Needle shedding in conifers
Pests and tree stress can contribute to needle shedding in the fall.
Each fall, Michigan State University Extension gets several calls about interior needles shedding on various species of Christmas trees. For some species such as white pine and Scotch pine, yellowing of older needles is a natural occurrence. Typically, they hold only one to two years of needles. In years where this happens over a longer period of time, it becomes more noticeable and can cause concern.
In other trees like Fraser fir, Concolor fir, Douglas-fir and spruces, shedding of interior needles in late September or October is unusual. What we find is that shedding is occurring on the oldest interior needles; this could be caused by a number of things. Typically, it can be due to either damaged needles from spider mite feeding, needlecast diseases, nutrient deficiency or a symptom of trees under stress (heat or drought).
To help you diagnose what may be going on in your field, here are some things to look for.
Mite damage. Needles will show the characteristic “stippling.” When mites feed, they empty the cell of the green chlorophyll. This causes yellowish spots along the needles, referred to as stippling.
Needlecast diseases. With a hand lens, look for fruiting bodies on the needles. These can be black and in rows (Rhizosphaera, Mycosphaerella , Swiss Needlecasts), black fruiting bodies on the needles (brown spot, Lophodermium, Sirrococus) or banding (Lophodermium, Dothistroma or Rhabdocline).
Nutrient deficiencies. Nutrient deficiencies symptom can be anything from a pale green color to needle chlorosis (yellowing) of the needle tips to the entire needle, but can also lead to needle shed. Look for other signs of deficiencies and for patterns of damage across the plantation.
Heat or water stress. Shedding leaves or needles is a common tree response to heat or drought. Needles may desiccate during extended dry or hot periods in late August and September, turn yellow/tan, but remain on trees until fall then come off during harvest and handling. Often, this occurs in pockets or areas in a field and usually affects only 1 to 5 percent of the trees.
If the cause is spider mites or needlecast fungus, you will want to develop a plan to manage these pests next year. For other stress possibilities, evaluate the site and maybe take a soil or foliar sample this fall to determine if you need to adjust your fertilizer program.
Dr. Cregg's work is funded in part by MSU's AgBioResearch.