Peppermint (cv. Black Mitcham) is a high-value crop grown for both its desirable oil composition and high yield. However, this crop is highly susceptible to Verticillium wilt, a devastating disease caused by Verticillium dahliae. While this perennial crop could traditionally be grown for five years, in recent years mint stands have begun to decline after only three years, after which oil yields declines to such an extent that the crop needs to be replanted in another field. All mint cultivars grown commercially are sterile hybrids and therefore traditional breeding for Verticillium wilt resistance has not yet been possible. Broad scale fumigants can help suppress this disease but are prohibitively expensive. Moreover, these products can negatively affect beneficial soil microbes that provide key agroecosystem services.
This project will work with mint growers to evaluate their current cultural practices and design new production systems which integrate more holistic and sustainable practices that will increase yield, reduce the need to move fields or new land, and improve soil health and their economic bottom line. We will survey and interview Midwestern mint growers, buyers and processors and gather data to elicit farmers’ decision-making processes, production costs, selling arrangements, access to markets and price premiums, and perceptions towards land availability, and technology adoptions. Farm fields will be surveyed to identify practices contributing to Verticillium wilt incidence and severity. Participatory on-farm research trials will be conducted investigating biological approaches to remediate soils currently infested with V. dahliae, cover crops and anaerobic soil disinfestation (ASD). A farmer-to-farmer education and outreach component will include field days, workshops with farmer panels and a visit of Indiana growers to Washington farms. Farmer experience will guide the development of resource publications which will be used across the state and Midwestern region. In an effort to increase farmer participation in this project, a farmer advisory committee will be formed to guide research and outreach components.
Midwestern mint growers will gain knowledge about alternative solutions for Verticillium wilt management. Knowledge gained from growers, and research and outreach efforts will translate into educational programs that are better equipped to fully support mint growers regionally with the adoption of enhanced sustainable agricultural practices.
Mint growers are running out of suitable land that is free from Verticillium wilt. Meanwhile, consumer demand for more sustainable products are increasing and mint growers are ready to embrace it. This initiative will have farreaching effects on the Midwest mint industry.
The project will document current mint production practices to identify key factors contributing to Verticillium wilt and identify opportunities to integrate more sustainable practices that will ensure long-term profitability. The potential for anaerobic soil disinfestation (ASD) and cover crops to cost-effectively mediate Verticillium wilt will be determined. Mint growers will learn how to conduct on-farm research and information sharing through a farmer-tofarmer network will ensure that results are disseminated and adopted. Adoption of improved production innovations recommended from this project will lead to longer crop cycles, enhanced soil health and maintenance, improved economic viability, and better use of existing farmland.
Mint is one of the most important specialty crops in the U.S. Midwest. Approximately 18,000 acres are currently planted to mint in this region, with an estimated annual value of $22 million. Traditionally mint was grown for five years, however, in recent years the production cycle has become limited to just three years, after which a new (virgin) field has to be identified for subsequent planting. This is mainly due to the soil-borne fungus (Verticillium dahliae) that causes Verticillium wilt, the most serious disease of mint. Verticillium wilt is a disease of the plant’s vascular system. The initial inoculum consists of microsclerotia (hardened fungal bodies), which are barely visible. Microsclerotia are produced internally in the plant and are released once the plant material decomposes. The fungus infects the roots and plugs the water-conducting tissue, which prevents water from reaching the upper plant parts. In the presence of the root lesion nematode (Pratylenchus penetrans), plants may develop earlier and more severe symptoms. Microsclerotia can be produced in large numbers and can survive in the soil for up to 15 years (Sunseri and Johnson, 2001). Other factors such as weed species serving as a pathogen reservoir, movement of contaminated equipment between mint fields, reintroduction of the pathogen following application of spent mint waste, and declining soil quality could also play a role in mint decline and susceptibility to Verticillium wilt. The most widely grown peppermint cultivar, ‘Black Mitcham’, is particularly susceptible to this disease. Verticillium wilt is a high priority for the Mint Industry Research Council. They are investing in programs at Washington State University and Oregon State University that investigate breeding of a Verticillium wilt resistant cultivar, mapping of the mint reference genome and analysis of V. dahliae diversity. However, because commercially grown cultivars are sterile, this is difficult using traditional breeding techniques and progress has been slow. Crop rotation is likely to be of little value since the disease can survive in the soil for many years. Currently the only effective solution is to find new (virgin) land for mint production. Although this has less negative environmental consequences compared to any chemical treatment, it is not sustainable.
