Cauliflower brings growers in coastal California about $168 million each lyear. On 52,000 acres, these growers raise nearly 80 percent of the cauliflower produced in the United States for both fresh and processing markets.

"But a sudden and widespread increase in Verticillium wilt has taken several fields out of production in the Salinas Valley," says Krishna V. Subbarao, a plant pathologist with the University of California, Davis. "Growers suffer extensive losses on crops harvested between April and October."

Since there is no effective control, Subbarao says that the wilt, caused by the fungus Verticillium dahliae, presents a significant threat to other cool-season vegetable crops as well. Verticillium wilt attacks more than 300 kinds of plants. The fungal microsclerotia live in soil or on decaying organic matter in the soil and can survive for up to 15 years.

Subbarao and colleagues have found a way to control the devastating wilt on cauliflower without using chemicals: working chopped-up broccoli into the soil before planting cauliflower.

"Although cauliflower and broccoli are related, broccoli resists the wilt even whe n it is planted in soil heavily infested with microsclerotia. Even broccoli roots are pathogen free, and since broccoli growers haven't complained about the disease, it evidently isn't a problem," Subbarao says.

He says there is also a compound in broccoli (absent in cauliflower) called glucoraphanin that possesses antimicrobial properties. As broccoli decomposes, it produces volatile chemicals that may have an effect on soilborne pathogens.

"We tested the pathogenicity of V. dahliae isolates from several crops," Subbarao says. "Our conclusions show that rotations of cauliflower with any crop other than broccoli, Brussels sprouts, and perhaps lettuce, would likely increase the incidence of wilt."

Salinas Valley growers routinely rotate cauliflower with lettuce, maintaining levels of microsclerotia in the soil. Therefore, rotations with lettuce will not control the fungus on cauliflower or other susceptible crops in the short term, according to Subbarao. He and colleagues tried broccoli to reduce the microsclerotia in soil and wilt on cauliflower. They chose cauliflower as a model system because of its importance to California agriculture and because broccoli is grown extensively in the Salinas Valley. Most of the funds for this research project came from several California cauliflower growers, with some financial support from ARS.

Subbarao conducted his research in experimental and grower fields. His 2-year study demonstrated that broccoli residue reduces the number of microsclerotia in the soil and decreases the incidence of Verticillium wilt, making broccoli an ideal candidate for rotation with cauliflower.

"We applied freshly cut broccoli shoots to experimental plots and immediately worked them into the soil," Subbarao says. "Our results show that less wilt developed on cauliflower in these plots than in plots fumigated with synthetic chemicals."

Tarping had no effect on controlling the fungus. One of the important findings was that the numbers of microsclerotia declined throughout the season and stayed low the following season. However, in conventionally fumigated plots, the numbers declined initially but increased toward the end of the season. This suggests that while fumigation gives short-term control, the effects of broccoli residue are longer term.

Subbarao isn't sure just how broccoli protects cauliflower from wilt. It could be a chemical or a biological mechanism, or both. Applying fresh-cut broccoli could increase the numbers and types of beneficial soil microorganisms such as actinomycetes, bacteria, and fungi, which would selectively inhibit the activity of the fungus.

"The results so far indicate that it may be a combination of both chemical and biological. Any effect of the chemicals is transient since they are volatile and stay in the soil for only a short time," Subbarao notes.

"In soil amended with broccoli, the number of actinomycetes and bacteria increased 1,000 fold, while the numbers of fungi increased only slightly compared with soil not treated with broccoli residue." These microorganisms may have a direct deleterious effect on the microsclerotia; indirectly, the secondary metabolites produced by these microorganisms may also have an effect.

Subbarao's results indicate that after commercial harvest, growers could plow under broccoli residue to manage Verticillium wilt in subsequent cauliflower and possibly other crops as well. "This would not only reduce the wilt, but would also help the environment by cutting the use of synthetic pesticides," he says. "We think that broccoli could be an economically viable substitute for chemical fumigants. The number of years a crop needs to be rotated with broccoli depends on the magnitude of soil infestation by the pathogen."

Broccoli's applicability to other cropping systems needs to be thoroughly researched, Subbarao says. Although it would probably work as a biocultural control on strawberries and other crops, its success should be demonstrated commercially for these crops before it is recommended.

"We propose that broccoli could be a possible alternative for methyl bromide, the fumigant now used by vegetable and strawberry growers in California," he says. Methyl bromide is scheduled to be banned by the U.S. Environmental Protection Agency by 2001.