Cucurbit crop growers in the North Central Region struggle to control cucumber beetles and the bacterial wilt pathogen they spread. The bacterium kills plants, depresses yield, and drains profits. Insecticide sprays are widely used against the cucumber beetles but damage pollinators and other beneficial insects, so their sustainability is in question. Muskmelon is highly susceptible to bacterial wilt, and organic growers often avoid planting this crop because yield losses can exceed 80%. Alternative management tactics such as biochemical lures, perimeter trap cropping, delayed planting, and crop rotation have not been reliably effective. Low tunnels covered by spunbond polypropylene fabric work well until they are removed to allow pollinator access, but then muskmelon can collapse and die from mid- to late-season wilt outbreaks. We have been testing an innovative variation on the protective tunnel theme that we call mesotunnels. Mesotunnels are more than twice as tall as low tunnels and are covered by a breathable nylon-mesh fabric that remains in place all season, with bumble bee colonies inserted underneath for pollination. In small-plot trials in Iowa in 2016 and 2017, mesotunnels increased marketable yield of organic muskmelon by an average of 450% compared to low tunnels or insecticide-only treatments – without using any insecticides. This approach therefore has promise to deliver consistently high marketable yields without the risks associated with insecticide use – a change that could open up new growing and marketing opportunities for organic cucurbit growers. Our proposed project will take the essential next steps to make mesotunnels into a workable alternative for North Central Region growers. Two years of field experiments under organic management will assess living mulches for weed control and soil quality improvement, and optimize the efficiency of purchased bumble-bee colonies for pollination under commercial-size mesotunnels (Objective 2). We will compare profitability of the mesotunnel system to current management strategies and work closely with growers to understand the factors that impact their views of the new system (Objective 2). Our intensive outreach program, mixing on-farm demonstration trials and field days in Iowa and Missouri with electronic features such as webinars and an online manual for growers, will ensure that the project’s messages reach cucurbit growers throughout the NC Region (Objective 3). Our results will have application to additional cucurbit crops that are susceptible to bacterial wilt, and for conventional producers seeking to minimize reliance on insecticides.
• 250 NC Region muskmelon growers become aware of the potential value of mesotunnels to suppress pest insects and bacterial wilt.
• 80 growers learn how to use mesotunnels on their cucurbit crops
• noo growers understand the economic tradeoffs of using mesotunnels for cucurbit production.
• 60 growers (including 30 organic growers) plan to try mesotunnel systems on their own farms within 2 years.
• 30 growers try mesotunnels in their muskmelon fields during the project period.
Bacterial wilt, caused by Erwinia tracheiphila, causes yield losses for cucumbers and muskmelon that can exceed 80%, and also decimates pumpkin, winter squash, and zucchini. Bacterial wilt-resistant cultivars of muskmelon and cucumber are nearly non-existent.Stopping cucumber beetles is currently the only effective way to combat bacterial wilt on highly susceptible crops like muskmelon, honeydew, and cucumber. Neonicotinoid insecticides are widely used against striped cucumber beetle (Acalymma vittatum) and spotted cucumber beetle (Diabrotica undecimpunctata). However, these and other pesticides weaken or kill pollinators and reduce populations of insectivorous birds. Synthetic pyrethroid insecticides are also widely used but kill natural enemies. Yield losses from bacterial wilt jeopardize growers’ sustainability. CSA customer dissatisfaction, unfulfilled contracts with cooperatives, and quality issues with wholesale clients can threaten economic viability and resilience of cucurbit growers. This problem is especially acute for organic growers because there are no consistently effective organic management strategies against bacterial wilt. With funding from NIFA Organic Transitions (2015–2018), our ISU team began evaluating an innovative protection system called “mesotunnels” to overcome disadvantages of low tunnels. Mesotunnels are covered with a tough nylon-mesh fabric that is far more durable and breathable than spunbond fabrics. Mesotunnels are also much taller (3.5 ft.) than low tunnels (1.5 ft), so there is more room for plant growth and no overheating of plants. Unlike low tunnels, mesotunnels can protect cucurbit crops for the entire growing season. Despite encouraging preliminary results, several key questions must be answered before mesotunnels can be recommended for use by growers:
- Can mesotunnels be scaled up effectively from small-plot experiments to commercial-size muskmelon plantings?
- What is the most cost-effective way to deploy purchased bumble bee colonies for pollination under full-season mesotunnels?
