Michelle Grabowski, University of Minnesota Extension Educator
Throughout Minnesota, purplish brown to rusty brown needles can be seen on spruce trees. A variety of problems can result in needle discoloration in spruces including insects, disease, and problems associated with environmental conditions. This time of year two common problems are Rhizosphaera needle cast and winter injury. Rhizosphaera needle cast is caused by a fungal pathogen. Winter injury is the result of environmental conditions. It is important to be able to distinguish between these two problems, since very different action is required to maintain tree health depending on the cause of the problem.
Rhizosphaera needle cast is caused by the fungi Rhizosphaera kalkhoffii and is most commonly seen on Colorado blue spruce, which are highly susceptible to the disease. White spruce and Norway spruce have greater resistance to the disease but can become infected when stressed. With the drought conditions present in Minnesota the last few summers, Rhizosphaera needle cast is showing up in a wide variety of spruce trees.
Photo 1: Notice how the lower branches of this Colorado blue spruce are most severely discolored by Rhizosphaera needle cast. Michelle Grabowski
Trees suffering from Rhizosphaera needle cast can be recognized by browning of the older needles. The older needles are located at the base of the branch closest to the trunk, while the new needles grow from the tip of the branch. Diseased spruce trees often have branches with green needles at the tip of the branch and brown needles towards the base. In addition, the branches closest to the ground tend to be more severely infected, because humidity is highest there. Later in the summer the discolored needles will fall off. If a spruce tree has been suffering from Rhizosphaera needle cast for several years, it may appear sparse and have dead branches at its base.
Photo 2: This branch infected with Rhizosphaera needle cast shows discoloration of the older needles, while younger needles remain green. Michelle Grabowski
The fungal pathogen of Rhizosphaera needle cast can be seen on infected spruce needles. Use a hand lens to closely examine discolored needles. Tiny black pimple like spore producing structures can be seen arising from the stomates, or air holes in the needle.
Spruce trees suffering from winter injury often have needle discoloration on the needles at the tips of the branches. Frequently this damage occurs on the south or west side of the tree due to excess wind and sun on those sides. In some cases winter injury is observed on trees receiving reflective light from a nearby building or car. The discolored needles often appear bleached or faded, with the tip of the needle most severely discolored. Winter injury can occur under several conditions. Needles can be killed by cold temperatures, desiccated by the wind, or bleached by the sun. If a spruce tree did not have time to harden off properly in the fall or is not fully adapted to Minnesota’s winters, complete browning of all needles may be observed.
Photo 3: Spore producing structures of Rhizosphaera needle cast on infected needles M.Ostry USDAFS Bugwood.org
If the problem is clearly winter injury, not much can be done for the tree at this point. Luckily winter injury rarely kills the buds of the tree and as weather warms, new growth resumes improving the overall color of the tree. Make a note to water trees throughout the summer to prevent drought stress and help the tree harden off next fall. In very exposed areas, spruces can be protected from future winter injury with a simple burlap barrier to block the wind and sun. Read ‘Protecting trees and shrubs from winter damage’ (http://www.extension.umn.edu/distribution/horticulture/DG1411.html ) to learn more.
Photo 4: Notice how the south side of this tree is most severely affected by winter injury. Michelle Grabowski
If the problem is clearly Rhizosphaera needle cast, management strategies should be implemented to protect this year’s needles from infection. When new needles are half the length of mature needles, spray the tree with a fungicide whose active ingredient is Chlorthalonil. Completely read the label and follow all instructions when using a fungicide. Apply the fungicide once more at the interval recommended on the fungicide label (typically 3-4 weeks later). These two sprays will protect the needles from infection.
Photo 5: Concolor fir is a good alternative to Colorado blue spruce. David Zlesak
In addition several cultural practices will help to reduce the risk of future problems with Rhizosphaera needle cast. When planting new spruce trees choose Norway spruce or white spruce instead of Colorado blue spruce because they are more resistant to the disease. Concolor fir (Abies concolor) is another alternative to Colorado blue spruce and has relatively large, blue-green needles. Reduce moisture on spruce needles by controlling weeds around the base of the tree and redirecting lawn sprinklers to avoid wetting the needles. Reduce stress on spruce trees by mulching the soil around the tree and providing trees with water during periods of drought. Avoid planting new spruce trees near old infected spruce trees.
Photo 6: The needles at the tips of this branch are most severely affected by winter injury. Michelle Grabowski
If unsure what is causing needle discoloration in the spruce tree, send a sample to the UMN Plant Disease Clinic (http://pdc.umn.edu) before implementing any management strategies.