Verticillium wilt is a an occasional problem on many species of trees and shrubs in Iowa. It is most common on maples, but also occurs on ash, redbud, smoketree, and other tree and shrub species. Because its highly variable symptoms mimic those of declines caused by environmental stresses, Verticillium wilt is frequently misdiagnosed. Over 300 other woody and herbaceous plant species are known to be susceptible to Verticillium wilt.
Symptoms of Verticillium wilt can develop at any time during the growing season. Symptoms can be chronic (long-term) and acute (relatively sudden and severe). Acute symptoms include premature fall coloration of leaves or leaf margins; wilting; defoliation (leaves fall from the tree); branch dieback; and death. Leaves turn yellow in some species; in others, such as green ash, leaves often fall from the tree while they are still green. Chronic symptoms typically reflect damage from earlier infections; these symptoms include slow growth, sparse foliage, stunted leaves and twigs, leaf scorch, and abnormally heavy seed crops. Acute and chronic symptoms often occur together on the same plant. Sometimes only one branch, or one side of a branch, is affected at first. Symptoms may or may not reoccur in subsequent years; sometimes, symptoms reappear after an absence of several years. In other cases, the entire plant wilts and dies rapidly soon after the disease appears. Because the symptoms of Verticillium wilt are so variable, it is often confused with decline due to environmental stresses, which may show similar symptoms. Streaking of the sapwood (the outer, water-conducting wood immediately under the bark) is common in some trees, such as maples, but is not present consistently. Streaking is a greenish to brown discoloration of the normally light-colored vascular tissue in the sapwood. It is more likely to be present in the trunk and lower limbs, but sometimes appears in twigs as well. In recent infections, streaking will appear only in the current year's (outermost) wood, but streaking resulting from earlier infections can sometimes be seen in inner wood layers. To look for streaking, remove the bark and examine the outer sapwood. In addition, cut cross- sections of larger branches or trunks and examine these for arcs or rings of discolored wood. Streaking is often absent in certain types of trees, such as ash.
The causal organism of Verticillium wilt is the soil-dwelling fungus Verticillium dahliae. Verticillium albo-atrum, a closely related fungus, can also cause the disease, but is rarely found in woody plants.
How Verticillium Causes Disease
Verticillium invades plant roots as they grow through the soil. Inside the invaded roots, spores of the fungus are carried upward in the xylem (the vascular tissue that transports water), spreading the disease to the trunk and branches. Spores can germinate and infect tissues anywhere in the xylem. Verticillium releases toxins (poisons) that can damage plant cells further down the xylem passageways, even where spores have not yet reached. As a result, the fungus often cannot be isolated from affected branches, even though streaking or wilting may already be visible there.
Plants respond to Verticillium infections by producing swellings, called tyloses, in the xylem vessels in an attempt to prevent the invading fungus from spreading further. However, tyloses also pinch off the upward flow of water to branches and leaves. The combined effects of toxins, cell invasion by the fungus, and tyloses leads to the development of typical Verticillium wilt symptoms.
The stop-and-start pattern of symptom appearance that sometimes characterizes Verticillium wilt results from the plant's attempts to compartmentalize (contain) the fungus behind barriers of new wood. When this strategy is successful, the fungus fails to grows into a new season's sapwood and symptoms of the disease may moderate or even diappear for some time. However, if the fungus later manages to grow through the barriers, symptoms will reappear, sometimes after an absence of several years.
The fungus can survive for many years in soil in the form of tiny, black structures called microsclerotia. Microsclerotia can even form on and in the fine roots of many species of resistant plants without causing symptoms. Therefore, crop rotation is not a very useful control option for this disease. Another result is that the microsclerotia can spead long distances when attached to the roots of nursery-grown plants that may be shipped nationwide. Microsclerotia can also spread long distances in soil attached to plants and equipment. When stimulated by nutrients from decaying organic matter or roots, microsclerotia can germinate and infect plants. If no host plant roots are nearby, these hardy structures can go dormant again, waiting for another opportunity to attack.
Management of Verticillium Wilt
Management of Verticillium wilt is a challenging task. The first step is to confirm a suspected diagnosis. The presence of both typical symptoms and streaking is strong evidence, but a laboratory culture should be done to confirm the diagnosis. The Plant Disease Clinic (mailing address: 351 Bessey Hall, Iowa State University, Ames, IA 50011) will do this test. A satisfactory sample should include several branch segments, with attached leaves, showing a range of symptom severity if possible. Dead or dried-out branches usually cannot be diagnosed reliably.
No fungicides are effective against Verticillium wilt. Soil fumigants can reduce or eradicate the fungus in soil, but are too hazardous for use in most home landscapes.
In addition to the genetic susceptibility of the host plant and the strain of the fungus, the environment plays a major role in the severity of Verticillium symptom development. Regularly watering woody plants during dry periods can help them to contain the infection and reduce symptom severity. A soil nutrient balance that is somewhat high in potassium and low in nitrogen appears to help plants resist the disease.
Trees showing severe, widespread wilt and dieback cannot be saved. The encouraging part of the story is that many woody species are resistant or immune to the disease (Table 1). When replacing Verticillium-infected plants, select a species from this list and avoid species that are known to be susceptible (Table 2). Proper fertilization and watering after transplanting will help the new plant to overcome transplant shock and resume vigorous, healthy growth.
If Verticillium-infested branches or trees are chipped after removal, avoid using the chips as mulch around Verticillium-susceptible plants until they have been heated in a pile for at least one month. The heat from the piled chips effectively kills the Verticillium fungus.
Table 1. Trees and shrubs resistant or immune to Verticillium wilt.
Oak, white and bur
Table 2. Trees and shrubs susceptible to Verticillium.
Cherry and other stone fruits
Coffee tree, Kentucky
Currant and gooseberry
Oak, pin and red (rare)
1 Resistance or susceptibility of these species depends on the cultivar and/or the strain of Verticillium present in the soil.
This article originally appeared in the March 13, 1998 issue, pp. 19-21.