The first step in overcoming these challenges is to document current production practices on mint farms. Detailed surveys of farmer practices and how these could contribute to the incidence and severity of Verticillium wilt will be critical to helping researchers and Extension educators begin to work with mint growers to develop and deliver cost-efficient solutions to this disease, as well as other production related problems that not only improve their profitability but ensure long-term sustainability. For example, we expect that integrating cover crops and green manures, or incorporating other organic amendments prior to planting, could help reduce this disease. Cover crops such as sorghum sudangrass, broccoli, and various mustards have been demonstrated to help suppress Verticillium wilt in potato and cauliflower (Davis et al., 1986; Subbaro et al., 1999; Dung and Weiland, 2018), but the direct link between cover crops and Verticillium wilt in mint has not yet been established.
A more aggressive strategy to revitalize soils currently infested with V. dahliae is ASD, a relatively new soil rehabilitation technique developed concurrently in Japan and the Netherlands. This approach involves amending soil with a labile organic substrate, saturating it with water and covering it with plastic to create anaerobic conditions. The process is thought to generate compounds that are toxic to soil borne pathogens such as V. dahliae as well as weed seeds and facilitate recolonization of beneficial soil microbes that enhance plant growth. ASD has recently been tested and demonstrated to be effective in mediating Verticillium wilt in U.S. strawberry production systems, which is also caused by V. dahliae. It is also currently being tested for its potential to mitigate soil-borne pathogens in Midwest high-tunnel production systems in a NCR-SARE funded proposal led by S. Miller at Ohio State University.
Verticillium wilt (V. dahliae) is a devastating disease for peppermint growers. Current control practices include the use of virgin land for new plantings and the use harsh chemical fumigants. Anaerobic soil disinfestation (ASD) is a novel technology that was developed for the control of soil pathogens, parasitic nematodes and weeds. Labial organic sources are used to rapidly deplete soil oxygen, while creating compounds that help to increase mortality of soil-borne pathogens and nematodes. We are confident that ASD could be a very useful tool for growers in their quest to control Verticillium wilt.
Purdue Extension will work in partnership with scientists and growers to design, test, and conduct a mint production survey among Midwestern mint growers to collect information on current production, processing, and marketing practices, and decision-making data. With support from Indiana Mint Market and Development Council, we will survey ~40 commercial mint growers, buyers and processors in the Midwest. The survey will gather data to elicit farmers’ decision-making processes, production costs, selling arrangements, access to markets and price premiums, and perceptions towards land availability, and technology adoptions. For instance, we will be able to capture the strategies farmers take to gain access to local and high-value markets, the cultural practices adopted to prevent the spread of Verticillium wilt, and technologies adopted to add value to mint. The data collected will shed light on the most salient barriers and drivers to the use of sustainable practices for mint production.
We expect that production and marketing challenges will vary by farm size, processing capability, crop diversification, value-added processes, and market access. The decisions to address mint pathogen pressure is complex. Moreover, the lack of available land has created additional challenges that mint farmers have had to address. The survey will help us determine the decision-making processes followed by successful mint operations and will shed light on the record-keeping practices and costs for mint operations, which we would expect to depend on the size of the operation, market access, and added-value technologies adopted. The survey will use structured questions to allow researchers to analyze across the groups of growers and identify marketing opportunities for mint farmers in the Midwest. The development of the survey questionnaire will involve Extension educators, farmers from the advisory committee, and field experts. The information collected will help researchers, extension personnel, policymakers, and farmers to improve the sustainability of mint production, determine which sustainable growing practices to use, and how best to harvest and market their mint or mint oils.
On-Farm Participatory Research
Soil and plant samples will be collected from peppermint fields representing the range of production practices identified using the surveys described above to determine how such practices affect soil health and potential for Verticillium wilt incidence and severity. Soil physical, chemical and biological properties will be determined using techniques described in Rudisill et al. (2015) to characterize soil health. Abundance of V. dahliae populations in soil will be determined using quantitative PCR as described in Bilodeau et al. (2012), and plant infection will be confirmed by plating tissues on Sorenson’s NP-10 selective media (Kabir et al., 2004).
