This 'Purple Rain' eggplant, despite sufficient fertility, moisture and insect control, lacked vigor throughout the season and produced only three undersized eggplants. Pennsylvania State University diagnosed verticillium wilt.
PHOTO BY BOB FERGUSON.
Numerous plants are susceptible to verticillium wilt, including cabbage, cauliflower, radish, celery, lettuce, eggplant, pepper, potato, tomato, cucumber, pumpkin, melons, grape, caneberries, strawberry, spinach, olive, peach, pistachio and plum, along with various field crops, flowers, shrubs and even weeds.
Unfortunately, the list of hosts has been expanding. Moreover, the pathogen can be seed-transmitted.
This broad array of host plants complicates the use of one of the most successful cultural control practices for soilborne pathogens: crop rotation. The pathogen's ability to survive in the soil for a decade makes control even more challenging.
Verticillium albo-atrum and Verticillium dahliae live in the soil. The fungus usually infects the host by entering young roots and growing into the water-conducting vessels. The vessels become plugged and collapse, blocking the plant's water supply. Consequently, the leaves wilt. This wilting may be in the top or bottom leaves. Leaf tips often dry and turn pale or brown, but frequently no symptoms are seen until the season is well under way. The visible wilting may partially recover, particularly during nighttime.
Even with some recovery, the plants typically appear stunted or weak, often only on one side of a leaf or plant. With severe infections, the entire plant may die. The weakened plants succumb more easily to other lethal pathogens, notably Fusarium, Phytophthora and Pythium. Production is diminished and of low quality. Symptoms vary with different species and the cultivar.
Top control recommendations are avoiding the pathogen and crop rotation of about four to six years. However, as noted, the number of unsusceptible crops is scant. Recommendations for rotation often include cereal crops and grasses.
Additional cultural controls include: planting resistant cultivars of the appropriate race; choosing healthy, disease-free transplants and pathogen-free seed; practicing strict sanitation by removing volunteer plants and end-of-season roots and plant debris; disinfecting tools; keeping the crop plus any cover crop and rotation crop weed-free; destroying infested plant material immediately; and maintaining plant vigor with fertility and irrigation.
Soil sterilization has reportedly not been shown to be a permanent solution to Verticillium spp. infestations. In addition, many regions and sites are not conducive to this treatment.
In the past, methyl bromide boosted production considerably, but it depletes the stratospheric ozone layer, and its phaseout is now nearly complete. Alternatives such as chloropicrin, dichloropropene and metam sodium are considered less effective. In addition, these chemicals are restricted and must be used with caution.
As pointed out by plant pathologists Dr. Krishnamurthy Subbarao, University of California, Davis, and Dr. George Lazarovits, Agriculture and Agri-Food Canada, most research has focused on managing the populations of the microsclerotia produced by V. dahliae. Microsclerotia are the resting structures critical to the pathogen's survival. Analysis of nonchemical methods has identified several promising practices.
Studies with soil amendments of blood meal, fish meal, liquid swine manure, poultry manure and soy meal have noted reduced severity and incidence of verticillium wilt. Some have even shown reductions in more than one season. However, results have been variable, site and soil-specific, and difficult to reproduce in the field. Also, verticillium wilt reductions were lower than with chemical treatments, but the added fertility of the amendments did increase yields. In addition, these organic amendments combined with solarization often produced a synergistic effect that improved the efficacy of both control treatments.
Broccoli excels as a rotational crop. While evaluating verticillium wilt on cauliflower in coastal California, Subbarao and his colleagues observed broccoli plants on an adjacent field that was also infested with the pathogen. He noted, "Broccoli was not affected." Coupled with its consumer popularity as a vegetable and its ability to reduce V. dahliae in the soil, broccoli can be a desirable rotational crop.
What's more, incorporating broccoli residue into the soil suppressed V. dahliae. Interestingly, fresh broccoli residue suppressed the microsclerotia more than dry broccoli residue over every temperature range in the trials. In greenhouse trials with cauliflower, the fresh broccoli residue-treated plants grew taller and had greater root and shoot mass. In addition, the trials demonstrated that to achieve the lowest wilt incidence, the broccoli residue should be incorporated when soil temperatures are at least 68 degrees Fahrenheit. Field trials also showed reductions.
Rotating with broccoli and incorporating broccoli residue shows promise in managing the pathogen in susceptible crops. The practice should be successful regardless of the irrigation methods or regimes followed. Trials with rotation of broccoli and Brussels sprouts also appeared to effectively manage verticillium wilt in strawberry production.
For some vegetables, advances in breeding resistance to the pathogen offer encouragement. Potatoes and tomatoes are two examples, although the specific race and species of Verticillium must be considered. Growers and researchers also note that particular cultivars seem somewhat tolerant, producing a crop before the damaging yield effects of the pathogen surpass their economic threshold. Local and regional extension experts can provide advice.
Asians have grafted susceptible varieties onto resistant rootstock for over 50 years. In the U.S., with the development and usage of robots, healing chambers and more rootstocks, grafting has become another cultural method to manage soilborne diseases. Eggplant, peppers, watermelon and heirloom tomatoes continue to benefit from research and refined methods. Increased yields, greater plant vigor and longer cropping seasons have been reported. Limitations include cost, the need for compatible rootstocks, and the possibility of resistance breaking down under high pathogen populations or the evolution of new races of the pathogen.
Steve Groff of Cedar Meadow Farm in Lancaster County, Pennsylvania, grafted high tunnel tomatoes in 2008 with impressive yields. Groff reported that grafting can delay the onset of verticillium wilt, but the plants will eventually succumb to it. Last season, he moved the tunnels to soil that did not have tomatoes for many years and was rewarded with record yields. Groff said, "[There's] nothing better than rotation to stave off soilborne diseases."
Current research reinforces the concept that verticillium wilt management demands a combination of methods.
The author is a writer and researcher specializing in agriculture.