- Can living mulches be used to control weeds under mesotunnels while also enhancing soil health?
- Will higher and more consistent marketable yields increase profits despite the higher cost of mesotunnels?
Our 2-state (Iowa and Missouri), 2-year project will answer these questions in organic muskmelon, for which bacterial wilt is especially threatening, with an integrated program of field experiments, economic and stakeholder assessments, on-farm demonstration trials, and outreach.
- In 30-ft-long mesotunnel plots of organic muskmelon, living mulches of annual ryegrass plus red clover, or red clover alone, will a) suppress weeds between rows as effectively as weed fabric or Miscanthus residues, b) result in marketable yield equivalent to that of the weed-fabric and Miscanthus-mulched treatments, and c) increase soil content of reactive carbon compared to a bare-ground (non-mulched) treatment.
- In 150-ft-long mesotunnel plots of organic muskmelon, a) inserting two purchased boxes of bumblebees inside the mesotunnels during flowering will result in higher marketable yield than inserting a single bumblebee box under the fabric.
- In 150-ft-long mesotunnel plots of organic muskmelon, removing mesotunnel fabric for 2 weeks during flowering will result in increased incidence of bacterial wilt and reduced marketable yield compared to treatments in which bee boxes are inserted under the mesotunnel fabric.
Weed management with mulches in mesotunnels. At the Iowa State University Horticulture Research Station near Gilbert, Iowa, during 2019, muskmelon (cv. Athena) seedlings were transplanted into 30-ft-long, 3-row subplots spaced at 6 ft on black plastic mulched 2-ft centers with drip tape irrigation system, under full-season mesotunnels (3.5 ft tall tunnels with ProtekNet nylon-mesh fabric supported on conduit hoops), one bumblebee box was placed during bloom in each subplot for pollination, in a randomized complete block design. There were five mulch treatments applied to soil between black plastic strips: 1) red clover living mulch, 2) red clover and ryegrass combination (both seeded at the time of transplanting the crop), 3) shredded Miscanthus residue, 4) landscape fabric, and 5) bare ground (control). Rationales for using these treatments: 1) They encompass a wide range of the weed suppression tactics used by organic cucurbit-crop growers in the North Central Region. 2) Weed control under mesotunnels poses special challenges because there are no opportunities for mowing or other organic tactics during the growing season, so we sought to test tactics with potential to provide full-season weed suppression.Yield data were taken on the center row of each 3-row subplot; the number and weight of marketable fruit were recorded at harvest. Above-ground weed biomass was assessed immediately before crop harvest began. Reactive soil carbon was measured using a permanganate oxidizable carbon (POxC) method. Disease incidence and severity were rated weekly on the center row of each subplot.
Pollination strategies in mesotunnels. At the Iowa State University Horticulture Research Station near Gilbert, Iowa, during 2019, muskmelon (cv. Athena) seedlings were transplanted into 150-ft-long, 3-row subplots on 6-ft row spacing in a randomized complete block design. All plots were covered with mesotunnels (3.5 ft tall, with nylon-mesh fabric (ProtekNet) on conduit hoops) immediately after transplanting. The first two treatments were: 1) one bumble bee hive (Koppert Inc.) per subplot or 2) two bumble bee hives per subplot. In these two treatments, the ProtekNet® remained in place all season, except when the hives were inserted under the fabric when female flowers began to appear; the hives remained under these tunnels for the rest of the season. In contrast, Treatment 3 used no purchased bumblebee hives; instead, ProtekNet was removed when female flowers appeared, and replaced two weeks later, in order to allow pollination by bees from ambient sources on the farm. Rationales for selecting these treatments: 1) we were scaling up from 30-ft-long plots used in our previous research, so we needed to determine how pollination could be optimized; and 2) we realized that some growers might not be able to purchase bees or they might be too costly, so we wanted to include a presumable lower-input-cost treatment (Treatment 3). Visual observations of bee activity on muskmelon flowers were made twice a day in all treatments between 8:00 and 11:00 AM on three days per week during two weeks after female flowers appeared. Disease incidence and severity were rated on the center row of each subplot and recorded weekly. Number and weight of marketable fruit data were recorded at harvest on the center row of each subplot.