On-farm trials to evaluate the efficacy of ASD for mediating Verticillium wilt in mint will be conducted at four farms with high levels of V. dahliae (two mineral and two muck soils) and funding will be provided to compensate growers for their participation. Criteria for the identification of on-farm research locations will include soil type, the type of mint produced, soil health, level of Verticillium wilt infestation, and geographic location. Each on-farm trial will include seven treatments. Trials will be arranged in a randomized complete block design with four replicates. Each experimental plot will be 18 ft. in width, to accommodate a standard swather, and 30 ft. in length. Treatments will include ethanol, composted poultry manure, a mustard cover crop (cv. Caliente 199), two locally available substrates that were effective in our on-going greenhouse trials evaluating the potential for ASD to remediate field soils infested with V. dahlia, an untreated negative control, a chemical fumigation, and an untarped mustard cover crop treatment. The cover crop treatments will be mowed and disked prior to treatment and the molasses and two alternative ASD substrates in the ASD plots will be amended and mixed into soil at a depth of 6 inches at rates determined to be effective in our greenhouse trials. Soils will be saturated with water and covered with clear polyethylene mulch film for a period of three weeks during the summer (August). The chemical fumigation treatment (Vapam® HL) will be conducted using standard industry practices. All treatments will be tarped, except the negative control and untarped cover crop treatments. Weeds will not be chemically controlled. In one block at each farm, oxidation-reduction potential and temperature sensors will be installed in each treatment to monitor soil redox and temperature during ASD treatment. After the soil treatment is complete, dormant mint stolons (cv. Black Mitchum) will be transplanted in late fall (November) into each plot at a row spacing of 30 inches. Soil samples will be collected from each plot prior to treatment, at the conclusion of the three-week treatment, and early in the season during year 2. Plant samples will also be collected at mid-season during years 2 and 3 of the study and just prior to harvest in each plot in each year of the study. Soil and plant samples will be evaluated as described above for the survey component of this study. In addition, weed and mint biomass will also be quantified in each plot during each year using standard protocols. Mint quality will be determined following distillation and GC-MS analysis. Statistical differences of all parameters quantified will be determined using ANOVA and means separation determined using Tukey (P<0.05) in SAS software. With farmer input, results will be interpreted and presented to their farmer-to-farmer network.
Like all field trials, natural disasters may impact the outcome of the work. Our project working group will minimize the chances for such impacts by identifying four farms in different parts of northwestern Indiana for the on-farm trials. Two locations will represent peppermint grown on muck soil and the other two on mineral soil to represent the range of mint farms in the Midwest region. The chance of a disaster affecting all four locations will be rare.
We were unable to implement our on-farm participatory research during 2019. The very wet extended spring followed by a very hot and dry period during June and July did not allow us to seed the cover crop on time. A project extension was requested. Approval was granted to initiate the on-farm participatory research during 2020 and continue with it until 2023. Field plots should be established during April 2020 and the ASD and fumigation treatments will be implemented during the last week of July 2020. This will allow us to plant mint in March 2021.
Results from our greenhouse potted study will be available soon.
- Workshops at the Annual Midwest Mint Conference covering relationships between soil health and pathogen dynamics, introduction to ASD and on-farm participatory research, and results of greenhouse experiments.
- Farmer panel discussions to share project results at workshop and regional grower meeting.
- Presentations of project findings at two field days, workshop at annual grower meeting and scientific conferences.
- One farmer-to-farmer event with 10 participants.
- Professional development session at annual Purdue Extension ANR retreat on soil health and sustainable agricultural practices for mint production.
- Post Doc and graduate student trained in the characterization of Verticillium wilt and ASD experiments.
- Extension bulletins and journal publications.
- Website dedicated to hosting mint research informaton.
Educational & Outreach Activities
An annual SARE project workshop is conducted in conjunction with the Midwest Mint Conference. Indiana, Wisconsin and Michigan growers come together annually to discuss issues and learn more about mint production. This has been very successful. It is the best time of year to get all growers in one room.
The 2020 Annual Mint Industry Research Council Conference was held in Scottsdale AZ. Is was very well attended. Growers from the main mint producing states in the U.S. (California, Washington State, Oregon, Idaho, Indiana, Wisconsin and Michigan), mint oil buyers (Labbeemint, A&M Todd, Callisons, Northwest Ingredients), end users (Wriggles, Colgate) and input suppliers were present at the meeting. Dr. Langenhoven’s presentation presented information to growers about opportunities for the integration of more sustainable practices in the quest to control Verticillium Wilt in peppermint. Its also gave him the opportunity to explain to attendees what we are trying to do in our SARE project.
The project is in its establishment phase and therefore we do not have any success stories yet. Farmers are however very enthusiastic about the project and the potential benefit it could have on their farming operations.