Keep an eye out for rust mites! As the weather begins to cool and become very dry, rust mite can flair up. Here's a photo of some rust mites and rust mite damage from a field of Canaan I was in yesterday. Remember, rust mites can be orange, yellow, or clear. The threshold for treatment is 8 mites on one needle or 80% of the shoots with mites on them.

The NC State Christmas Tree Portal has very good information on this pest.