Answer: Unfortunately, based on your description, I am unable to identify the problem with any certainty. It is possible you are decribing the join encircling the trunk where the plant was grafted, in which case there is nothing you need to do. If the bark was damaged by say a weedwhacker, the tree may be showing some oozing through the wound. If this is what happened, take steps to prevent such damage in the future and hope that the tree can heal quickly before pests discover the opening. If the tree was pruned, the branch collar at the base of the removed branch may be healing over the cut, sometimes this looks a bit like a bubble (or in early stages, a donut) as it closes from the outside inward over the hole. Finally, cherries are subject to Cytospora infection or gummosis. The tree will show a sort of crystallized amber colored oozing in spots on the trunk -- these could look like bubbles in a sense. This is a rather severe problem and should be attended to. Your county extension should be able to diagnose the problem (if it is one) and suggest what steps to take next. I'm sorry I can't be more specific long distance. Good luck with your tree.
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