Conifers such as pine, spruce, fir, and arborvitae shed their oldest (innermost) needles in the fall, a process known as seasonal needle loss or fall needle drop. Samples showing seasonal needle drop are commonly submitted to the Plant Disease Clinic each fall, with concern that the trees are dying.
The pattern of yellowing or browning and the time of year can help distinguish seasonal needle drop from infectious diseases, environmental stresses, insect, or mite problems. Natural drop of needles occurs in the fall and occurs uniformly throughout the tree. Infectious diseases, on the other hand, often affect the lower branches first or affect random, scattered branches.
Natural needle drop is often most dramatic on white pines, which tend to keep only a year's worth of needles on the tree. Needle loss on arborvitae also can be alarming because the oldest foliage turns dark brown.
Disease problems on conifers are common in Iowa, so it is important to determine if the browning of needles is normal or the result of a disease. The Iowa State University bulletin Pm-1528 "Common Diseases of Conifers in Iowa" is a useful resource for diagnosing disease problems. If you suspect a disease problem, you may submit a sample for confirmation. Send in several branch segments showing both green and discolored needles to the Plant Disease Clinic, 323 Bessey Hall, Ames, IA 50011. You may also take the sample to your local Extension office.
This article originally appeared in the September 13, 2002 issue, p. 119.