Verticillium wilt is a fungal disease of the vascular tissue of many plants. The fungus infects the plant’s vascular system and thereby inhibits flow of water and nutrients. Usually when Verticillium infects, the plant wilts and dramatically dies. Sometimes, however, decline occurs over several years. Often Verticillium is blamed for the death or decline of a tree merely because wilting and death occur suddenly. In fact, many factors that can cause Verticillium-like symptoms.
One of the most diagnostic features of Verticillium infection is the appearance of a stain or streaking of the vascular tissue. This discoloration is usually brown or dark green. It can be seen as a ring of discoloration (or parts of a ring) when an infected branch is cut and the cut end examined. The stain has a streaked appearance when the bark is peeled back from the wood and viewed along the length of the stem. A positive diagnosis can be made by culturing the stained wood in a laboratory, isolating and identifying the Verticillium fungus. Verticillium does not always cause vascular streaking. It has been documented that infected ash trees do not always show vascular streaking.
Staining of the wood can also result from canker infections. In most cases, that staining is in the center of the stem rather than the vascular tissue but not always. Canker stains tend to be localized around a wound or canker, and the stain does not extend to wood very far from the canker. Staining from Verticillium extends into new and old wood alike.
Sudden wilting and decline may occur as a result of root injury or root trauma. Extreme drought or flooding injures roots and can cause foliage to wilt. Look at the weather patterns before symptoms occurred to confirm or rule out such injury.
Construction injury (mechanical injury) to roots or compaction as a result of heavy equipment can also injure roots and result in wilting and decline. Unfortunately, such injury does not always appear until construction has been completed.
Deep planting can cause a slow decline of trees, with Verticillium sometimes blamed for the damage. Look for vascular discoloration before you blame this disease. Lack of trunk flare at the ground line and the presence of the first root 5 or 6 inches underground should make one suspect deep planting as the culprit.
Girdling/circling roots have been shown to be involved in the decline of many trees. They are difficult to diagnose because they are below the soil and often give no clue other than tree’s decline. Look for trees that lean, with flare on only one side of the trunk, and poor growth for several years.
It is important to know whether Verticillium is present when considering replant options.
Verticillium is able to survive in the soil for many years and may infect hundreds of landscape plants. For more information on Verticillium wilt, refer to Report on Plant Disease, no. 1010, “Verticillium Wilt Disease,” available in University of Illinois Extension offices or on the Web at http://www.ag.uiuc.edu/%7Evista/horticul.htm.