Weed management with mulches in mesotunnels. RESULTS: Mulch treatments did not differ significantly in marketable yield (Table 1). Landscape fabric and shredded Miscanthus had significantly less weed biomass than the two living-mulch treatments (red clover only and red clover + ryegrass combination) or the bare-ground control (See Table 2). There were no significant differences among the five treatments in post-season concentration of permanganate-oxizidizable carbon (reactive carbon) in the soil. DISCUSSION: The living mulches were ineffective at suppressing weeds compared to Miscanthus or weed fabric, but the much greater weed pressure in the living-mulch treatments did not suppress yield significantly – possibly due to the presence of black plastic mulch, which suppressed weeds in the strips where transplants were located. Therefore, our hypotheses about negligible effect of the different mulch treatments on yield was supported, but not our hypothesis about weed control for living mulches being equivalent to that for landscape fabric or crop residue. Despite having minimal impact on yield, the severe weed infestations that we observed in the living-mulch treatments – to the point that weeds overtopped the seeded mulches long before harvest – could undermine a central tenet of organic farming, which is to minimize the weed seed bank. Therefore, our living-mulch treatments could lead to an explosion of weed seeds in the soil that could severely inhibit productivity of subsequent crops due to weed competition. It could be therefore be argued that the living-mulch treatments we tested are unsuitable for organic vegetable farms in Iowa. However, there are other living-mulch options. For example, our colleagues at University of Kentucky have had considerable success with using teff as a living mulch; in their studies, teff grew rapidly after seeding and suppressed weeds very effectively in organic melon production systems. Therefore, we intend to change our living mulch species to teff for our 2020 experiments.
Pollination strategies in mesotunnels. RESULTS: Natural pollination (with ProtekNet removed for 2 weeks during bloom) resulted in about the same marketable yield as permanently covered mesotunnels that had one or two purchased hives per subplot. The natural-pollination treatment had the most flower visits by bees and other insects, followed by the two-bumblebee-hive treatment and then the one-bumblebee-hive treatment No bacterial wilt symptoms were seen during this trial. A fungal disease, Alternaria leaf spot, was observed late in the season; severity of this disease was low and did not differ among treatments. DISCUSSION: In the absence of cucumber beetle pressure and bacterial wilt, full-season mesotunnels did not offer an advantage over removal of covers for 2 weeks during bloom. Although these results are only for one site-year, they suggest that it may be more cost-effective in mesotunnels to use pollination strategies that do not include purchasing bees. Therefore, 2020 experiments will use purchased bees only for a control treatment and focus on utilizing on-site, “free” pollination services for treatments. This strategy is not without risk, however, since it is possible that cucumber beetles could infest the mesotunnel plots during the non-covered period, potentially leading to crop damage from bacterial wilt.
On-farm demonstration trials.
- Iowa. An on-farm trial was held at Bode’s Moonlight Gardens, in Algona, IA, owned by Beany and Joanne Bode. They grew butternut squash in three treatments: 1) under mesotunnels uisng ProtekNet nylon-mesh fabric, 2) under low tunnels (18 inches high, using Agribon, which is a spunbonded polypropylene row cover material), and 3) without any row covers. Due to wet conditions, planting was delayed. Plastic was laid and 4-week-old transplants of ‘Waltham Butternut’ butternut squash were planted on 20 June 2019. Agribon and ProtekNet were installed on 26 June 2019. Each treatment had a 33 ft. long raised plastic mulch bed with 13 plants. There were three replications for each treatment. Agribon and ProtekNet were removed on 22 July, 2020 to allow for pollination. ProtekNet was replaced on 10 August 2019 but Agribon was not replaced. A single harvest was conducted on 13 October 2019. Use of ProtekNet significantly increased the number and weight of marketable fruit compared to the other two treatments, whereas marketable yield in the Agribon (low tunnel) treatment was not significantly different from that in the non-covered control. Air temperature data was continuously collected during the growing season. For the month of July, average daily minimum and maximum temperature,as well as average daily temperature, were significantly higher under Agribon than under ProtekNet or in the non-covered control. There were no statistically significant differences in temperature among treatments for August or September. We also collected grower feedback on plant vigor, difference in insect damage and pest pressure, ease of installation, management challenges, and cost associated with row covers and Protek net treatments. A major advantage of using Agribon or ProtekNet was the protection from wind. Northwestern Iowa is windy. Wind protection from row covers resulted in more vigorous plants than in the non-covered treatment. As indicated by the grower, one of the biggest challenges was to manage weeds under the row covers and Protek net, especially on the edges of the black plastic mulch. It is not economically feasible to remove the ProtekNet and the conduit hoops when weeding is needed. Alternate strategies such as use of straw mulch between rows could help with weed management throughout the growing season. In this study the grower had applied Dual Magnum (s-metolachlor), a pre-emergent herbicide to manage weeds. Grower also expressed benefits of Protek netting as an insurance against hail events. Hails can damage crop, reduce productivity, and negatively affect farm profitability.
