Rotations with Broccoli: A Sustainable Alternative to
Soil Chemical Fumigants

Krishna V. Subbarao

Because of the environmental, regulatory, economic, and political concerns of using synthetic, highly toxic agrichemicals, current research is attempting to find viable alternatives to these chemicals. Such alternatives must be effective, economical, and not harmful to the environment. The use of rotations with broccoli for controlling soilborne pathogens perhaps offers a novel means of controlling such pests.

Even though the benefits of crop rotation in maintaining crop yields and suppressing diseases have been recognized and exploited for centuries, the availability and cost-effectiveness of chemical fertilizers, pesticides, and soil fumigants have rendered its use limited. However, because of the detrimental effects of chemical inputs on groundwater supply and the environmental and health risks they pose, the wisdom of their continued intensive use has been called into question. This has rekindled an interest in crop rotations to improve profitability, stability, and sustainability of farming systems.

Verticillium dahliae is a soilborne pathogen that produces survival structures called microsclerotia, which can persist in the soil for many years. This prolonged survival, in combination with a very broad host range, makes crop rotations for its management impractical. However, we noticed that broccoli, which is closely related to cauliflower, was never infected with V. dahliae even when planted in the highly infested fields where cauliflower suffered severe yield losses. When broccoli was inoculated with V. dahliae isolated from cauliflower, it did not develop Verticillium wilt and microsclerotia were not observed on broccoli roots. We then began a series of studies to determine if rotations with broccoli control Verticillium wilt and other soilborne diseases of vegetable crops.

First, we evaluated the effect of broccoli under various temperatures. After placing 25 grams of V. dahliae-infested soil into plastic bottles, we mixed either fresh broccoli (8% wt/wt of broccoli/soil), dry broccoli (equivalent to the fresh broccoli treatment), and no broccoli. Added broccoli corresponded to the fresh weight of broccoli residue remaining after a typical commercial broccoli harvest (25 tons/acre). The soil and broccoli treatments were incubated at 10, 15, 20, 25, 30, and 35 C for 45 days, and soil was evaluated for microsclerotia survival. For each temperature, both dry and fresh broccoli significantly reduced the numbers of living microsclerotia. Fresh broccoli was better than dry broccoli at all temperatures below 30 C. The pathogen was virtually eliminated by the fresh broccoli treatment above 25 C.

Second, we conducted a two-year study in Verticillium-infested field plots in the Salinas Valley. Each year, the main treatments for the randomized, replicated experiment were broccoli residue incorporated into plots, no residue in the plots, and a fumigated (50% methyl bromide + 50% chloropicrin at 370 lb/acre). These treatments were also tested under furrow and drip irrigation methods and rates (deficit, moderate, and excessive). Three weeks after residue incorporation, plots were prepared and cauliflower was planted in all plots. Levels of microsclerotia and disease incidence and severity were recorded in each plot. Over a two-year period, a 95% decline in the number of microsclerotia in plots rotated with broccoli and a five-fold increase in non-broccoli plots were observed. In both years, Verticillium wilt incidence and severity were significantly reduced in the broccoli residue treated plots compared with no residue plots regardless of the irrigation methods or rates.

Tarped and non-tarped broccoli treatments were compared in a grower’s field as the volatile gases from decomposing broccoli and other crucifer crops have been implicated in disease suppression along with Vapam (60 gal/acre) and chloropicrin (485 lb/acre). Both tarped and non-tarped broccoli treatments were comparable to chloropicrin fumigation and Vapam treatments. Tarped and non-tarped broccoli treatments had equivalent low disease, suggesting that the effectiveness of broccoli residue was not dependent on volatile substances released by decaying material.

Our experiments using broccoli residue have demonstrated its effectiveness in significantly reducing populations of V. dahliae. This technique is economical because the crop generates revenue and the remaining residue contributes to disease control. It can be adapted by both large and small acreage growers as well as by both conventional and organic producers. Plastic tarps, that add significantly to soil treatment costs, are not necessary for the effectiveness of broccoli residue. Finally, broccoli residues pose no threat to the environment, other crops, wildlife, or people.

A number of growers in the Salinas Valley have adapted this practice and have observed similar beneficial effects from rotations with broccoli. We are currently investigating the role of broccoli residue in controlling soilborne diseases of other crops (lettuce – lettuce drop and strawberry – Verticillium wilt). In addition, growers have observed for many years that when broccoli residues from processing plants were incorporated into fields, reduced weed growth in subsequent seasons occurs. Thus, broccoli residue may be suppressive to weeds and perhaps other types of pests as well.

Krishna V. Subbarao
University of California
c/o U.S. Agricultural Research Station
1636 E. Alisal St.
Salinas, CA 93905
Tel: (831) 755-2890
Kvsubbarao@Ucdavis.edu

[Table of Contents]