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Spruce Needle Cast

May 8, 2002

One of the most common diseases of spruce in the landscape is caused by a fungus, Rhizosphaera kalkhoffii. The disease is Rhizosphaera needle cast. Blue spruce is most often infected; and Norway spruces are resistant to this fungus. Infection is prevalent in wet, springtime weather. By fall or the next spring, 1-year-old needles are reddish brown or yellow. The newest needles remain green. Although infection starts near the bottom of the tree, infected branches can be scattered throughout the tree.

Many other noninfectious problems can mimic this disease: for example, an imbalance in soil pH, poor fertility, fertilizer or chemical burn, root injury, root rot, drought stress, and spider mite infestations. To confirm the presence of the pathogen, look for fruiting bodies (pycnidia) on the discolored needles. It might be necessary to place some of the affected needles in a moisture chamber (plastic bag with moist toweling) overnight to encourage growth of fruiting bodies. Look for pinhead-sized, black structures poking out of the needle through stomates. A hand lens is usually required to observe these structures, which occur in nice rows on the needles. They do not easily rub off the needles because they are embedded in the tissue.

Many diseases, including this one, occur more readily on plants under stress. Do not stop looking for causes of poor growth just because you find Rhizo-sphaera. It is possible that site or environmental stress is the true problem, and Rhizosphaera has followed. If that is the case and you control Rhizosphaera, the growth problem will persist. Investigate soil type, drainage, injuries to the trunk, compaction possibilities, root injury, etc., so that stress factors can be identified and alleviated.

Fungicides may be used to help fight Rhizosphaera needle cast. Once the disease has been identified, the focus lies on protecting the new growth. Two sprays are recommended--one when the bud cap has fallen off and another about 2 or 3 weeks later. Chemical options for commercial growers include Daconil, Kocide, Manicure, PathGuard, Protect T/O, Spectro, Thalonil, and TwoSome. Home growers can choose from Bonide Fung-onil, Dragon Daconil, and Ortho Daconil. Rake and remove fallen needles if that is possible. Remove dead wood and prune surrounding plant material to allow better air movement in the area. Water trees in periods of extended drought.

A University of Illinois Report on Plant Disease (RPD) discussing Rhizosphaera needle cast is under development. For photographs of this disease, consult The Ohio State University fact sheet found on the Internet at http://ohioline.osu.edu/hyg-fact/3000/3059.html.

Author: Nancy Pataky


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