Verticillium wilt is a serious disease caused by two fungal species, Verticillium dahliae and V. albo-atrum, which altogether can infect over 400 plant species. In gardens it is common to see it on tomato, pepper, eggplant, and potato. This year we have also diagnosed Verticillium on lettuce, blackberry, and woody ornamentals.
In general, the most noticeable symptom is wilting and yellowing of leaves but this can be easily confused with lack of water and other vascular diseases. Symptoms may start in the lower part of the plant or on one side of the plant only, and leaves can show browning of the margins or V-shaped lesions at the tips. Wilted plants may look worse under hot and sunny conditions but may recover overnight; however, in most cases infected plants die. A characteristic symptom of Verticillium is a tan or brown discoloration in the vascular system but a diagnostic test is needed to confirm the fungus.
Verticillium species live in the soil and infect plants through the root system. These fungi have a great survival strategy; they produce compact structures called microsclerotia that can remain in the soil for many years. Microsclerotia can survive in plant debris and under the right conditions, these structures germinate and infect nearby plant hosts. After entering the plant these fungi move into the vascular system, the plant’s waterway, and cause clogging of the water-conducting vessels, which eventually leads to wilting. As infection progresses, plants become stunted, fail to produce any fruit, and finally die.
Verticillium wilt is difficult to control. Management relies mostly in preventing the disease and use of resistant varieties or cultivars, especially in vegetable crops. Unfortunately, this is not a possibility for those interested in ‘heirloom’ vegetables, as these varieties do not carry resistance genes against Verticillium. Weakened and stressed plants are more prone to become infected, which could be the reason why we are seeing a lot more Verticillium wilt this year. Therefore, make sure you plant in areas with good drainage and provide enough water. Other recommendations include long crop rotations with non-hosts and weed control.