ISSUE 6 June 14, 2007
PLANT DIAGNOSTIC LAB REPORTS
Periodic reports of the NDSU Plant Diagnostic Lab are available at the following website:
DISEASES OF SPRUCE AND WINTER INJURY IN 2007
As in several consecutive previous years, spring weather conditions have been conducive for two common diseases of spruce: needle cast diseases and Cytospora canker. In addition to disease, winter injury (as a result of weather conditions in winter) was a common problem diagnosed on spruce this year.
Spruce Needle Cast
Needle cast disease is caused by Rhizosphaera kalkhoffi. Another fungus, Stigmina lautii, may also be causing a needle cast disease in North Dakota. More data is needed to confirm whether this second fungus is in reality causing injury to spruce. Observational (anecdotal) evidence suggests Stigmina lautii causes a needle cast on blue spruce, but the extent of injury on blue spruce is not well documented. In 2006, Stigmina lautii was detected on samples with needle cast symptoms, and Rhizosphaera kalkhoffi was rarely observed on samples. For a more in-depth discussion of this possible new needle cast fungus, see the December 2006 issue of Tree Talk (a newsletter edited by Dr. Joseph Zeleznik, NDSU extension forester):
The Colorado spruce is especially susceptible to needle cast disease caused by Rhizosphaera kalkhoffii. Rhizosphaera needle cast often begins in the lower part of a tree, killing interior needles. The buds on the limb usually remain viable, and in more dramatic cases, the buds are accompanied by only 1-3 years’ worth of needles on a limb. Severe cases of the disease can eventually lead to death of lower limbs, if left unchecked over several years. Interior needles show symptoms that can range from a purplish to brown color (symptoms are not observed on current growth). Black fruiting bodies of the fungus can usually be observed with a 10x hand lens, and these fruiting bodies cause the normally white stomates along an infected needle to appear black (see photo).
Cultural practices that can reduce disease severity include promoting better aeration within the tree canopy. This may involve selective pruning within a tree or, more dramatically, the selective removal of closely planted, unthrifty trees. Planting spruces too close together should be avoided. Fungicides are effective, but timing is absolutely critical to protect the newly developing buds, and two applications in each of 2 consecutive years are required for the fungicide to be effective. The first application should occur shortly after bud break, when needles are 50% elongated (usually in early June), and the second application should be made when needles have reached their full length (about 3-4 weeks later). These two applications must be repeated the following year. It is probably too late to begin this spray regimen this year, but plans can be made to implement the spraying next year.
Spruce Cytospora Canker
Cytospora canker is another common spruce disease in North Dakota. This disease is often suspected if a tree shows a dead limb here and there in a tree. This disease does not usually manifest itself as many dead limbs in one area of a tree or in a spiral pattern around the tree. This disease can sometimes be difficult to diagnose, since fruiting bodies in cankers may be difficult to find (see photo). The canker can be difficult to see, but it is usually accompanied by abundant bluish-gray sap. Mechanical wounds, such as from hail or mowing, also have abundant sap, and these wounds are sometimes mistaken for cankers. The symptoms of mechanical injury and Cytospora canker are similar, but discoloration of needles associated with Cytospora canker sometimes occurs more gradually than symptoms associated with mechanical injury or breakage. Observation of actual fungal fruiting bodies is needed for confirmation of Cytospora canker.
No fungicides will cure a tree infected with Cytospora canker. Pruning about 6-8 inches below a canker, or all the way back to the trunk, may lead to eventual eradication of the fungus from the tree, but if a canker is missed or not pruned out entirely, the battle will continue. The best time to prune is before the spring rains, when the spruce is still dormant (late winter is ideal), before the fungus becomes active and spreads to another limb. Pruning tools should be sanitized between cuts to prevent spread of the fungus. Cultural practices, as described above to manage needle cast diseases, apply here as well.
Needle cast and Cytospora canker have each been observed on at least one sample submitted to the lab this spring, but neither disease was observed on samples nearly as often as what appeared to be winter injury. One way to distinguish Cytospora canker or mechanical injury from winter injury is to look for new growth. If new growth is present on a limb suspected to be infected with Cytospora canker, then winter injury is more likely the culprit rather than disease. If buds are still viable and abundant new growth is on a limb where all older needles have apparently died, the limb will usually recover and look normal within a few years. Winter injury rarely kills limbs, but considering the weather conditions in many parts of North Dakota in February, followed by a severe cold snap, this may be the year when winter injury was severe enough to kill individual limbs and possibly even entire trees. Extent of winter injury can really only be assessed by evaluating amount of new growth this summer.
The links below have more information on winter injury and diseases of spruce:
NDSU Plant Diagnostic Lab