This is the time of year evergreens arrive in the Plant Disease Clinic showing symptoms of seasonal needle loss. People often mistake this normal discoloration and loss of needles for an infectious disease problem.
Evergreens do not keep their needles indefinitely. Older, inner needles senesce, turn yellow or brown, and drop from the tree after one to several years depending on the tree species.
White pine are the most dramatically affected. By November of most years, only one year's needles may remain attached to the tree. Austrian and Scots pine usually retain their needles for three years. Spruce and fir needles also yellow and drop with age, but since these trees retain their needles for several years, needle drop is often not noticeable. The needles on arborvitae and some cedars usually turn brown rather than yellow with age.
These general patterns of needle drop may vary from tree to tree and year to year. Adverse conditions in the summer and fall may lead to more pronounced needle drop in the fall.
We have received evergreen samples that have shown both seasonal needle loss and an infectious needle disease. If symptoms of browning were observed earlier this year, possibly on the older needles on the lower portion of the tree or on this year's growth throughout the tree, an infectious disease such as Rhizosphaera needle cast or Diplodia tip blight may be involved. These and other diseases are described and pictured in Pm-1528 "Common Diseases of Conifers in Iowa". As mentioned in the bulletin, infectious needle diseases are controlled by applying fungicide sprays in the spring.
If you suspect a disease problem, you may submit a sample for confirmation (several branch segments showing both green and discolored needles) to the Plant Disease Clinic, 323 Bessey Hall, Ames, IA 50011. You may also submit the sample to your local Extension office.
This article originally appeared in the October 13, 1995 issue, p. 140.