Verticillium wilt is a name associated with death of mature trees, shrubs, perennials, and vegetables alike. Once the causal fungus enters the vascular tissues of the host plant, it is likely to spread quickly, block the water-conducting vessels, and cause plant death. Tips on how to sample for Verticillium testing were discussed in issue no. 5 of this newsletter. We have seen several cases of Verticillium wilt in the lab recently, possibly due to heat and moisture stress and because of its association with stressed plants.
One host that seems to be more frequently infected with this fungus in the past several years is magnolia. If your magnolia tree shows branch-by-branch decline or an overall thinning of the branches, cut into a few finger-sized branches and look for staining of the wood.
A diagnostic clue for detecting Verticillium wilt is the presence of vascular streaking or vascular discoloration. The disease is common on maple. On that host, the stain is dark green to brown and usually found in a circular pattern when the stem is viewed in cross-section, as seen in the left image. On redbud (another common host) the staining is darker and very distinct, as seen in the middle image. Infected magnolia stems appear to have more of a general brown color of the sapwood, as seen in the third image. In a recent Plant Clinic magnolia sample, tissues showing the distinct type of staining and the general discoloration of the sapwood both yielded the Verticillium fungus in the lab. Take a close look at your magnolias to be certain you are not missing this wilt disease.
Because Verticillium lives in the soil for many years, even without a host plant, the replacement plant needs to be resistant to this fungus. The list of plants not yet known to be infected by Verticillium is short. Therefore, if you plan to replace the dying plant, it is important to confirm a case of Verticillium wilt.
For information about Verticillium wilt, refer to the Report on Plant Disease, no. 1010, “Verticillium Wilt Disease.” It is available for downloading from the Internet at http://www.ag.uiuc.edu/%7Evista/horticul.htm. It is also available in Illinois Extension offices.