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Published May 11, 2015, 08:53 AM

Growing Together: Area spruce trees ravaged by new disease

FARGO -- A potentially devastating disease epidemic is attacking our region's most common evergreen trees. Colorado, Black Hills and Norway spruce have long been planted in yards and shelterbelts. Most of the tree-type evergreens in our region are spruce, ranging in color from silvery blue to deep green and growing 40 or 50 feet high. And now a relatively new disease called Stigmina needlecast is causing serious damage.

By: Don Kinzler, Forum News Service

FARGO -- A potentially devastating disease epidemic is attacking our region's most common evergreen trees.

Colorado, Black Hills and Norway spruce have long been planted in yards and shelterbelts. Most of the tree-type evergreens in our region are spruce, ranging in color from silvery blue to deep green and growing 40 or 50 feet high. And now a relatively new disease called Stigmina needlecast is causing serious damage.

Today's topic was prompted by a reader whose spruce branches were losing needles. When asked who might provide a diagnosis, I didn't hesitate. Jim Walla researched tree diseases at North Dakota State University for 36 years until semi-retirement. Now, through his business Northern Tree Specialties, Jim offers tree health consulting.

When Jim diagnosed Stigmina needlecast, I knew we needed to inform the public. Last year, Jim was hired by the North Dakota Forest Service to perform a spruce health assessment. As an example of his findings in Cass, Stutsman, Grand Forks, Ramsey, and Pierce counties, needlecast was found on 72 percent of spruce examined, and was severe on 40 percent.

The frequency is alarming. I asked Jim to summarize the disease, its diagnosis and its control. Jim indicated that few people are aware of the disease, which was first noted in North Dakota in 2006. A disease called Rhizoshpaera has been in the region for years, but the new disease, Stigmina needlecast, is more widespread, causing more needle loss, and killing more branches.

An obvious effect of the disease is the thinning of the tree's appearance as needles are lost. Spruce tree health can be assessed by "crown porosity," which indicates how much light can be seen as you look at the tree. A healthy spruce has an average 20 percent porosity. A tree with needlecast disease thins out, causing average porosity of 40 percent, and severe infections cause thinning to 60 percent porosity, as more needles are shed. For spruce trees, that's devastating.

Needles showing symptoms of the disease become discolored--yellow, purple, tan to reddish brown. Once discolored, needles usually remain attached to the tree for a year before they drop.

How can you tell if a spruce has Stigmina needlecast? The only way to diagnose the disease is to see the "fruiting bodies" of the fungus, which are tiny black masses that produce spores capable of causing more infections. You can see the black fruiting bodies on needles using a magnifying glass of at least 10x magnification. They are most common on the oldest branch needles, whether needles are green or have turned brown, and can be on all sides of needles. Jim showed me how to spot the black fruiting bodies with a magnifying glass, and they're easily seen. Samples can also be sent to the Plant Diagnostic Labs of either North Dakota State University or University of Minnesota.

How can this disease be controlled? Fungicide sprays should not be applied unless you are sure Stigmina needlecast is present, because other problems like spider mite damage, winterburn, heavy shade and high soil salts cause similar symptoms.

But if Stigmina is found, fungicide applications are needed to sustain tree health and prevent further decline. There are two approaches to using fungicides to combat needlecast, depending on how many times per season you are willing or able to spray.

The minimal approach is to spray twice per year, once soon after buds open, and again one month later. This option must be repeated every year as long as you want to maintain a healthy tree appearance.

A second, more intensive option maintains the fullest tree density. Spray five to seven times per growing season beginning when spores form in spring, which is usually mid-May. (This year spores formed by mid-April.) Continue at approximately monthly intervals until mid-October. This option should be continued for four to five years until infected needles are no longer found. Then a break in treatments is possible, but trees must be checked each year.

Chlorothalonil is the fungicide commonly used for spray treatments, and is effective when used at the appropriate time following label concentrations. It's sold under many brand names at numerous garden outlets. Check the fine print of the label for "Active Ingredient: chlorothalonil."

Cultural practices can help prevent or lessen Stigmina infection. Because moist conditions favor fungal diseases, strive to promote air circulation. Prune up lower branches, thin out trees that were planted too closely, and avoid wetting spruce with lawn sprinklers.

People who own or manage spruce trees need to learn about the disease. We don't know how long this epidemic will continue, or how devastating will be its effects. Watch for an upcoming North Dakota Forest Service publication: Spruce Health Assessment in North Dakota, 2014. Additional information and photos are available in NDSU's Extension bulletin F1680 "Two Needle Diseases of Spruce in North Dakota," viewable online and in print.

Thank you, Jim Walla, for guidance. Besides tree health advice, Northern Tree Specialties provides tree and shrub propagation, and customized multivariety fruit trees. Jim can be reached at NorthernTrees@Outlook.com or his Facebook site "Northern Tree Specialties w/Jim Walla--Tree Doctor."

Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler's Greenhouse in Fargo. Tune in to his weekly radio segment from 11 to 11:30 a.m. Fridays on WDAY Radio 970. Readers can reach him at forumgrowingtogether@hotmail.com.

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