This fungal disease causes vascular tissue to be plugged, effectively blocking the movement of water in the plant and causing foliage to wilt and branches or plants to die. Maple, smoke tree, redbud, magnolia, and ash are some of the more common trees affected in Illinois, but more than 300 plant species are susceptible to this fungal disease. The list includes annuals, perennials, trees, shrubs, fruits, and vegetables. Report on Plant Disease No. 1010 discusses Verticillium wilt and contains lists of plants that have been reported as hosts of the disease.
Symptoms of Verticillium wilt include wilting and yellowing, and death of leaves, branches, or entire plants. Chronic symptoms may include stunted and chlorotic foliage, leaf scorch, slow growth, abnormally heavy seed crops, and dieback of shoots and branches. Vascular tissue is discolored—usually brown, black, or light to dark green. Only ash does not produce some type of vascular discoloration. In terms of diagnosis and confirmation of the disease, vascular discoloration is the most significant symptom. With the exception of ash, samples taken for laboratory culturing must contain this symptom for valid results. Tissue must be alive but showing active wilting. The ideal branch section is thumb thickness, 8 to 10 inches long, alive, and contains vascular discoloration. This fungus is relatively slow growing. Fungal isolates that develop in laboratory cultures usually grow for about seven days before the fungus can be identified positively.
Most plant species do not readily recover from this disease. In fact, it is probably more typical for infected plants to die. Still, some fast-growing trees have been able to “wall off” the fungus through compartmentalization and continue to grow well for many years. I have seen this happen on a few maples and ash. There are no chemical cures for the disease, and resistant varieties are available for only a few plant species, such as strawberry and tomato.
Management recommendations include removing dead wood, watering trees in periods of drought lasting two weeks, and fertilizing in the fall to improve tree vitality. Do not grow susceptible crops on land where crops that proved susceptible to Verticillium wilt were grown previously. A rotation of five years or more for vegetables and flowers may help reduce the amount of inoculum in the soil. The Verticillium fungus is soilborne and can survive for five years or longer in the soil.