Answer: It's possible that the lilac has suffered from "weed whacker blight" and the bark is scarred but still intact. Since the stems and branches are producing foliage, they are healthy - on the inside, at least. What you can do for your lilac is protect it from additional weed whacker damage so the damaged bark can callous over. I'd form a basin from the main stems out to about 8-10 inches and maintain it as bare earth or mulch over it with bark chips. This will keep excess moisture from invading the damaged bark, and will protect the bark from being damaged by string trimmers.
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