This disease is often associated with stress. We have not seen many positive cases yet this season, but I expect to soon. Hot temperatures put a huge demand on the roots to absorb water. If roots are injured (by excess water, compaction, clay soils, etc.), they are not able to supply the demand. This stress causes foliar wilt, branch dieback, and often infection by the Verticillium fungus. Of course, the same is true of susceptible herbaceous hosts. The fungus may have been present in a planting for years but not infect until plants are stressed--another good reason to maintain good horticultural practices.
This year, many trees, shrubs, and even perennials show weather-related problems that can mimic Verticillium wilt. 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 trees more commonly affected in Illinois; more than 300 plant species are susceptible--including annuals, perennials, trees, shrubs, fruits, and vegetables. We often see the problem on tomato in Illinois, so we recommend the VFN hybrids--resistant to Verticillium, Fusarium, and nematodes. Report on Plant Disease (RPD), no. 1010, discusses Verticillium wilt and lists reported hosts.
Symptoms 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. Still, the most diagnostic feature is vascular tissue 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 this discoloration. Samples for laboratory culturing must contain this discoloration for valid results (except on ash). Tissue must be alive but showing active wilting. The ideal branch section is thumb thickness, 8 to 10 inches long, alive, with 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 confirmed.
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 and grow well for years. I have seen this happen on a few maples and ash. There are no chemical cures; and resistant varieties are available for only a few plant species (for example, strawberry and tomato). Management recommendations include removing dead wood, watering trees in drought lasting 2 weeks, and fertilizing in the fall to improve tree vitality. Although we may not be able to save an infected plant, identifying the problem has implications. This fungus is soilborne. If a susceptible species is planted as a replacement, it too will become infected. Do not grow susceptible crops 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, consult Report on Plant Disease (RPD), no. 1010, available in Extension offices or on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm .