Biofumigation, the managed release of degradation products from plant residues, is an alternative to soil fumigation. We evaluated traditional biofumigant crops like rapeseed and mustards, and others including forage pearl millet. Our intent was to combine crop amendments with solarization for managing Potato Early-dying Disease (PED) pathogens, but we found soil covers were not crucial for forage pearl millet. Forage pearl millet is frost sensitive so is best used in the upper Midwest as a full season cover crop or to follow early season vegetables. The mechanism for suppressing soilborne pathogens appears to be related to decomposition rather than to specific plant compounds.
Potato production systems are important in most states in the North Central region and potato is prominent in diets typical of the Midwest and the world. Potato production accommodates a range of cultural practices and strategies, but there is one practice that is common to 80% of the potato acreage in Wisconsin and a majority of acreage elsewhere – soil fumigation. Fumigation is done in the fall preceding potato for the purpose of decreasing two soil pathogens, the fungus Verticillium dahliae and the nematode Pratylenchus penetrans and the disease that they cause, potato early dying. Metam sodium is the most commonly used chemical in the North Central Region, but there is a growing interest in chloropicrin. Both metam sodium and chloropicrin are general biocides with the potential to harm a broad range of organisms, including humans if accidental exposure occurs. Soil fumigation is effective, profitable despite a relatively high cost (approximately $150-$300 per acre), and is accomplished as a one-time event during a period that doesn’t interfere with other farm activities. Despite the advantages of soil fumigation, there is a clear and growing interest among farmers to develop additional strategies for managing potato early dying disease.
Biofumigation, the managed release of degradation products from plant residues that are toxic to soil organisms, is an effective practice for pest and pathogen control. The principal components of biofumigation are appropriate plant material, tools to chop and incorporate the material into soil, and manipulation of the soil environment to affect the timed release of decomposition products. For all of the human effort involved, the success of biofumigation also rests with the soil microbial community. It is the activity of the rich parade of litter- and soil-dwelling microbes that release volatiles, break down organic matter, and change the fabric of the soil community to the credit of the microbial consortium responsible for litter breakdown, nutrient cycling, disease and pest suppression, and filtering ground water. These ecosystem services are an integral part of agriculture and are supplanted, in part, by anthropogenic inputs such as fertilizer, pesticides, and tillage.
Biofumigation might not match the profitability of fumigation in the short term, but developing practices that are both profitable and sustainable is a critical activity for improving the profitability of farmers. All of the cover crops used for biofumigation return organic matter to soil, and thus help sustain important ecological services that can supplement current management activities. Unlike soil fumigation which causes a fast sharp spike in biological activity of select organisms able to capitalize on dead colleagues, biofumigation promotes a continued release of volatiles in combination with carbon and nitrogen resources that support a diverse community. Biofumigation is a pest suppression practice that poses no risk of accidental release or exposure of the public to noxious chemicals. Accidents with metam sodium, chloropicrin, and other soil fumigants are rare, but such occurrences gain immediate and wide-spread attention that has an impact on public perception disproportionate to their real risk.
Pesticides are an important tool for sustaining profitable farming systems, but absolute reliance on pesticides increases the vulnerability of farms and farmers to changes that can be mandated with little warning or full consideration of consequences. Our long-term goal is to equip farmers with knowledge, confidence, and tools to maintain agricultural production systems that utilize the ecological services provided by the soil community to augment pest control and promote crop productivity. The short- and intermediate term objectives for this project were to:
1) Build a knowledge base among growers and agribusiness representatives about ecosystem services available in the soil and the organisms involved.
2) Increase the expertise of farmers about cover crop management and the practice of biofumigation.
3) Increase awareness of farmers, crop consultants, county agents, and industry representatives of the value of biofumigation relative to soil fumigation with synthetic chemicals.
4) Increase the use of cover crops in vegetable production systems in Wisconsin
5) Increase the number of growers who consider ecological principles when making management decisions.
6) Increase public awareness of biologically-based pest management alternatives and efforts by farmers to promote land stewardship.
1) Determine why covering soil enhances the efficacy of biofumigation.
2) Evaluate different cover crops, timing of biofumigation, and soil covering strategies
Increasing grower awareness about ecosystem services, adoption of cover crops, and use of biologically-based pest management:
Two groups of potato farmers were the primary focus groups for our efforts. A group of progressive potato growers producing fresh market potatoes in the Healthy Grown program were reached using presentations, field days, and on-site farm visits. Dialog at meetings, interviews, and review of the pest management practices were used to evaluate changes in their attitudes and adoption of new practices. The second group was composed of seed potato growers. Interactions with this group were primarily in the form of farm visits with much of the activity coordinated with a crop consultant. We collected data from their on-farm trials with different cover crops, with a focus on helping them design and execute their own evaluations of crops to plant during the late summer and fall preceding potato.
Both of these focus groups also participated in state-wide meetings for potato and vegetable growers. In this large venue, we used audience response systems to collect ideas and monitor growers’ attitudes.
