Rotations with Broccoli - A Sustainable Alternative to Soil Chemical Fumigants

Final Report for SW99-009

Project Type: Research and Education
Funds awarded in 1999: $145,750.00
Projected End Date: 12/31/2001
Region: Western
State: California
Principal Investigator:
Krishna Subbarao
University of California, Davis
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Project Information


The effects of vegetable crop rotations and residue amendment on Verticillium wilt of strawberry and marketable yield were compared with methyl bromide+chloropicrin fumigation. After four crops of vegetables and two crops of strawberry, the numbers of V. dahliae microsclerotia in broccoli plots were consistently low, while the lettuce plots were a potential reservoir of microsclerotia. At both sites broccoli rotation plots had higher strawberry plant diameter and lower Verticillium wilt severity than other treatments. At, final harvest, yield loss was 23% in broccoli rotation treatment and 39% in lettuce rotation treatment relative to the standard fumigation treatment. Data on the soil microflora suggest that rotations with broccoli increased the overall bacterial and actinomycete populations in soil by nearly a 1000-fold while rotations with lettuce did not increase these populations over the baseline levels. Mycorrhizal associations on strawberry roots under the different rotation treatments were non-existent. The cost-benefit analysis of the different rotation treatments compared with the standard fumigation treatments suggested that despite not being able to plant strawberries each year, rotations with broccoli would be profitable over the long term. Rotations with broccoli were equally effective under both conventional and organic production systems. Under moderate disease pressure, broccoli rotation has the potential to be a feasible alternative for reducing Verticillium wilt severity in strawberry.

Project Objectives:

1. Demonstrate the effects of rotations with broccoli, brussels sprouts, cauliflower, and lettuce on strawberry yield, root infection, systemic vascular colonization, incidence of soilborne diseases on strawberry, and pathogen survival in soil.
2. Determine if the broccoli-mediated pathogen propagule attrition and wilt suppressiveness on strawberry is due to altered soil microflora.
3. To determine the impact of crop residue on the occurrence of arbuscular mycorrhizal symbiosis in strawberry roots.
4. To calculate costs and benefits of sustainable alternatives such as broccoli rotational crop to chemical fumigants and to determine their relative profitability in strawberry crop system.


