It is difficult to know for sure without being able to look over a sample. If it was an insect then the rest of the foliage should be relatively problem free for the winter season. However cutting it back may promote new growth before winter which would be tender and subject to winter injury. If this happens cut the damaged parts back in late winter.
If you see this problem next spring or summer I suggest you take a sample of the plant and a photo if possible to your County Extension Office for identification, and if necessary, control recommendations. It could be that the plant was suffering from a root problem which in time will lead to dieback of part of the above ground parts of the plant.
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