Verticillium wilt is caused the fungi Verticillium albo-atrum and Verticillium dahliae. These fungi live in the soil and attack plants whose roots are stressed. These fungi may attack more than three hundred woody and herbaceous plant species. Plant susceptibility or resistance may vary from one region to another since the virulence found in the different strains of Verticillium sp. is usually different as well as the genetic resistance of the plant. Cultural practices and environmental conditions can influence the infection of susceptible plants with this disease.
Barberry, boxwood, stone fruit trees such as peach and plum, Kentucky coffee tree, horse-chestnut, Ohio buckeye, magnolia, maple, peony, privet, redbud, serviceberry, smoke tree/bush, spirea, sumac, tulip tree, viburnum and many herbaceous plants such as tomato, strawberry and many weeds are examples of susceptible plants. Some examples of resistant plants include all monocots, all gymnosperms, apple, crabapple, mountain ash, beech, birches, dogwood, hackberry, hawthorn, linden, honey locust, oaks, sycamore, popular, walnut, and willow.
The plant symptoms that result when this disease attacks may be confused with other plant problem symptoms such as fusarium wilt, bacterial wilt, root rots as well as drought and damage due to excessive soil moisture. In Illinois, the disease is more severe in cool to warm weather and is not as prevalent in hot weather.
Plant leaves may curl, wilt, yellow or redden interveinally, die and defoliate. Individual branch die back may also be noticed. The disease starts in the roots and then progresses upward. As the vascular system becomes plugged due to the release tyloses or gums, the above ground symptoms begin to appear.
Internally, discoloration or streaking of the sapwood (xylem vascular tissue) occurs in most plants. The sapwood discoloration may appear as a striping of the wood when viewed on a branch with the bark peeled away. If the branch or plant is cut and the cross section examined, the discoloration may appear as a ring. The vascular discoloring often occurs with the advance of the fungus or the fungal spores through the sapwood except sometimes in the early stages of infection. Vascular discoloration varies with the host. In all ashes, internal discoloration is rare. In black locust the color is a dark reddish brown, it is a yellowish brown in cherry and smoke trees and is a light- to dark-green in maple, magnolia and sumac.
Although some plants may die quickly, more commonly it takes one or more years to die. Trees and shrubs with only a few wilted branches during a growing season may become more severely infected the following year. Some may recover and show no more symptoms in the following years or the disease may cause the plant to develop symptoms years later.
In herbaceous plants, the older and lower leaves turn yellow, wilt and wither. In young herbaceous plants, plants are stunted and die prematurely. Wilting during the day with recovery at night may be common for a while.
Grow plants adapted to the site. Grow resistant varieties and species. Avoid root and collar injury. Keep plants vigorous. Trees, shrubs and herbaceous plants killed by Verticillium spp. should be removed with as much of the roots intact as possible. Plants showing early symptoms should be watered and fertilized. Use fertilizers lower in nitrogen and higher in potassium. Sterilize tools between pruning and removal of infected plants and pruning healthy plants. Allow several years before growing a susceptible plant in an infected area. Do not plant back into the same hole! There are no proven cures (chemical or cultural) for this disease.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Pesticide Applicator Training and Plant Pathology, University of Illinois at Urbana-Champaign.