In California, strawberry is harvested from approximately 23,600 acres for fresh market and processing accounting for nearly 80% of the total U.S. production with an annual farm gate value of $612 million (6). In the conventional production systems, pre-chilled strawberry transplants are planted during first week of November. The crop establishment and maintenance requires preplant fumigation, plastic mulch, drip irrigation, fertilization with slow-release nutrients, foliar application of synthetic pesticides and concentrated intermittent hand labor throughout growing season. Strawberry harvesting begins from late March, continues until final harvest and plant removal some time during August-September. Nearly all strawberry acreage in California is fumigated to control weeds, soil pathogens, and nematodes. Approximately 4.5 million pounds of methyl bromide are used annually in California for pre-plant fumigation of strawberries, representing roughly 35% of the total use of methyl bromide in California (4).
Atmospheric methyl bromide is believed to be the principal source of stratospheric bromine, which is highly effective in converting ozone to oxygen and is about 50 times more potent than chlorine in destroying stratospheric ozone (3). The U.S. Environmental Protection Agency, in response to the concern over methyl bromide as an ozone-depleting chemical, has proposed to eliminate its production and use by the year 2000, pursuant to section 602 of the Clean Air Act. The implications for California’s strawberry crop are very serious. Strawberries have the highest net return per pound of methyl bromide used (58) and are among the most expensive crops to grow, with a cost to establish the stand at approximately $7,700 per acre and yearly production cost totaling roughly $30,000 per acre (5). According to the comprehensive cost-benefit analysis by the National Agricultural Pesticide Impact Assessment Program (NAPIAP), strawberry fresh-market business would lose an estimated $110 million without methyl bromide.
Soilborne diseases are a major impediment to strawberry production in coastal California. Among the soilborne pathogens, diseases caused by Pythium, Phytophthora, Cylindrocarpon, and Rhizoctonia spp. can be important causes of reductions in growth and yield, while Verticillium wilt is the most important pathogen due to death of plants and the lack of good genetic resistance in the germplasm. It is caused by the fungus Verticillium dahliae, which is widely distributed in the agricultural soils in California affecting such diverse crops as artichokes, cotton, pepper, pistachio, potato, strawberry, tomato, watermelon, and a number of crucifer crops. Especially strawberries are highly susceptible to V. dahliae infection. As few as 2 microsclerotia can result in 100% disease incidence (31).
Because of the availability of effective soil fumigants such as methyl bromide and chloropicrin (55), strawberry-breeding programs concentrated on horticultural characteristics and neglected disease resistance. The imminent loss of methyl bromide (1,2,54) leaves few alternatives for effective management of soilborne diseases in strawberries. Strawberry has a narrow genetic base and all the presently available commercial cultivars are genetically very similar and susceptible to most soilborne diseases, particularly to Verticillium wilt. Attempts, however, are being made to increase resistance to Verticillium wilt in strawberries.
The long-term risks to human health, the environment and to agricultural crops from the use of methyl bromide (3) far outweigh any short-term monetary benefit. The effectiveness and suitability of many of the existing and potential alternative chemicals have been thoroughly tested in the past 6-8 years and a few chemicals have shown efficacy equal to that of methyl bromide. Further complicating implementation of alternative control strategies is that a dependence on chemical alternatives is becoming increasingly socially unacceptable and may seriously effect the sustainability of the U.S. agricultural production (1,54). Developing safe, environmentally sound integrated soilborne pest management strategies, and identifying, evaluating, and developing naturally occurring chemicals to manage soilborne plant pests were considered as two high priority, long term components in the search for alternatives for methyl bromide at a USDA workshop help at Arlington, VA, during 1993 (1).
Solarization of soil for soilborne disease management is effective (30) but is restricted to warm, sunny climates. In other crops where host resistance is either unavailable or the use of fumigants is not economically viable, crop rotation is considered as a means of reducing the soilborne propagules and minimizing losses from Verticillium wilt (12,21,23,33) and other diseases (15). Monoculture of a crop increases the populations of pathogenic microorganisms of that crop. In contrast, crop rotations generally reduce the populations of pathogens because pathogens of one crop are unlikely to infect and multiply on an unrelated crop (16,17). For example, of the Rhizoctonia species that infect strawberry, none attack broccoli and only one out of four attacks lettuce. Absence of pathogenic root fungi disease outbreak in a specific organic strawberry system, in part, has been attributed to the site’s long cropping to non-host Brussels sprouts (28). Other mechanisms by which crop rotations suppress soilborne diseases may include changes in microbial interactions (38) or changes in soil physical factors (9,24,44,49,56).


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  • Zahangir Kabir
  • Frank Martin


Materials and methods:

Experimental site. The field experiments on vegetable crop–strawberry rotations were conducted. Experimental plots were located at Monterey Bay Academy (MBA), near Watsonville, CA on an Elder sandy loam soil and at the Spence field site, Salinas, CA on a Chualar loam soil. This is the first time strawberries have been planted at the Spence field site. At both locations, commercial growers managed strawberry and vegetable production. Inoculum density was moderate to high at the MBA site with an average of 10 V. dahliae microsclerotia g-1 soil but below detectable levels for the Spence site. Both locations were naturally infested with other soilborne strawberry root pathogens (Pythium, binucleate Rhizoctonia, and Cylindrocarpon spp.). Over the past year, the experiment was also repeated on larger plots at the MBA site. The plots were established in an area with very high levels of V. dahliae microsclerotia. One rotaiton cycle has been completed at this new site and the second cycle is in progress.

