Answer: What you describe could be a canker caused by a bacterial infection or it could simply be a result of bud proliferation which manifests itself as a gnarled swollen area on the trunk. Bud proliferation is unattractive but harmless; disease cankers can cause eventual decline and death of the crabapple. Since I can't examine the problem, I can't really give you any specific controls. You might want to take a close-up photo of the problem and then take the photo into a local nursery. Someone there should be able to identify the problem and suggest controls. Best wishes with your crabapple!
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