Verticillium wilt, a damaging disease of cauliflower, was successfully managed in a multiple-year field study by incorporating broccoli residues into infested soil. In a study conducted from 1993 to 1995 in the Salinas Valley of California, cauliflower disease incidence and severity were consistently and significantly reduced in the broccoli residue plots when compared with no broccoli. The commercial standard plots fumigated with methyl bromide plus chloropicrin had the lowest disease incidence and severity. In both years of our tarping study, Verticillium wilt severity was lowest in the metham sodium treatment. The cauliflower-Verticillium host-pathogen system, therefore, can act as a model for controlling soil-borne diseases without the use of synthetic chemicals.
Broccoli Residues Can Control Verticillium Wilt of CauliflowerThis article by Steven T. Koike and Krishna V. Subbarao appeared in "California Agriculture," Volume 54, pp. 30-33.
Disease Control Alternatives
Controlling Verticillium wilt of cauliflower, like Verticillium diseases on most other crops, is difficult. The fungus produces survival structures (microsclerotia) that can persist in the soil for many years. The ability of this fungus to survive for long periods, in combination with a very broad host range, makes crop rotation impractical. Resistant cauliflower cultivars are not available, and soil fumigants such as methyl bromide and chloropicrin provide some control but are too expensive to use for cauliflower production, and may not even be available in the future for agricultural use. Therefore, there is an urgent need to discover alternative soil-borne pathogen controls that are effective, affordable, and not harmful to the environment.During our initial work on cauliflower Verticillium wilt, we noticed that although broccoli is closely related to cauliflower, broccoli crops were never infected with V. dahliae, even when planted in the same highly infested fields. When greenhouse-grown broccoli was inoculated with V. Dahliae isolated from cauliflower, the broccoli did not develop Verticillium wilt, and microsclerotia were not observed on broccoli roots. In addition, researchers have been interested in the possible fungicidal properties of chemical substances present in many cruciferous crops. Substances such as glucosinolates and other toxic volatile compounds, released during crop residue breakdown, could possibly be exploited to control soil-borne pathogens, or otherwise suppress soil-borne diseases.
The quality and amount of glucosinolates differ among various crucifer plants. Because of broccoli’s apparent status as a non-host of V. dahliae and the known fungicidal properties of the particular glucosinolates in this plant, we conducted a series of studies to investigate the usefulness of broccoli residues in suppressing Verticillium wilt of cauliflower.
Over the two-year period, we observed a significant decline in the number of microsclerotia in soil treated with broccoli residue. In contrast, soil not treated with the broccoli residue experienced a five-fold increase in the number of microsclerotia. Irrigation methods (furrow vs. drip) had no consistent, significant effect on the microsclerotial populations between the two treatments. Likewise, differing amounts of irrigation water (deficit, moderate, and excessive volumes) generally did not significantly influence pathogen numbers in broccoli or non-broccoli plots.
When we assessed the percent of Verticillium wilt infection and severity during the two-year study, we generally did not see significant differences between the furrow and subsurface drip irrigation plots. However, Verticillium wilt incidence was usually higher in the excessive irrigation regime when compared to moderate- and deficit-watering treatments. For both years of the study, disease incidence and severity were consistently and significantly reduced in the broccoli residue plots when compared with no broccoli. Not surprisingly, the commercial standard plots fumigated with methyl bromide plus chloropicrin (the control plots with no V. dahliae microsclerotia) had the lowest disease incidence and severity under all irrigation regimes.
Tarping Doesn’t Affect Activity
Fumigants are usually more effective in controlling soil-borne pathogens if the treated soil is covered with plastic tarps after application. We were interested in measuring the effectiveness of broccoli residue under tarped and non-tarped conditions. We established another two-year study (1993 to 1994) using a randomized, replicated experiment in a field naturally infested with V. dahliae. Treatments for the two years were the following: broccoli residue (equivalent of 25 tons/acre) with tarp; broccoli residue (25 tons/acre) without tarp; untreated control with tarp; untreated control without tarp; two consecutive cauliflower plantings; cauliflower followed by fallow; metham sodium (Vapam) (60 gal/acre); chloropicrin (485 lbs/acre). Tarps were removed after two weeks, cauliflower was planted into all plots, and Verticillium wilt severity was then evaluated in all plots near cauliflower maturity.In both years of the study, we found that Verticillium wilt severity was lowest in the metham sodium treatment. Both broccoli treatments (tarped and non-tarped) were comparable to the chloropicrin fumigation treatment. Most interestingly, the tarped and non-tarped broccoli plots had equivalent disease severities, suggesting that the effectiveness of broccoli residues was not dependent on volatile substances released by the decaying material. Untreated controls (both tarped and non-tarped) had the highest disease severity for both years.
Novel Method of Disease Control
In the laboratory study, fresh broccoli residue demonstrated significant activity in reducing microsclerotia viability of V. dahliae over a broad temperature range. The material was most effective at temperatures greater than 77 degrees F. When we applied this finding to commercial agricultural settings, we found that broccoli residue consistently resulted in reduced populations of microsclerotia and decreased disease incidence and severity. Yield data showed that cauliflower plants grown in broccoli-amended plots produced greater numbers of larger, marketable heads compared to untreated plots. Irrigation methods and rates did not account for the observed differences in V. dahliae populations in broccoli and non-broccoli plots. Of particular interest was the finding that the broccoli residue effects were unaffected by the presence or absence of plastic tarps. If growers are able to control disease by using broccoli residues without tarps, they would have the added benefit of not being concerned with obtaining or disposing of the plastic tarps.Because of the environmental, regulatory, economic, and political concerns of using synthetic, highly toxic agrochemicals, agricultural industry and research teams are attempting to find viable alternatives to the materials. Such alternatives must be effective, economical, and not harmful to the environment. The use of broccoli residue for controlling soil-borne pathogens perhaps offers a novel means of controlling such pests. Our experiments using broccoli residue have demonstrated its effectiveness in significantly reducing populations of V. dahliae. If sufficient volumes of broccoli residue are used as a regular component of crop rotation cycles, this suppressive activity may be even further heightened.
This technique is economical because the broccoli effect does not require the harvestable heads; hence, the crop generates revenue, and the remaining residues contribute to disease control. This utility of having the crop harvested is an important feature of this method, as other disease-suppressing crops -- such as cover crops -- are not harvested and, therefore, do not generate revenue for the grower. Plastic tarps, another economic factor, add significantly to soil-treatment costs and are not necessary for the broccoli amendment effects. Finally, broccoli residues are natural materials and pose no threat to the environment, surrounding crops, wildlife, or people.
The use of broccoli residue in this way is particular well-suited and available to vegetable growers in the Salinas Valley because of the extensive plantings already grown in this valley. In 1997, over 56,000 acres were produced, having a value of $225 million. This technique can be exploited by large- and small-acreage owners as well as conventional and organic producers.
The usefulness of broccoli residue as an integrated pest-control option is further highlighted by its potential versatility in managing multiple-target pests. We, as well as industry groups, are already investigating the role broccoli residue might play in controlling soil-borne diseases of other crops, such as lettuce, pepper, and strawberry; and we are investigating its effect on pathogens other than Verticillium, such as Sclerotinia. In addition, growers have observed for many years that when leftover broccoli residues from processing plants were incorporated into fields, those locations had reduced weed growth in subsequent seasons. Broccoli residue is most likely suppressing weeds and, perhaps, other types of pests as well.
In summary, broccoli residue incorporation, as a disease control practice, is an innovative means of managing Verticillium wilt of cauliflower that is effective, economically feasible, and non-polluting to the environment.
