The ring collar is the tree limb's natural foundation upon which wound closure is built. When pruning, do not remove the ring collar. Make your cuts nearly flush, but not quite. So, if your tree has stubs after pruning, or if it has been flush-cut, you are the problem. There are chemicals now for almost everything in your yard.
Weed killers can do the most damage to trees.
In fact, of all the thousand s of sick-tree calls that I have answered, week killers have been the largest single factor. Then, so many gallons of the mix have to be applied to so many square feet of lawn area. There are a good many chances here to misfigure, so watch it. Granular weed killers can get you in trouble too, especially the kinds that are combined with fertilizer. Some of them are not to be used in the vicinity of tree roots, so read the label carefully.