Verticillium wilt has a number of different symptoms. External symptoms may be acute or chronic in their expression. There are also internal symptoms. These numerous symptoms can make diagnosis difficult. Only laboratory examination can positively diagnose the disease.

Acute symptoms are seen in mid-season and often found on scattered branches throughout the host plant. These symptoms include changes in leaf color (red, brown or yellow, depending on host), wilting, defoliation and dieback of stems. On woody plants, one or more branches may wilt suddenly. Sometimes the leaves turn yellow before they wilt, or leaf margins turn brown and appear scorched.

Chronic symptoms are usually seen early in the growing season because they arise from stress due to death of sapwood that occurred in the previous year or years. These symptoms can include slow growth, sparse or small foliage, leaf scorch, abnormally heavy seed crops and dieback of branches.

The internal symptom is discolored sapwood in the recent annual rings. When bark is stripped from an infected branch, streaks of discoloration may be seen. In most woody plants, the discoloration is brown. In maples and magnolias, the streaking is green. In ash trees, no streaking is found.

The fungal organism that causes Verticillium wilt lives in the soil as small resting structures. These structures may lie dormant in the soil for years. When soil conditions are conducive to the fungus and the roots of susceptible plants are growing nearby, the fungus germinates and infects the roots of the plants. While healthy roots can be infected, damaged or compromised roots are more vulnerable to infection. The fungus may enter through wounds or natural openings. It then spreads into the plant’s vascular system. The xylem becomes blocked, not only by the fungus, but by the plant’s production of gums intended to stop the fungal growth. Once the xylem is infected, it becomes so clogged that water can no longer move through the plant and reach the leaves. The fungus is able to produce the resting structures in the dead tissues. These dead tissues can re-infect the soil.

The appearance of streaking helps to identify the disease but does not guarantee that the tree has Verticillium. Sometimes other factors or diseases cause discoloration of sapwood. Only laboratory examination can positively diagnose the disease. Samples should be sent to a diagnostic laboratory as soon as Verticillium wilt is suspected. The University of Illinois Plant Clinic will confirm Verticillium wilt for a nominal charge. Some arborist companies have their own labs or have access to other labs and can assist with obtaining a proper diagnosis. Having a precise diagnosis does not save the infected tree. Knowing that Verticillium wilt is the issue and that it lives in the soil, will help direct the decision on which plants to replant in the area.

Cultural Management:

Verticillium wilt is difficult to control because it persists in the soil for long periods of time, even if no host plant is present. Dead branches should be pruned out to help overall plant vigor. The disease can be transmitted on pruning tools. It is recommended that tools be sterilized between cuts and between trees.

To avoid stress, trees should be planted in sites that are favorable to their growth. Water thoroughly during dry periods. Use a three to four inch layer of organic mulch to retain moisture and prevent soil temperature fluctuation. Fertilize properly and avoid injuries to the roots, trunk, and branches. Severely infected trees should be removed and replaced with plants that are not susceptible to Verticillium. Trees that are not known to be susceptible include: arborvitae, bald cypress, beech, birch, boxwood, crabapple, ginkgo, hackberry, hawthorn, hazelnut, hickory, holly, honey locust, hornbeam, ironwood, Katsura tree, mulberry, oak, pine, serviceberry, spruce, sweetgum, walnut, willow, and yew.

Chemical management:  

At this time, there is no known chemical control for this disease.

For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).