Answer: I think the problem requires a more thorough inspection. Blistering bark can be a sign of insect feeding (blister bark beetles), a result of injury (mechanical or environmental), or be due to constantly wet conditions. Peel away some of the bark to see if you can find tunnels or holes (evidence of insects). If so, the problem can probably be controlled. Or, take some of the peeled bark to your local cooperative extension office or nursery and ask the professionals there for their opinion. Dormant spray is lime-sulfur which can act as a fungicide but won't have any effect on insects. It shouldn't hurt your ash tree, but I don't think it will help, either. Again, either have an arborist come inspect the tree or take some samples of the damage to the extension office or local nursery. I'm sure someone there can help diagnose the problem and recommend actions to control the problem.
Best wishes with your tree!
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