Verticillium wilt is a fungal, vascular disease that causes branch-by-branch decline or sudden death of trees. Usually, the affected tree dies slowly during the season, with winter striking the final blow. 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? The reason is that Verticillium can 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.|
Verticillium wilt is caused by Verticillium dahliae or Verticillium albo-atium. Symptoms include wilt, branch death, and quick decline of plants. Hundreds of plant species, including trees, shrubs, vines, fruits, vegetables, groundcovers, herbaceous ornamentals, and flowers may become infected. We see symptoms anytime in the growing season. Research confirms that stressed plants are more susceptible to infection.
This pathogen may be soilborne and can survive for decades in the soil. It may also be present in apparently healthy plants and can be brought to the landscape on such plants. Roots of susceptible plants come into contact with soil contaminated with the pathogen, which then enters wounds in roots and grows into the cortex. The fungus produces spores in the roots. These spores are transported systemically upward in the xylem. Spores then lodge throughout the plant, and new hyphae grow and spread. If the stem remains alive, Verticillium grows close to the meristematic regions and allows continuous infection each year, as long at the plant lives. Verticillium survives as small, resting bodies in diseased plants, eventually returning to the soil.
Spore germination and hyphal growth cause a staining of the vascular tissue in streaks. This symptom is characteristic of the disease 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.
There is no cure for Verticillium wilt. Still, there are many cultural and preventive strategies to manage it and help the tree live with the fungus. Always start with healthy plants and avoid susceptible species. Supply balanced fertilization and adequate irrigation to improve the health of stressed plants; you can help the tree “wall off” infections. Branches and trees with wilt symptoms should not be removed immediately, as they may recover with fertilization and watering. Remove dead wood to avoid problems with wood rots and decay. When dead wood is removed it should be burned, not chipped and 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 this disease in maple, redbud, smoketree, ash, magnolia, and catalpa. Suggested 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. It is advantageous to control weeds in the landscape because many can serve as sources of inoculum. Dandelions, pigweed, horsenettle, and velvetleaf are all susceptible to Verticillium.