Verticillium wilt is a serious fungal disease that causes injury or death to many plants, including trees, shrubs, ground covers, vines, fruits and vegetables, and herbaceous ornamentals. It is a disease of the xylem, or water-conducting tissues, in the plant. Commonly infected woody plants include maple, smoke-tree, catalpa, and magnolia, among others.
One or more branches, usually on one side of the tree, wilt suddenly. Sometimes the leaves turn yellow before they wilt, or leaf margins turn brown and appear scorched. In some instances, there is a slower decline in new twig growth, or dead twigs and branches appear. On maples and tulip trees, elongated dead areas of bark, called cankers, may appear on diseased branches or trunks. In Illinois, these symptoms usually occur in July, but can be seen as early as May or as late as October.
Internally, diseased trees may exhibit discolored sapwood in the recent annual rings. In maples, Verticillium produces greenish streaks; in smoke-tree, the streaking is yellow-green. In other woody plants, the discoloration is brown. In some trees and on younger twigs, discoloration does not occur or is found several feet below the point where leaves are actually wilting. This makes identification difficult.
There seem to be two forms of the disease, one in which plants die slowly over several years and another where they die rapidly within a few weeks. Trees that show minor branch wilt one year may show more the next year or may not show symptoms again for several years. There is some evidence that unbalanced fertilization (too much or too little nitrogen, for example) exacerbates this disease.
Verticillium wilt is caused by a soil fungus called Verticillium dahliae. Another species, Verticillium albo-atrum, is less common. This fungus lives in the soil as small, darkened structures called microsclerotia. These microsclerotia may lie dormant in the soil for years. When the roots of susceptible plants grow close to the microsclerotia, the fungus germinates and infects the roots of the plants through wounds or natural openings. The fungus spreads into the branches through the plant’s vascular system and simultaneously causes the plant cells to “plug” themselves. Once the xylem is infected, it becomes so plugged that water can no longer reach the leaves. Verticillium can also be spread to plants through wounds on branches or trunks.
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.
For laboratory identification, select twigs that are about 1/2-inch in diameter and approximately eight inches long. The twig must be from a branch that is actively wilting, but not yet dead. Wrap the samples in wax paper or other material that will keep the sample from drying out. Mail the sample (overnight, if possible) with your name, address, and a history of the problem to the University of Illinois Plant Clinic. Phone 217-333-0519. There is a charge per sample.
Verticillium wilt is difficult to control because it persists in the soil indefinitely. Infected trees that are not yet dead sometimes “outgrow” the fungus. 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 by dipping them in a diluted cleanser, such as Lysol, Pinesol, or household bleach, 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 fourinch 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, baldcypress, 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.
At this time, there is no known chemical control for this disease.