Research activities to better understand and select biofumigation practices for managing PED pathogens:
In small plot field studies, cover crops of forage pearl millet (Pennesetum glaucum) and rapeseed (Brassica napus) were sown in May and grown as a cover crop until the first week in August when they were chopped and incorporated as a green manure. Non-seeded plots with weeds were maintained as a fallow control. Five biofumigation treatments were established at incorporation: 1) millet as a green manure, 2) rapeseed as a green manure, 3) millet green manure + solarization, 4) rapeseed green manure + solarization, and 5) rapeseed green manure + modified solization. In the plots with modified solarization, soil was placed around the periphery of the plot so that irrigation and rain water puddled on top of the polyethylene film.
A laboratory experiment was conducted to study the response of P. penetrans and V. dahliae to decomposing plant tissues. Air-tight plastic containers were filled with field soil, coarsely cut millet leaves, millet leaves plus field soil, and millet roots plus field soil. Millet was chosen as the crop residue based on earlier work showing the efficacy of this crop in reducing nematode population densities. Rates of the amendments were based on the shoot growth obtained in our field trials for 60-day forage pearl millet. Nematodes and Verticillium were placed in water-filled dishes suspended above the soil and plant material, so that they contacted volatile gases but not organisms.
In the laboratory, a follow-up experiment was conducted to determine the impact to nematodes of elevated, but non-lethal temperatures. Mixed stages of P. penetrans in soil-filled containers were immersed in a water bath maintained at 36 C for 1 to 90 minutes and then assayed for viability.
Our original intent was to develop practices that combined green cover crop amendments with solarization for PED pest management. In our controlled experiment we found that soil covers were not crucial for the millet treatments. In both the controlled experiment, and the on-farm studies, we confirmed that rapeseed, Oriental mustard, and other Brassicas are excellent hosts for Pratylenchus spp. so that population densities are often greatly increased by the time plant tissues are incorporated for biofumigation. Those crops do benefit from solarizing crop residues to insure maximum efficacy. By the end of the project we were promoting forage pearl millet as a cover crop without soil covering, with the caveat that the crop must be planted early because of its sensitivity to frost. In the grower field trials, one frost event in early September killed the cover crop even though temperatures remained above freezing for the next 60 days.
In the on-farm trials, growers compared forage pearl millet to a commercial biofumigant mustard mix (3 sites), oat (2 sites), and legumes (5 sites). The results were encouraging for the forage pearl millet, except for the frost sensitivity. This crop has not been used by farmers in Wisconsin, so the trials provided opportunity to evaluate it under real world conditions.
We had mixed success with our research experiments. Our attempt to modify solarization so that plots were covered but not elevated in temperature was partially successful. The modified solarization treatment achieved the desired effect when the design was executed properly. The number of hours when soil temperature at a 3-cm depth exceeded 40 C was 18 hours for the successfully modified tarped plot versus 69 hours for a tarped plot without surface water. Soil temperature exceeded 50 C only in plots with no surface water. We were unable to maintain puddled water on solarized plots for a long enough period to justify treatment comparisons.
Laboratory experiments showed that field soil amended with cut millet shoots was lethal to nematodes. Mortality was induced by the indirect effects of decomposition because the nematodes were not in the soil. Field soil alone, cut millet shoots alone, or cut millet roots mixed into field soil did not affect the survival of nematodes. An additional control treatment of cut corn shoots in field soil was also lethal. There was a trend for similar results with Verticillium, but the data were more variable and not statistically significant. Factors associated with the soil plus shoot treatments that might contribute to nematode mortality included volatile gases, anaerobic conditions, and elevated temperature. These studies confirmed what we observed in the field: biofumigation is a general phenomenon that can be achieved using a wide range of crops.
Controlled trials showed that exposing P. penetrans to temperature ranging from 34 – 36 C for as few as 3 hours decreased mobility and survival. Our treatments were consistent with the heat generated from decomposing residues and may explain, in part, why buried amendments suppress nematode population densities.
The two focus grower groups were interested and invested in the project. More behavioral changes were seen in the group of potato seed growers than the group growing Healthy Grown product. The Healthy Grown program has been successful at moving the product into the marketplace, but has not been able to command even a modest premium over the conventional product. New environmental standards for the management of natural areas surrounding Healthy Grown fields were added to the label, so growers saw an increase in their costs (seed of native plants, managed burns, etc.) with no change in profit. In light of these constraints, the organization that provides oversight for the standards and certification postponed fumigation reduction goals until the Healthy Grown product is economically viable. The growers remain committed to the goal of increasing the use of biofumigation and other fumigation alternatives, but currently lack the financial incentives to do so.
The reaction of the state-wide group of growers to the audience response system was very positive. It provided a valuable source of feedback and it also stimulated interactive presentations in audiences of more than 100 people. Knowledge about fumigation alternatives among growers increased over the course of this project.
Forage pearl millet is beneficial for suppressing the pathogens that cause the potato early dying disease.
Crop residues of forage pearl millet have a biofumigation effect on potato early dying pathogens.
