Blue spruces are a favorite conifer of Iowans and several reasons. In addition to being a beautiful tree that maintains color year round, these trees make an excellent wind break and for the most part grow quite well here. When we see diseases in blue spruce, which is not often, it usually is one of two different types--Cytospora Canker and Rhizospharea Needle Cast.
Cytospora Canker of Spruce
The most common canker disease observed in the Iowa State University Plant Disease Clinic is Cytospora canker on blue spruce. Cytospora canker is observed most often on older trees, especially those that are planted in poor sites. Trees weakened by environmental stresses, such as drought, freeze injury, or high temperatures, also are more susceptible to canker diseases. The Cytospora canker fungus may attack many different species of hardwood trees, conifers, and shrubs.
Spruce trees infected with the Cytospora canker fungus typically show scattered branch dieback, often starting on the lower branches. A close look at the dead branches usually reveals the presence of sticky white sap. Infected trees produce this resinous sap in response to the infection by the canker fungus.
The Cytospora fungus gains entrance into branches or twigs of trees through wounds or branch stubs. Over time, the fungus encircles or girdles branches, causing death. Brown needles can be observed on killed branches, but they eventually fall off, leaving bare branches.
As with many diseases, the best control for Cytospora canker is prevention. Plant trees in a good site, one that is well-drained and allows unrestricted growth as the tree matures. Adding mulch around trees increases overall health in many ways, including reducing competition from turfgrass. If dry conditions occur, water deeply if feasible. Any cultural practice that promotes good tree vigor helps prevent canker diseases.
Pruning out diseased branches is the primary means of treating trees showing symptoms of Cytospora canker. Scout declining trees closely for cankers. Prune at least 4-6 inches below any visible cankers. Some branches may need to be pruned back to the trunk. To minimize spread of the disease, prune only during dry weather. The fungal spores of Cytospora can be easily spread when conditions are wet. Fungicide sprays are generally not effective at controlling canker diseases.
To learn more about common canker diseases observed on trees in Iowa, refer to the Iowa State University bulletin SUL 11, Fungal Cankers of Trees (pdf). You can also get this publication at the extension office in Logan.
Rhizosphaera Needle Cast
Blue spruce trees are susceptible to an infectious needle disease caused by the fungus Rhizosphaera. The disease, referred to as Rhizosphaera needle cast, is the most common problem seen on blue spruce samples that are submitted to the Plant Disease Clinic. White spruce are classified as intermediate in susceptibility to the disease and Norway spruce are relatively resistant.
Symptoms - The disease is usually first evident on lower branches and then works upward gradually. Second-year needles turn a purple or brown color and eventually fall from the tree. After several successive years of needle loss branches may die. In general, trees appear to die from the bottom upward. In some cases, however, infections start higher on the tree, giving the appearance of scattered dead areas.
The disease can be diagnosed by looking at the discolored needles with a magnifying glass or hand lens. Small black spots (fruiting structures of the fungus) appear in rows in the infected needles. The fungus is actually emerging from the stomata (natural pore-like openings) that occur in lines on all sides of a spruce needle. Green needles may show these small black fruiting structures.
Keep in mind that environmental or site-related stresses can also cause discoloration and loss of needles on spruce trees. The extended wet weather of 1993, for instance, has been responsible for needle browning and even tree death in some cases. (Fruiting structures of the fungus are not evident on these trees.)
Spread - Rhizosphaera over winters in infected needles on the tree and on needles that have fallen to the ground. The fungus is spread by splashing and dripping water beginning in spring and continuing into the fall. Newly emerging needles can become infected during wet spring weather.
Control - If symptoms appear, diseased trees should be sprayed with a fungicide in the last 2 weeks of May and again 4 to 6 weeks later. Be sure to read the product label for specific rate and timing instructions. Good coverage and proper timing of applications are critical for successful disease control. Fungicides labeled for Rhizosphaera needle cast include Daconil 2787, Daconil Ultrex, Terranil 90, Thalonil 4L, Thalonil 90, Manicure Flowable, and Twosome Flowable.
In addition to fungicide sprays, other control measures include spacing trees adequately to promote good air circulation, improving tree vigor through mulching and watering when needed, and not shearing trees when the foliage is wet.
Rhizosphaera needle cast is pictured and discussed in Pm-1528 "Common Diseases of Conifers in Iowa.”