Is My Spruce Diseased?
|April 18, 2006|
Over the past few months, I have received phone calls about spruce trees with brown needles. The growers are concerned that a disease is or possibly was involved. Here are a few tips to determining whether your spruce has been invaded by a pathogen or affected by other factors.
In our area of the Midwest, we usually see only two spruce diseases: Rhizosphaera needle cast and Leucostoma (Cytospora) canker. I have occasionally seen spruce needle rust in the northern counties of Illinois. We are also watching for a rather new but weak pathogen associated with spruce needle drop. That fungus is Setomelanomma holmii. If it is present in Illinois, it has not been identified as a major problem.
Rhizosphaera needle cast is a fungal disease that we usually don’t notice until June, but reports from Morton Arboretum (northern Illinois) confirm its presence this year. One-year-old needles turn a purplish brown color, but the newest growth remains green. The appearance of new, green growth in mid-May helps us notice this disease. Affected branches occur in patches around the tree, usually worst at the bottom of the tree. The presence of the fungus can be confirmed by placing some of the brown/purple needles in a moisture chamber (plastic bag with moist paper toweling) overnight and observing the characteristic fruiting bodies emerging in rows down the needles the next day. These fruiting bodies, which look like black pinheads sticking out of the stomata, are indications of a fungus, Rhizosphaera kalkhoffii, the cause of Rhizosphaera needle cast. Tree species also helps in narrowing the cause of symptoms. Blue spruce is most often infected, while white spruce is moderately susceptible and Norway spruces are resistant to this disease. Keep in mind that there may be more than one problem on a tree. Rhizosphaera often accompanies other problems. For some very nice pictures of the symptoms and fruiting bodies, visit this Forest Service Web site: http://www.na.fs.fed.us/spfo/pubs/pest_al/rhizo/rhizo.pdf.
Leucostoma (Cytospora) canker also occurs frequently on spruce trees in Illinois, especially trees on stressful growing sites or in stressful environmental conditions. In fact, this disease is most common on stressed trees at least 15 years old. Because drought-stressed trees seem to be most susceptible, and most of Illinois was under drought conditions in 2005, we would expect to see more of this disease in 2006. Cankers are dead areas of branches or trunks. They may girdle and kill the branch, or they may only limit movement of water and nutrients. On spruce, there is usually a sappy exudate associated with the canker; but this sap is a thin layer, not the large blobs of sap associated with some insect pests such as pine shoot moths. The wood under the bark of a tree with Leucostoma canker is brown, rather than green or white. Species affected include mostly Colorado blue and Norway, but other spruce species (as well as Douglas-fir, balsam fir, hemlock, larch, and red and Eastern white pine) may have this canker disease. The symptoms start on the lowest branches and move upward, branch by branch, over a number of years. Unlike Rhizosphaera, Leucostoma causes both old and new needles to be discolored. A University of Illinois fact sheet, “Cytospora or Leucostoma Canker of Spruce, Report on Plant Disease (RPD), no. 604, is available on the Internet at http://www.ag.uiuc.edu/%7Evista/horticul.htm. Most Illinois Extension offices also have copies of this fact sheet. Two species of the Chrysomyxa rust fungus usually cause needle rust on spruce. The disease is common on black, white, and Colorado blue spruce in Minnesota and occasionally occurs on Norway spruce. It is not very common in Illinois.
Spruce needle rust causes yellowing of the foliage. On close inspection, needles have small, raised pustules about one-third the needle thickness. These pustules contain orange spores. Often the edges of the pustules have a white, paperlike appearance, so you may see white, paperlike structures sticking out of the needles. These structures may be as long as the needle is wide. Yellowing and sporulation does not occur until midsummer, so you probably won’t mistake this disease with early-season problems. A USDA fact sheet available at http://www.fs.fed.us/r10/spf/fhp/leaflets/Sprneerus.htm provides images of this disease and pathogen.
Plant diseases are not always the cause of spruce decline. The spruce samples that I have worked with this spring have not been infected with the above-mentioned fungal pathogens. Incubated tissue did not yield disease pathogens. Instead, environmental stress factors have been implicated. It is often difficult to prove environmental stress. Look at the pattern of brown needles. The noninfectious cases we have seen exhibited needle necrosis that was somewhat scattered on branches but was most intense on the south and west side of the trees. All of the spruce trees in the landscape were affected, though not necessarily in neighboring landscapes. The south and west sides are usually desiccated by sun and wind. The drought of 2005 may also have contributed to the needle necrosis. Look for live buds and green tissue in new stems as indicators that the tree is still healthy. Such trees may need supplemental water this season in periods of drought. There are many other noninfectious problems that can mimic disease. A few possibilities include an imbalance in soil pH, poor fertility, fertilizer or chemical burn, root injury, root rot, drought stress, and spider mite infestations.
If you are uncertain of your diagnosis, consult a University Extension office anytime or the Plant Clinic (http://plantclinic.cropsci.uiuc.edu/) after May 1.
||Nancy Pataky |