Verticillium wilt continues to be a relatively common problem among shade trees such as maple, ash, and many others. It is important to remember, though, 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 include root rot, cankers, deep planting, girdling roots, damaged branches or roots, poor soil-water drainage, and drought stress. The only way to prove the symptoms are caused by Verticillium wilt is to culture for the fungus (see issue No. 11 of this newsletter). To further confuse the issue, laboratory culturing is not 100% effective at identifying the presence of the Verticillium wilt fungus, and a primary field diagnostic symptom, vascular streaking, may not always be present even when the fungus is killing the tree. The bottom line is that this disease is not always easy to diagnose.
You are probably thinking, “Who cares why the tree is dying or dead? The outcome is the same.” Yes, but can we learn something from the tree or perhaps save it? Recently, I attended the Shade Tree Wilt Conference held in St. Paul, Minnesota. A highlight of the meeting was the root collar excavation work by Gary Johnson of the University of Minnesota. Using a nondestructive method, Johnson has examined over 40 species of “sick” trees, many diagnosed in the field as having Verticillium wilt. He found that Verticillium wilt is overdiagnosed in the field. Many times, these so-called “vert” trees are declining or dying from a problem that is preventable and possibly curable--namely, deep planting or girdling roots. Even if Verticillium wilt is positively identified in the tree, you should investigate the site for stress factors that may have allowed the fungus to enter the roots and cause disease.
Trees that have been planted too deep 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 (well below the soil grade), or have one or more flat sides at the base. Johnson has observed Littleleaf linden ‘Greenspire’ to be one of the more problematic trees, although Norway maple and members of the Prunus genus commonly have problems as well.
His diagnostic tools? A steel rod, three-pronged hand rake, small shop vacuum, and portable generator! Johnson demonstrated his work on two linden trees located at “Plant Pathology Headquarters” in St. Paul. The trees were 10 to 15 years old, about the same size, and planted on either side of a sidewalk in a protected area. However, one tree was leaning somewhat and the crown was much thinner than its “twin.” In this case, the girdling roots should have been obvious to the casual observer, unless they were covered by mulch. But the extent of the girdling could not be determined until some of the soil was removed.
Before our arrival, Johnson spent about 50 minutes examining the root system nondestructively. After probing the soil to find the depth and direction of the first roots, he carefully removed the soil from around the base of the trunk using a hand rake and vacuum. In this case, he found that 90% of the trunk was compressed (under pressure) by girdling roots. He estimated that this tree would die or blow down within two years. His recommendation? The tree will die, and it is a hazard, so cut it down now.
Does this sound like a waste of valuable time to you? Johnson believes that trees with girdling roots can be saved if the compressing roots are identified early and removed. As a rule, he says that if 50% or more of the circumference of the trunk is girdled and compressed, the tree, in an urban setting, will die well before its time. When that happens, how will the problem be diagnosed--Verticillium wilt, root rot, insects, or high winds?