Jeff Owen sent me the following information to further discuss what's happening with fall needle shed:
Stressed Fraser firs will shed interior needles in late September or October. It usually occurs right in line with fall color on hardwood trees. Usually it is a sign of drought stress, but there may also be a fertilizer deficiency or toxicity contributing to the problem. We have not been able to point to a single nutritional problem contributing to interior needleshed. But I have seen internal needle shed in fields that have low phosphorus, or calcium, and/or very high manganeses or sulfur.
I have observed several fields where the needle shed is widespread on the southwestern aspect and very light as the field shifted to more of a northern aspect. Even a subtle difference in aspect made a big difference in this problem. In a drought stress year, most growers will have 1 to 5% of this problem in their older trees, but I have seen one or two fields with as much as 30% of the trees affected.
Needle loss problems seldom show up before trees are of a marketable age and size. Trees greatly increase the amount of foliage with each progressive year. This creates an increasing demand for water and nutrients. Factors which may have been adequate or marginal for a smaller tree become limiting as the tree grows. The failure of symptoms to be expressed in younger trees has often lulled growers into complacency only to be surprised by needle loss in trees already tagged for market.
When needles shed out to the new buds in the fall, I think it is a different scenario. In that case I have observed 3 different factors in play: salt, calcium deficiency, and Phytophthora root rot.
Salt injury at some time in the current year can cause needle loss all the way out to the bud typically without killing the branch or bud (although sometimes they die too). I have seen late fall needle loss and grey-black necrosis in the bark and wood of branches that were spring fertilized with "plops" of 10-10-10. Sometimes the whole tree sheds, but sometimes only one side or a coil of branches climbing up the stem from damaged roots will lose needles.
Calcium deficiency will induce fall needle loss in the top or middle of a mature Christmas tree. I have seen white pine and Frasers with similar symptoms in the same field. All the needles on a branch can turn bright yellow, then brown, then drop off. The branch and buds live although growth can be stunted in the following year. I have seldom seen more than 1% or 2% of trees with symptoms of calcium deficiency. Yet these trees may reflect a field-wide deficiency. The concern has been that other trees in the field that showed no symptoms prior to harvest could shed needles after harvest either on the retail lot or in the home. Dr Hinesley and Dr. Shelton conducted unpublished research that suggested this was the case.
The third root cause of total needle loss that I have seen was Phytophthora root rot (PRR). Usually, Fraser firs that die of PRR hold on to their needles a long time, but sometimes in the fall when the diseased trees have been stressed, green trees will shed out just like they do with a calcium deficiency. Only healthy roots can actively take in calcium, so it makes sense that PRR can contribute to the expression of calcium deficiency.
In all of this there are few proactive steps to take. In terms of the imediate harvest, buy a shaker machine to remove interior needles on trees that are still salable. For the future, take a soil sample and fertilize your farm according to the reports. Don't let market trees go to harvest short on calcium.