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Verticillium Wilt of Trees

August 1, 2006
Here’s another disease that can kill mature trees. It is also a disease that may be confused with general decline and dieback. This fungal disease infects via the roots and eventually becomes systemic in a tree. Some disagree, but if you can catch infection early, branch removal and pampering may actually save a tree.

Verticillium wilt is caused by a fungus, either Verticillium dahliae or Verticillium alboatium. Symptoms include wilt, branch death and quick decline of plants. Hundreds of plant species, including trees, shrubs, groundcovers, vines, vegetables, fruits, herbaceous ornamentals, and flowers may become infected. We see symptoms throughout the growing season.

The most diagnostic feature on a suspect tree is the presence of vascular staining. You can see this staining as a ring of discoloration in a cross-section of an infected branch or possibly as streaks of discoloration visible on the wood when bark is peeled. Streaking can be caused by a cankers, some insects or injuries, and a few other wilt pathogens, so culturing of infected tissue is necessary for a positive ID. Staining from Verticillium continues down the stem toward the trunk, whereas cankers and other localized injuries have limited staining of the wood past the injured area. Staining of the center of the stem (the pith) is not characteristic of Verticillium wilt. That stain usually indicates an injury further down the branch. Usually the fungus can be isolated from these stained tissues. Samples should be alive, showing vascular streaking, thumb thick, and 8 to 10 inches long. Culturing can be done at the University of Illinois Plant Clinic for a fee of $12.50 per sample. See http://plantclinic.cropsci.uiuc.edu/ for details about the Plant Clinic.

Research confirms that stressed plants are more susceptible to infection. To complicate matters, symptoms of site and environmental stress can mimic Verticillium wilt symptoms. Stress, however, does not cause vascular streaking as described above. The image shows vascular streaking in a smaple that tested positive for this disease.

There is no cure for Verticillium wilt. Still, there are many cultural and preventive strategies to manage the disease and help infected trees live with the fungus. Always start with healthy plants and avoid susceptible species. Supply balanced fertilization and provide adequate irrigation to improve the health of stressed plants. This may help the tree “wall-off” infections and resist attack. Remove dead wood to avoid problems with wood rots and decay. When dead wood is removed, it should be burned, not chipped, and not reused in the landscape. Because the disease is soilborne, use only resistant species to replace Verticillium-infected plants. At the Plant Clinic, we usually see Verticillium wilt in maple, redbud, smoketree, ash, magnolia, and catalpa. Some suggestions for replacement plants can be found in Report on Plant Disease, no. 1010, “Verticillium Wilt Disease,” available in Extension offices or on the VISTA Web site, http://www.ag.uiuc.edu/%7Evista/horticul.htm. It is advantageous to control weeds in the landscape because many can serve as sources of inoculum. Dandelions, pigweed, horse nettle, and velvetleaf are all susceptible to Verticillium. Do not move soil from an infected area of the garden. This pathogen may be soilborne and can survive for decades in the soil. Laboratories cannot positively identify this disease on a dead tree but can isolate the causal fungus from live, symptomatic wood. Why should you bother if the tree will be removed anyway? The reason is that Verticillium is able to survive in the soil for many years without a host, and it may infect hundreds of landscape plants. It is important to know whether this fungus is present when considering replant options.