Treatments and experimental design. There were three rotation treatments at each location. The crops were planted in the following sequence 1). lettuce-lettuce-strawberry; 2). broccoli-broccoli-strawberry; and 3). Brussels sprouts-strawberry (MBA only) or cauliflower-cauliflower-strawberry (Spence only). The treatments were laid out in a randomized complete block design with four replications. At MBA, the individual plots consisted of two single beds of 7.6-m length and at the Spence location there were eight single beds of 9-m length. The treatments at the new site included 1). lettuce-lettuce-strawberry; 2). broccoli-broccoli-strawberry; and methyl bromide + chloropicrin as the standard fumigation treatment. The plots here were 6 beds wide and 9-m long. Standard grower production practices were followed during each crop cycle. The rotational crops were transplanted during mid-April and the planting cycles were timed to include two rotational crops of lettuce, broccoli, or cauliflower but only one brussels sprouts cropping cycle. At the new site, two rotational corps of lettuce and broccoli were completed before planting the first crop of strawberries.

After harvest, all crop residues were flail shredded, air dried on the surface for a minimum of two days and incorporated into the soil using a rototiller. Four weeks after incorporation, the beds in all plots were reworked for the next crop cycle. Strawberries (cv. Selva or Aromas) were planted during November in all plots (including a replicated plot fumigated with methyl bromide+chloropicrin (67+33%) for comparison with rotation treatments). Again standard grower production practices were followed during the strawberry crop cycle.

Soil samples to determine the densities of V. dahliae propagules and the soil microflora were collected at beginning and at end of the rotational crop, and every month after the start of strawberry crop. Samples were assayed using the modified Anderson sampler technique and semi-selective NP-10 medium. Total actinomycetes, bacteria, and fungi in different rotation treatments were determined using standard microbiological techniques (7,8,10,11,18,19,25,29,40,51).

Plant canopy, root and shoot weight, yield and disease determinations. To determine the relative effects of different rotation treatments, plant growth was monitored by recording plant canopy diameter on 30 plants in each replication every month. Twenty plants were randomly chosen from each treatment replication and carefully uprooted. The roots were washed free of soil and the root and shoot weight were separately determined for each treatment replication. Twenty plants per plot were visually rated for Verticillium wilt severity to monitor disease progress every other week from May. The disease severity estimate was done on the scale of 0 to 8, where 0 = healthy plant, 2 = moderately stunted, 3 = moderately stunted, slight rosette of dead leaves, 4 = moderately stunted, moderate rosette, 5 = significantly stunted, slight rosette, 6 = significantly stunted, moderate rosette, 7 = significantly stunted, significant rosette, 8 = dead plant. Yield data for marketable yield and culls were obtained in the plots once a week at MBA and twice a week at Spence site.

Strawberry root samples were collected from different treatments to determine mycorrhizal mycorrhizal associations by direct microscopic examination and an MPN technique (22,46,47).

During the course of these studies, economic data for the cost-benefit analysis were also collected from growers as well as our experimental plots.

Data analysis. Differences between treatments for strawberry plant canopy diameter, root and shoot weight, disease severity and marketable strawberry yield were determined by analysis of variance, and means were compared by the least significant difference test (P < 0.05). Numbers of microslcerotia in each treatment were expressed as microsclerotia g-1 of dry soil. Means and the corresponding standard errors were computed for each treatment and sampling time. Repeated measures analysis of variance was used to test disease severity from different treatments recorded over time. All analyses were done using SAS (release 6.12 ed., SAS Institute, Cary, NC).

Research results and discussion:

At the first MBA site, the soil inoculum levels of V. dahliae were moderate at the start of the experiment. However, the inoculum levels were affected by the vegetable-strawberry crop rotation. The lettuce rotation treatment was more conducive for build up of inoculum, with upper ranges of 10-17 microsclerotia g-1 soil being more frequent. Whereas in brussels sprouts treatment the inoculum build up was moderate to low and the numbers were not high as in lettuce treatment. Broccoli rotations reduced the inoculum significantly and the inoculum levels remained consistently low throughout the sampling period (Fig. 1). At the second MBA site, the average inoculum levels before the treatments were begun were as high as 40 microscleroti g-1 soil (Fig. 2). There was a significant increase in the number of microsclerotia following two crops of lettuce and a significant, nearly 50% decrease following two crops of broccoli. The fumigation treatment had the least amount of inoculum (Fig. 2). This reduction was consistent in both conventional and organic management systems. Soil samples from the second season from these sites are still being analyzed. No detectable microsclerotia were present in the Spence field soils during the four growing seasons.

