Although the Plant Clinic has not had a laboratory-confirmed case of Verticillium yet this season, it is likely to happen any day now. Suspect cases on strawberry, maple, and magnolia are in the works, and this is the time of year when Verticillium wilt rears its ugly head. Verticillium wilt can occur on more than 300 host plants--including many weeds--but in the landscape we usually observe this disease on maple, redbud, magnolia, ash, and tuliptree.
In Illinois the causal fungus could be Verticillium dahliae or V. albo-atrum. Both Verticillium spp. are soil borne, are now active, and tend to invade weakened or stressed plants. The fungus invades plants through wounds above or below ground. Once introduced into the soil, the fungus can survive for five years or more, so identification of the problem is important when considering replanting in the same spot.
The Verticillium fungus invades the vascular tissues; the plant reacts by partially blocking its own water-conducting system. Symptoms resemble those of a plant under water stress and include wilting, yellowing, and death of leaves, branches, or entire plants. Chronic symptoms may occur: stunted, chlorotic, and deformed foliage; leaf scorch; slow growth; abnormally heavy seed crops; and dieback of shoots and branches. The vascular tissue is discolored, usually light to dark green, brown, or black. Samples taken for laboratory cultures must contain this discoloration for valid results. Tissue must be alive but showing active wilting.
Because the fungus remains in the soil even after removal of an infected plant, resistant varieties are a desirable control option. Look for Verticillium-resistant varieties, which are available for hosts such as strawberry and tomato, at a reliable nursery.
Most species will not readily recover from this disease, but maples have been known to "wall off" the fungus within the wood when plant growth is rapid. For this reason, control suggestions often include watering plants in periods of drought that extend to two weeks, and fertilizing to "push" growth. Do not grow susceptible crops on land where other crops that proved susceptible to Verticillium wilt were previously grown. A rotation of five years or more for vegetables and flowers may help to reduce the amount of inoculum in the soil.
At a time when compost piles are promoted and chipping of dead trees is encouraged to recycle plant material, one final word of warning. Research suggests that wood chips from Verticillium-infected trees can serve as a means of infecting other trees when used as mulch around those trees. We still do not know how great that risk is, nor do we know how to assess the risk. It would be wise to burn or bury infected trees and to spread out bark mulch from unknown sources to allow it to dry before using it around your trees. For more information about Verticillium wilt, including lists of susceptible and nonhost crops, plus additional control measures, see Report on Plant Diseases No. 1010.