Verticillium wilt is caused by a soil-borne fungus, Verticillium dahliae. The disease can affect more than 350 plant species, including deciduous trees, vegetables, berries, and flowers. Once a plant has a disease, there is no fungicide, that’s why preventative measures are so important.
The diagnosis of verticillium wilt is tricky, too, because the symptoms are different in every plant species, and wilting can have many other causes, such as fusarium wilt, bacterial wilt, root rot, or drought or excessive soil moisture. Only a laboratory test can reliably determine whether it’s verticillium wilt.
There is a second verticillium pathogen, Verticillium albo-atrum, which affects a much smaller range of species, including hops, alfalfa, and cotton. It is less relevant to home gardeners and therefore not covered here.
Many different ornamental and shade trees and shrubs can get verticillium wilt, including maple, redbud, and magnolia. Perennial flowers susceptible to the disease include asters, mums, shasta daisies, coreopsis, dahlias, larkspur, bleeding heart, peonies, and phlox.
In the vegetable garden, the members of the nightshade family (peppers, tomatoes, potatoes, eggplant) are the most commonly affected. The disease is also found in strawberries, and to a limited extent in raspberries, especially black raspberries.
Symptoms of Verticillium Wilt
The symptoms vary depending on the type of plant.
In trees, symptoms can appear any time but often start in hot, dry weather. The margins of the leaves may brown, looking like they are scorched. The leaves are smaller than usual. Leaves may wilt on some large branches in the crown, or on the entire side of the tree. Eventually those branches die. The tree produces many more seeds than usual.
If you scratch the bark of a branch with wilted leaves, you’ll notice a streaky discoloration of the wood below. Its color varies, ranging from green to black in maples, and brown to black in black locust and other trees. The symptoms are not always consistent. One tree with the disease may show symptoms one year and then seem fine until symptoms restart years later, whereas another tree dies not long after the symptoms appeared.
In potatoes and other vegetables of the nightshade family, the first symptom is usually yellowing of the lower leaves and subsequent wilting. The leaves develop areas of dead brown tissue surrounded by larger areas of yellowing. These symptoms may only appear on one side of the plant. The brown discoloration inside a stem—cut one off and slice it lengthwise to inspect it—also gives you clues.
In strawberries, the outer, older leaves wilt, dry and develop reddish-yellow or dark brown areas at the leaf margins and between veins. The development of new foliage is scarce and new leaves are stunted, possibly also curled up. You might also notice brown streaks on the petioles, on the runners, and in the crown, which will decay in heavily infected plants. New root growth might be stunted, with the growing tips turning black.
How Do Plants Get Verticillium Wilt
The fungus can remain dormant in the soil for a decade or more in the form of resting structures called microsclerotia, which survive drought and cold. When a potential host is planted near the microsclerotia, the roots of that plant stimulate the microsclerotia to germinate and produce spores. They attack the plant, entering it through its roots. Roots with wounds are especially susceptible.
In the plant, the fungus moves upwards and plugs the vascular system of the plant that is responsible for transporting water. It’s the plugging of the vascular system that causes the typical wilt, and eventually leads to plant death.
The fungus can be transmitted to garden soil from infected but asymptomatic nursery plants, potato tubers (seed potatoes), and hardwood mulch from an infected tree. Once the fungus is in one location, it can be easily spread in the soil via tilling, digging, and moving soil around in any other way, and by water and wind.
Verticillium Wilt Control and Management
There is no fungicide that will cure a plant once it’s infected by verticillium wilt but there is a number of steps you can take to prevent it.
Some trees have natural resistance or immunity to verticillium wilt. If you—or a neighbor— had a problem with the disease, you might be better off planting a plane tree instead of a maple. If a tree died because of verticillium wilt, do not replant the same tree species in that exact location or nearby.
If a tree is only lightly infected, you might get away by pruning out the branches, and by boosting the tree’s vigor with fertilizer and watering during dry spells. Keep in mind however that the symptoms might take a turn to the worse any time, so you are merely prolonging the tree’s life. During that time it might be spreading around the disease to the surrounding soil.
For strawberries, choose cultivars that are resistant to the disease, such as Allstar or Earliglow. Diligently practice crop rotation, and do not plant susceptible strawberry cultivars in the same soil where other susceptible crops or perennial flowers were grown in the last five years.
Your garden soil should be rich in nutrients, loose, and well-drained. Poor drainage can stress the roots as much as drought, and make your plants susceptible to the fungus. Do not apply fertilizers that are high in nitrogen but use a balanced fertilizer, or one with a higher phosphorus percentage.
Mulch around your vegetable plants, which reduces weeds and minimizes the risk of damaging the roots when cultivating the soil. Do not use hardwood mulch from an infected tree.
Promptly remove plants that have been killed by verticillium wilt, and also remove the roots. Infected plant material must be thrown in the trash. Trees should be taken away by a professional, such as by a tree removal service.