Question
We have a maple tree that is rotting where a branch was removed several years ago. How and what should we use to treat it?

Answer
There is not anything that can be done for the decay fungi. This fungi infects the woody cells and over many years can make the tree hollow. It will not kill the tree since it does not infest any living cells. The loose bark can be removed this will help the tree heal over the wound better. I would watch the wound for sawdust or hole bored into the wound, These are caused by an insect called a borer. The borers can damage and may kill the tree. I would spray the wound area on the trunk with an insecticide called Merit or Onyx IF you see signs or you could spray for a preventive treatment.  This will kill the insect in the bark and wood. Borers usually attack weakened trees so I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Fertilize just before a rain storm and you will not need to water it in. Do this now and again in the Fall after the leaves fall. This will increase the overall health of the tree.