Decomposing shoots, but not roots of forage pearl millet are lethal to root lesion nematodes. The crop is frost sensitive so achieving adequate shoot biomass is a concern in Wisconsin for crops planted after September 1st.
Some of the more general effects of biofumigation, such as the elevation of soil temperature during decomposition may be, in part, responsible for the suppression of nematodes.
Growers were willing to incorporate replication and control treatments into their on-farm trials and benefited from information about designing evaluations of farming practices.
Participants in the Healthy Grown program reported learning beneficial information about biologically based alternatives to soil fumigation for disease management.
Starting in 2011, certification for the Healthy Grown program will require growers to make a plan for fumigation.
The Healthy Grown program expanded to include vegetables grown in rotation with potato and will stress the importance of cover crops, including biofumigation practices.
An important message to our program is that soilborne diseases such as the Potato Early Dying Disease need to be addressed throughout the entire rotation rather than during a short time frame prior to planting the susceptible crop. Soil borne pathogens are permanent residents in fields and all field activities should be viewed for their impact on pathogen population densities.
Forage pearl millet planted at a rate of 12 – 15 lbs per acre ranges in cost between $15 and $25 per acre for seed. The crop responds to nitrogen fertilizer and 40 – 60 pounds of available N per acre is suggested. We conducted preliminary studies, but did not determine, the impact of harvesting shoots to sell as a forage. The cost of using a green manure cover crop is negligible if soil fumigation, at a cost in excess of $275 per acre, is eliminated or practiced less frequently.
Growers report trying new cover crops including forage pearl millet as well as thetraditional biofumigant crops of India and Orientele mustards and rapeseed. Over 300 growers have been reached by this project and although only about 2-3% of the clientele have tried the practices we’ve studied, we are confident the number will increase. Interest in trying alternative practices to soil fumigation is increasing and new buffer zone requirements for the soil fumigants will force some growers to find alternatives in the near future.
Educational & Outreach Activities
2006 (July – December)
Presentation: Cover Crops and Potato Early Dying, presented by A. MacGuidwin at the Biointensive IPM Meeting, Hancock, WI.
Presentation: Fumigation Alternatives Initiative, presented by A. MacGuidwin and D. Knuteson at a meeting of growers participating in the Healthy Grown Potato Program
Presentation: Soil Health and Soil Communities, presented by A. MacGuidwin at a meeting of 20 growers, Hancock, WI.
Presentation: Research on Fumigation Alternatives , presented by A. MacGuidwin and D. Knuteson at the Biointensive IPM Meeting, Hancock, WI
Presentation: Cover Crops and Potato Early Dying, , presented by A. MacGuidwin and D. Knuteson at a meeting of growers participating in the Healthy Grown Potato Program
Presentation: Alternatives to Soil Fumigation, presented by A. MacGuidwin at a meeting of growers participating in the Healthy Grown Potato Program
Presentation: Top 10 Ways to Manage Potato Early Dying, presented by A. MacGuidwin at Wisconsin’s Annual Potato Meeting to about 150 sttendees.
MacGuidwin, A. 2007. Cover crops for potato early dying management.. Proc. of WI Annual Potato Meeting, 20:21-24..
MacGuidwin, A. E., and D. L. Knuteson. 2007. Using biointensive practices to manage two interacting pathogens. Journal of Nematology 39:104.
Presentation: Making a Soil Fumigation Plan , presented by A. MacGuidwin at Wisconsin’s Annual Potato Meeting
Presentation: Managing Potato Early Dying – Today and Tomorrow, presented by A. MacGuidwin at Wisconsin’s Annual Potato Meeting
Presentation: Fumigation Alternatives Initiative, presented by A. MacGuidwin and D. Knuteson at a meeting of growers participating in the Healthy Grown Potato Program
Presentation: Hot Topics for Nematodes, presented by A. MacGuidwin at the Hancock Research Station Potato Field Day
MacGuidwin, A. E. 2008. Cover Crops and Biofumigation For Managing Pratylenchus spp. Fifth International Congress of Nematology, July 13-18, Brisbane, AU.
Presentation: Maintaining Field Records for Potato Early Dying, presented by A. MacGuidwin at a meeting of growers participating in the Healthy Grown Potato Program
MacGuidwin, A. E., and D. L. Knuteson. 2009. What if you can’t fumigate every field? Wisconsin’s Annual Potato Conference, 22:137-138.
Presentation: Nematodes and the Soil Food Web, presented by A. MacGuidwin at the Soil Quality Workshop
MacGuidwin, A. E. Pratylenchus penetrans is a common and persistent pathogen of potato in the North Central Region. North Central Meeting of the American Phytopathological Society, Ames IO.
MacGuidwin, A. E. Perplexing potato problems: potato early dying. Phytopathology 99:S158.
Areas needing additional study
The limitation to forage pearl millet is that it is a warm season grass highly susceptible to frost. Additional crops with biofumigant properties and frost tolerance are needed. We are making progress in convincing growers that soilborne pathogens are permanent residents of fields that can be targeted at any point in the rotation cycle, but we need effective ways to demonstrate this concept.