- Missouri. An experiment was conducted with ‘Athena’ muskmelon at the Organic Farm of Lincoln University in summer 2019. The experimental design was a randomized complete block design with 2 treatments (non-covered control and mesotunnel); 3 sections; 3 blocks per section and 10 plants per block. Transplanting took place on June 12. Vcucmber beetle populations were extremely low, and bacterial wilt symptoms were not evident in the field. At harvest, total yield for the non-covered control was approximately twice that of the meotunnel treatment. However, statistical analysis of the yield data has not yet been conducted.
- In Iowa, a field day was held at Bode’s Moonlight Gardens on June 21, 2019, wth 60 attendees, who saw the treatments just after transplanting. A second field day was held at the ISU Horticulture Research Station on August 5, 2019. As part of the wagon tour for that field day, 50 growers viewed the experimental plots for the SARE project. ISU graduate student Kephas Mphande, who had primary responsibility for conduct of the field experiments, spoke to attendees about the purposes of the trials and provided them with a one-page handout summarizing the project.
- In Missouri, a field day that was held near Jefferson City, MO, on August 7, 2019, included a poster presentation by Dr. Touria Eaton on impact of mesotunnels on muskmelon production and pest-insect management.
Dr Ajay Nair (ISU co-PI) presented two formal talks relating project purposes and results:
- Title: Row covers and other netting materials for pest management
Event: Gardens Club of America
Number of attendees: 25
- Title: Integrated pest management in sustainable vegetable production systems
Conference: Illinois Specialty Crop Growers Conference
Number of attendees: 35
ISU graduate student Kephas Mphande presented posters summarizing his 2019 SARE-project research results as follows:
- Great Plains Growers Conference in St. Joseph, Missouri, on January 9-10, 2020.
- MOSES Conference, Wisconsin, February 27-29, 2020.
Mr. Mphande also submitted two research reports (one on each field experiment), summarizing 2019 results, that are awaiting publication by the ISU Agriculture Experiment Station as part of their Farm Reports series (they have not yet received URLs). These research reports are intended for grower use.
Educational & Outreach Activities
We are 1 year into a 2-year project, so conclusions are tentative at this point. But we have learned or suspect the following:
- We have become doubtful that using a crop-debris mulch (Miscanthus in the Year 1 Iowa field experiments) in mesotunnel systemsis a practical approach for many vegetable farmers in the North Central Region. The limiting factors are cost and availability. Many vegetable growers do not have ready access to these materials, particularly organically sourced mulches. In addition, they can be quite expensive to buy, and the equipment and labor requirements to deploy the materials may be too high for many smaller-scale growers. Therefore, living mulches would be preferable. However, we are not convinced that rye-clover mixes are preferable for the strips between black-plastic rows in cucurbit production. Our results with rye-clover mixes have been poor due to slow or no germination and rapid overtaking by weeds. As a result of these experiences, we plan to seed teff instead of a rye-clover mix in 2020 experiments. Our colleagues at University of Kentucky have had promising results for several years with teff as a living mulch. We feel that this grass has greater potential than rye-clover mix to suppress weeds, mitigate the weed seed bank, and enhance soil organic matter.
- We have also had doubts that adding purchased bees to mesotunnel systems is a sustainable practice. There are two barriers: cost (Koppert Inc. hives are about $125 each and prices have been rising rapidly, in part because Koppert has a near-monopoly in this product line), and handling (some growers are reluctant to handle bees themselves due to personal-safety concerns, and there are added management challenges with maintaining the bees under row covers). Therefore, for the 2020 field experiment we plan to use full-season mestounnels, with purchased bees, only as a control. A new treatment in our pollination trial will open both ends of the mesotunnels when female flowers appear and re-close them 2 weeks later. The idea behind this new treatment is to allow for natural pollination (via the open ends) while hopefully continuing to provide some deterrence of cucumber beetles.