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Spruce Needle Cast Confusion

April 21, 2004

There is only one major needle cast disease to deal with in Illinois, Rhizosphaera needle cast. We sometimes see spruce needle rust or another secondary fungus, but those instances are few. Rhizosphaera needle cast is most severe on Colorado blue spruce, with Norway spruce being resistant. The most diagnostic symptom is the yellow to brown color of 1-year-old needles, while newest needles remain green. Unfortunately, other problems can also cause these symptoms: spider mites, drought, and even scale insects. Rhizosphaera fungus readily forms characteristic fruiting bodies on affected needles placed in a moisture chamber for 24 hours. If you have seen the described discoloration followed by needle drop, place some of the brown needles (taken from the tree, not the ground) in a moisture chamber to determine whether fruiting bodies are present. You can also send a sample to the Plant Clinic. The clinic opens May 1 but can handle suspect Rhizosphaera samples now if sprays are being considered.

If you had problems with this disease in the past, keep in mind that spores will be infecting new growth as it emerges this spring. If the fungus is present, protective sprays can help keep the disease managed. Although the disease does not kill your spruce, repeated defoliation causes bare limbs, which soon result in an unsightly tree. Fungicides are applied when the new growth is 1/2 to 1 inch long and again when fully elongated. Stressed trees are more susceptible to infection. It is helpful to provide spruces with a well-drained site. Keep the tree mulched, well watered in drought, and fertilized annually. Prune surrounding plant material to promote air movement.

Pictures of infected needles and fruiting bodies of the Rhizosphaera fungus can be found in this forest service pest alert of the disease on fir: http://www.na.fs.fed.us/spfo/pubs/pest_al/rhizo/rhizo.htm. Another site to visit is the Kansas State fact sheet at http://www.oznet.ksu.edu/dp_hfrr/extensn/problems/rhizosph.htm.

Author: Nancy Pataky


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