At both locations, higher strawberry plant canopy diameter was recorded in broccoli rotation treatment (Figs. 3-7). Plants grown in lettuce treatments at both the locations had significantly lower plant diameter than the rest of the treatments (Figs. 3-7). At the new MBA site also, strawberry plant canopy diameter was significantly higher in the broccoli rotation treatment compared with the lettuce treatment at both times when the data were collected. The plants in the fumigated plots were more robust than in all other treatments early in the season but this difference compared with the broccoli treatment disappeared later in the season (Fig. 8). In mid-season, the shoot weights of strawberry plants in the fumigated control were greatest, followed by broccoli and lettuce treatments. The root weights, however, were not significantly different amongst the treatments (Fig. 9). Results from the second year were identical to the first year.

The rotation treatments had a significant effect on the strawberry disease severity rating during all of the observation points at both the locations during 1998, 1999, 2000, and 2001 (Figs. 10-15). At MBA, strawberry plants grown in lettuce rotation treatment plots had the highest disease severity rating, about 25% - 100% higher than in the broccoli treatment. At Spence, plants grown in broccoli rotation treatment showed the lowest disease severity rating among all the treatments, and at MBA, the broccoli treatment disease severity was next only to that of methyl bromide and chloropicrin control. Strawberry plants in broccoli rotation treatment showed a consistently lower disease severity than in the rest of the rotation treatments during all observation dates (Figs. 10-15). Petioles from diseased plants from MBA site when plated on NP-10 medium yielded V. dahliae. The diseased plants from Spence site did not yield V. dahliae, indicating other soil borne pathogens were responsible for growth and yield reductions. At the Spence site, the disease severity ratings were low and diseases observed were mostly powdery mildew and other non-Verticillium soilborne pathogens. However, towards the end of the season few diseased plant samples yielded V. dahliae. The emergence of Verticillium wilt in the plots with no initial detectable soil inoculum may have been due to a build up inoculum in the presence of susceptible host.

Methyl bromide and chloropicrin fumigation produced the highest marketable strawberry yields obtained in both test locations. Yield at final harvest at Spence was not significantly different among rotation treatments. However, at MBA, the broccoli treatment plot marketable yield was significantly higher than the lettuce treatments. The yield loss was 23% lower in broccoli plots and 45% lower in lettuce plots compared to the fumigated control (Figs. 16-21). Results from the second year at the new site are being analyzed.

Objective 2: We now have the complete data on the total actinomycete, bacterial, and fungal counts in the different treatments. These data suggest that relative to the lettuce treatment, the total actinomycete, bacterial and fungal counts were significantly higher in the broccoli treatment. The results were consistent between the two years and were similar to those obtained from a cauliflower-broccoli rotation system. The mechanisms by which crucifer residues act on plant pathogens have been assumed to be mostly chemical. Most of the previous studies have tested the effectiveness of crucifer residue, or of total or specific gases emanating from the residues on non-sclerotial fungal pathogens. Because most glucosinolate breakdown products are volatile, their retention in the soil environment is very short. Sclerotial fungi, however, survive in soil despite adverse weather conditions for prolonged periods of time. Thus, a transient exposure to the volatile gases may be insufficient to affect the viability of a significant number of V. dahliae microsclerotia. As the microsclerotia may be located both in the soil and in the organic debris, they may not be uniformly exposed to the volatile gases. It is most likely that other biological mechanisms in broccoli residue-amended soil operate in affecting pathogen propagule survival. Thus, data obtained in this study provide the first evidence that soil biology altered by specific crop residues are perhaps responsible for the suppressive nature of these rotations.

