Verticillium wilt is caused by a fungus, either Verticillium dahliae or Verticillium albo-atium. 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.
Start watching for this disease now. Peel off some of the bark on a symptomatic branch and look for staining of the wood in distinct streaks of brown, dark green, or yellow-green wood. The stain shows right away and should not be confused with the browning that occurs as the wood is exposed to the air. Also try cutting a finger-sized twig and looking at it end on. There may be a ring of discoloration much deeper in the wood, often as much as 1/4 inch under the bark. 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. Similar staining of the wood can occur from canker fungi. That stain is more localized and is not seen as a systemic staining. Verticillium-infected ash trees do not always show staining. Obviously, vascular staining is a symptom of Verticillium wilt but not proof of infection. Look for this staining in the wood as areas to be tested in a lab. The fungus can be most successfully isolated from these stained tissues. Samples should be alive, showing vascular streaking, thumb-thick, and 8 to 10 in. 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.
There are many research reports discussing Verticillium wilt and stress, but these findings vary by host, by soil type, by moisture availability and by geography. Disease severity has been reported to be worse in sandy loam, loam, clays, and soils high in organic matter. That seems to cover the range for Illinois. In addition, frequent irrigation of sandy soils increases the Verticillium infection possibilities; and in the landscape, sandy soils must be irrigated more frequently. Even fertilizer recommendations are conflicting. Various macro- and micronutrients interact with differing soil factors. There does seem to be agreement that low to moderate levels of nitrogen fertilization seems to produce the best results in terms of disease avoidance.
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 soil-borne, 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 Extension 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 soil borne and can survive for decades in the soil. Laboratories cannot positively identify this disease on a dead tree but can easily isolate the causal fungus from live, symptomatic wood. Why bother if the tree will be removed anyway? The reason is that Verticillium is able to survive in the soil for many years and may infect hundreds of landscape plants. It is important to know whether this fungus is present when considering replant options.