We saw a fair number of positive Verticillium cases at the Plant Clinic in 2003. A sample is considered positive when the Verticillium fungus is actually isolated from the plant sample in the lab. There is no doubt that Verticillium wilt continues to be a relatively common problem among shade trees such as maple, ash, catalpa, magnolia, redbud, and fragrant sumac. However, it is important to remember that the basic “Vert” symptoms such as wilted, yellowed, scorched or dead leaves; early fall color; dieback; and even discolored vascular tissue can indicate a number of different plant problems. These problems might include root rot, cankers, deep planting, girdling (encircling) roots, damaged branches or roots, poor soil-water drainage, and drought stress. All too frequently, Verticillium is blamed because that is the easy out. It might be interesting to note that of 55 woody samples submitted to the Plant Clinic for Verticillium culturing this summer, only 12 tested positive. The only way to prove the symptoms are due to Verticillium wilt is to culture for the fungus (see issue no. 6 of this newsletter for details). Laboratory culturing is reliable in identifying the presence of the Verticillium fungus, but only if the tissue is alive and vascular streaking is present. In many cases, woody tissue exhibiting vascular discoloration was not submitted. The fungus is not uniformly distributed throughout the plant; and it is most likely to be recovered from areas of vascular streaking.
To some, it may not matter why the tree is dying. In fact, however, we can often learn something from the dying tree. Root collar excavation work by Gary Johnson at the University of Minnesota showed that many trees diagnosed with Verticillium wilt were often misdiagnosed. In many cases, the trees were actually declining or dying from a problem that is preventable--deep planting and girdling roots. Keep in mind that even if Verticillium wilt is positively identified in the tree, you should investigate the site for stress factors that may have predisposed the roots to infection. Trees that have been planted too deeply or that have girdling roots may exhibit canopy symptoms such as early fall color, thin canopy, or reduced annual twig growth. In addition, the trunk may be leaning, have no noticeable flare roots, or have one or more flat sides at the base. Dr. Johnson has observed Littleleaf Linden ‘Greenspire’ to be one of the more problematic trees with these problems, although Norway maple and members of the Prunus genus are commonly affected as well. Deep planting has been observed on clinic samples on ash, maple, and even spruce.
When you plant a new tree or shrub, first locate the main root/trunk junction. The first root should be buried just below the soil line. When you get your tree or shrub for planting, loosen the burlap or root ball covering and carefully probe near the trunk to find the first large root. You may find that you will need to remove some soil (sometimes several inches) from the top of the root ball before figuring the depth of the hole and placing the plant. Do this carefully so as not to injure the trunk or the roots. This process should help prevent later problems associated with deep planting.
The Verticillium fungus lives for many years in the soil. If the fungus is confirmed in the plant sample, it is also present in the soil. Only plants with resistance to Verticillium wilt should be planted in infested sites. This is another benefit to knowing the cause of tree decline.
When making a statement that a plant is infected with Verticillium wilt, be certain that symptoms fit, vascular streaking is present, and other site, environmental, and mechanical possibilities have not been overlooked. For details about Verticillium wilt, refer to Report on Plant Disease, no. 1010, available in Extension offices or on the Vista Web site, which you can locate under links of interest on the Plant Clinic Web site: http://plantclinic.cropsci.uiuc.edu/.