Answer: It sounds like you've done all you can to figure out what's infecting your fir tree. If the Extension Service didn't see any signs of insect damage, then I'd agree that it's probably an environmental stress (wind, drought...) or herbicide drift. This spring check the buds carefully early, before you see the symptoms. Look for any small insects in the bud clusters. Perhaps by the time you showed the Extension Service the buds, the insects had left. If you see small, soft-bodied insects, consdierspraying the buds with insecticidal soap. I'm sorry I couldn't help further.
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