Cytospora Canker of Spruce
Cytospora Canker of Spruce
The principal hosts of Cytospora canker, which is caused by Cytospora kunzei, are the Colorado blue spruce and its varieties. The disease also occurs on Norway spruce, Koster's blue spruce, white spruce, Douglas fir and other spruces planted as ornamentals. The most commonly attacked trees are 15 to 25 years old and 20 to 40 feet high.
Symptoms: The most striking symptom of Cytospora canker is the death of branches, usually those nearest the base of the tree first, followed by a progressive dying of branches upward in the tree. The tree seldom dies outright because only a few limbs are killed each year, but the progression of dieback ruins the symmetry of the tree and reduces its aesthetic value.
Cause: The girdling caused by the fungus will occur on any part of the branch except the small twigs. Cankers are inconspicuous, with little or no bark deformation. The fungus grows throughout the inner bark, causing the death of the portion beyond the canker. A heavy pitch flow is characteristic of the disease. The pitch is a clear amber color when freshly exuded, but later it hardens to form a crusty whitish coat over the cankers. Lower, healthy branches may sometimes become covered with resin exuded from infected branches above. Needles on infected branches turn grayish green, then brown, and may drop immediately or persist on the branch for a year.
Shaving the bark carefully in the area between diseased and healthy tissue will reveal tiny, black fruiting bodies of the fungus. Spores ooze from the fruiting bodies during wet spring and summer weather and are capable of causing new infections if they land on freshly wounded wood.
Control: Early detection and removal of infected branches will reduce the probability of more extensive disease development. Infected branches cannot be saved and so should be pruned flush with the trunk. Prune only during dry weather to avoid spreading spores to healthy branches. Affected branches must be burned promptly. If not destroyed, the cankered part will continue to produce spores for many weeks. Continuous surveillance to detect new cankers is especially important.
Spruce trees subjected to drought or to other environmental stresses appear to be more susceptible to Cytospora canker than vigorous trees. Therefore, fertilizing and watering during dry periods are helpful in promoting tree vigor, though they probably have no direct effect on disease control.
No chemical sprays give satisfactory control of Cytospora canker on spruce.
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