It looks like a shelf fungus colony growing on your tree. Unfortunately this is usually a sign of rot inside the tree and the only treatment is to remove the infected branch and hope for the best In your case it looks like the trunk of the tree is infected so there is not much you can do. These fungi attack weak trees growing in damp soil so try not to wet the bark with a sprinkler and be sure you have good drainage if you add a new tree. If it makes you feel any better, know that there are hundreds of different fungi like this in the forest that do good work breaking down old wood and have also been used as medicine and as an art form when carved into jewelry - they are very hard. Keep growing, Marianne Binetti