For those of you familiar with this disease, be on the lookout now. Verticillium wilt has been confirmed at the Plant Clinic on maple already this year. We have also seen many trees, shrubs, and even perennials with weather-related problems that can mimic Verticillium wilt. Read this article for some tips on how to tell what’s Verticillium wilt and what’s not.
The Verticillium fungus causes vascular tissue to be plugged, effectively blocking water movement in the plant, resulting in wilting of foliage and death of branches or plants. You can imagine that a root rot, root injury, trunk damage, insect injury, or any other problem that inhibits water uptake might look like Verticillium wilt. In fact, the disease is often blamed for unexplained deaths of plants. Maple, smoke tree, redbud, magnolia, and ash are some of the more common trees affected in Illinois, but there are more than 300 plant species susceptible to this fungal disease. The list includes annuals, perennials, trees, shrubs, fruits, and vegetables. We often see the prob-lem on tomato in Illinois, which is why we recommend the VFN hybrids—those resistant to Verticillium, Fusarium, and nematodes. Report on Plant Diseases (RPD) 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. The vascular tissue is discolored in a striped or streaked pattern, usually brown, black, or light to dark green. As far as we know, only ash does not produce some type of vascular discoloration when infected by this fungus. The presence of vascular discoloration is the best method of distinguishing Verticillium wilt from some of the other look-alike problems listed above. In terms of diagnosis and confirmation of the disease, the vascular discoloration is the most significant symptom. Samples taken for laboratory culturing (except ash) must contain this discoloration for valid results. Tissue must be alive but showing active wilting. The ideal branch section would be thumb thickness, 8 to 10 inches long, alive, and containing vascular discoloration. This fungus is relatively slow growing. Fungal isolates that develop in laboratory cultures usually grow for about 7 days before the fungus can be positively identified.
Most plant species will 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 vi-tality. Although we may not be able to save an infected plant, identification of the problem has great implications for the future. The Verticillium fungus is soilborne and can survive for 5 years or longer in the soil. If a susceptible species is planted as a replacement plant, it too will become infected. Do not grow susceptible crops on land where Verticillium has been confirmed. A rotation of 5 years or more for vegetables and flowers may help reduce the amount of inoculum in the soil.
For more information about this disease, consult RPD No. 1010, which is available at an Extension office or on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.