Dakota State University -- NDSU Agriculture
7 Morrill Hall, Fargo ND, 58105-5655, Tel: 701-231-7881, Fax: 701-231-7044
May 23, 2003
Spruce Trees Turning Brown? Look for Fungi
Property owners are often alarmed at this time of year to find the needles of their spruce trees turning brown. In this region, the cause is often two fungal diseases that affect spruce trees, says a forester at North Dakota State University.
Rhizosphaera needlecast and Cytospora canker both cause needles to turn brown and both typically begin in the lower branches of a tree. The similarities end there, however, and differentiating between the two is important to ensure proper treatment, says Joe Zeleznik of the NDSU Extension Service.
Rhizosphaera needlecast, caused by the fungus Rhizosphaera kalkhoffii, is the more common of the two. "The disease gets its name because the tree ‘casts’ off its needles, often in the spring," Zeleznik notes.
Confirmation of this disease requires looking at the green needles on branches that also have some brown and often purple needles, or those branches just above the browning branches. "With a 10X magnification lens you can see small black spots that are lined up along the length of the needle in two rows. Normally, these rows of dots appear white. The black spots are the fungus’ fruiting structures that produce spores that cause new infections," he explains.
Needlecast often begins in the lower part of the tree, killing the interior needles as it progresses up the tree. The needles will initially turn yellow for a few days and later purplish-brown. Infected needles may turn brown within six to eight months after infection or remain green for several years. The infected needles may fall before browning or remain attached for several months after browning.
In severe cases only the current season needles remain green. Prolonged defoliation kills the lower branches first then continues up the stem, eventually killing the tree.
Management for Rhizoshpaera needlecast requires two applications of a chlorothalonil fungicide. Several formulations of chlorothalonil are available. Timing of the fungicide application is critical.
The first application should be made when the new needles are approximately 1/2-inch to 3/4-inch long, and should be done within a week of the time the spruce trees shed the little brown caps as the new growth emerges, or about when the lilacs are blooming (late May to early June).
The second application should be made 3 to 4 weeks later when needles have fully expanded.
"It is critical to do both applications to get the disease under control in a tree. These two applications of fungicide should be done for two consecutive years," Zeleznik says. As always, application of chemicals should be done according to label directions.
The other disease, Cytospora canker, is caused by the fungus Cytospora kunzei. When this disease is present in a tree, typically the lower branches are affected first. Unlike Rhizosphaera, the needle browning begins at the end of the branch and proceeds back toward the trunk of the tree.
Needles generally turn a light tan to brown and may remain attached for a year or two and often, sap is extruded from infected branches. It appears as sticky, white or bluish-white ooze on the infected branch or on branches immediately below infected branches. Drought-stressed trees are more susceptible to Cytospora canker.
"This disease is not managed through fungicide application because the fruiting structures produced by this fungus exist just below the bark of infected branches," Zeleznik says. "In this case, management requires judicious pruning out of infected material. All branches with the needle browning symptoms should be removed. It is often prudent to take branches that are immediately above the infected branches as well since they may have already become infected by spores released from symptomatic branches."
Additional information may be obtained from your local NDSU Extension Service County office, the North Dakota Forest Service, or at www.ext.nodak.edu/extpubs/plantsci/trees/f1192w.htm.