fertilize,water and prune the tree. A healthy tree could heal over the wound as it naturally grows new wood.Try to protect the tree from further damage. Lawn mowers or other vehicles may have damaged the tree. You can put mulch and maybe a rock ring in a large area around the tree and keep out other competing plants and grasses that need to be trimmed or weed-whacked. The tree can be wounded with the mower blade or string and this is where pathogen can invade.