I have a Lacy Bark Elm that was planted by a nursery about 4+ years ago. It has done beautifully until this past spring. I noticed that at the base of the tree, it had turned dark and there were nodules protruding from the bark. About 1/4 of the way up from the ground, the bark looks much darker than the rest of tree trunk. Other than this, the tree looks very healthy and is growing steadily. I had applied an organic fireant killer to the base of the tree as the little firey critters had staked a claim there. Should I be concerned?