Objective 3: No mycorrhizal associations on strawberry roots collected from different treatments were detected. Again, the results from both years were similar.

Objective 4: The cost-benefit analyses suggested that in the short term, rotation treatments appear to decrease the returns on the strawberry crop using standard fumigation practices. However, given the various other benefits from rotations with broccoli, in the long run, rotations appear to be highly profitable (Appendix 1).

Research conclusions:
Impact of Results

One crop that has the potential to significantly alter the way we manage soilborne diseases is broccoli. Working on other cropping systems, these benefits have been clearly demonstrated. Cauliflower Verticillium wilt, once considered a significant threat to the cauliflower industry in California has essentially been eliminated from production fields using rotations with broccoli. Results from our work on this project also show that the same benefits can be reaped in the strawberry production system. Another attractive aspect of our work has been the demonstration of the effectiveness of broccoli in both conventional and organic production systems. Thus, rotations with broccoli are likely to play a significant role in the post-methyl bromide era.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

Bull, C. T., K. G. Shetty, and K. V. Subbarao. 2002. Interactions between myxobacteria, plant pathogenic fungi and biocontrol agents. Plant Disease 86:889-896.

Kabir, Z., K. V. Subbarao, F.N. Martin, S. T. Koike. 2002. Crop Rotation for Verticillium Wilt Management in Conventional and Organic Strawberry. (Abstr.) Phytopathology 92:S40.

Kabir, Z., R. G. Bhat, and K. V. Subbarao. 2001. Optimizing polygalacturonic acid in NP-10 medium to improve Verticillium dahliae recovery from soil. (Abstr.) Phytopathology 91:S45.

Shetty, K. G., K. V. Subbarao, F. N. Martin, and S. T. Koike. 1999. Management of Verticillium wilt in strawberry using vegetable crop rotation. Workshop on alternatives to methyl bromide, San Diego, CA.

Each year in May, the California Strawberry Commission and the USDA hold field days to demonstrate all of the ongoing research on strawberries. As part of this effort, the benefits of rotation treatments and the results obtained thus far were presented to the attendees every year during 1998-2002. The response to this work has been overwhelming. The results of this work were also presented at the WSARE ‘Farming and Ranching for Profit, Stewardship and Community’ conference held during March 7-9, 2000, in Portland, OR. In addition, posters on this work were presented at the annual American Phytopathological Society meetings in Montreal, Canada in 1999 and the annual Methyl Bromide Alternatives meetings in San Diego, CA, the same year. The results of this work were also published in a newsletter published by the California Strawberry Commission that is distributed to virtually all growers, PCAs, and researchers in California (Appendix 2). Furthermore, the results were presented in two other talks tailored mainly to strawberry growers, handlers, and PCAs. Both talks had an attendance of at least 150 people.

Judging from the response of growers attending field days, it is clear that many of them are contemplating adoption of rotations with broccoli in their fields. This will not only provide them an opportunity to test the benefits of these practices in their own fields but also tailor the different production practices to their individual preferences before methyl bromide is completely withdrawn.

Reactions from Farmers and Ranchers

The comments that we have heard so far have all been very complementary. At least one strawberry grower stated that he has been practicing rotations with broccoli for a number of years and has observed similar beneficial effects demonstrated in this study. We have also heard from five other large growers inquiring details of how they might adapt rotations in their cropping practices. We will be assessing in the coming years whether these growers actually have implemented this practice in their production fields and their reaction to the results.

Education and Outreach Outcomes

Recommendations for education and outreach:

Areas needing additional study

Now that strawberry production is occurring more and more in the organic production systems, rotations with broccoli are likely to provide the greatest benefit also under this production system. Thus, there is a great need to compare the efficacy of rotations in conventional and organically managed production systems. Our preliminary evaluations in experimental organic plots have demonstrated the efficacy of broccoli under this system as well. We are likely to see the more widespread adaptation of rotations of broccoli with strawberries when methyl bromide use ceases completely in 2005.

Information Products

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.