Question
old large maple trees have white round spots under limbs.  Wasn't sure if this is some type of fungus.  Trees look healthy otherwise.  My lot is heavily wooded and I in Traverse City Michigan.  Thanks

Answer
Sounds like you are describing lichens, which are harmless reltationships between fungi and algae.  They cause no damage to the trees they grow on and are not parasitic to the trees.  They are benign, so dont worry about them.  You do not need to take any action.  Thanks.