Evaluation of Soil Solarization as a Sustainable Management Method for Pests, Pathogens, and Weeds in Southeastern High Tunnels

Solarization plot map

Objective 1

For this objective, experiments will be conducted in Years 1 and 2 at the University of Kentucky Horticulture Research Farm in Lexington, Kentucky, and at University of Tennessee East Tennessee AgResearch and Education Center in Knoxville, Tennessee. Solarization will be implemented in 30 x 96 ft high tunnels on each university research farm. We will impose treatments that vary in duration (2 or 4 weeks) and season (April, July, and September) of solarization in two high tunnels in each location – one tunnel will remain open but individual plots will be solarized while the second tunnel will be closed and individual plots will be solarized. Beginning in April 2023, we will bury data loggers at different soil depths (2, 4, and 6 inches) to record soil temperatures. Individual solarization treatments (2- and 4-week time periods) will be examined within plots that are 4 x 10 ft using 6-mil polyethylene clear plastic sheets covering moist, drip irrigated soil. Prior to solarizing, we will eliminate all aboveground plant material inside the high tunnels. This will require weeding, rototilling, and cultivating to make the soil surface inside the high tunnels smooth with few clods or clumps and no visible weeds. The soil will then be drip irrigated to 70% field capacity. Utilizing drip irrigation, rather than sprinklers, will help ensure that the soil surface across the high tunnel is evenly moist. We will measure the soil moisture at different locations in the high tunnels with soil moisture sensors at 2, 4, and  6 inches. The irrigation infrastructure will be removed before tarps are laid down. For each solarization plot, we will dig small trenches along the perimeter, lay the tarp over top of the plot, pound in sod staples to hold the tarp in place, and then cover all edges of the tarp with soil to seal it. There will be four replicates of each treatment and of the non-solarized control.

Non-solarized control plots will also be treated the same as the solarized plots with respect to tillage and irrigation. Because the non-solarized plots (plots without tarps) will be located in a tunnel that will be closed, and therefore, hotter than a tunnel normally would be, we will also have non-solarized plots in the open high tunnel. In the open tunnel, we will have plots that will be tarped/solarized to evaluate if the soil reaches high enough temperatures to manage pests, pathogens and weeds without fully closing the tunnel. By comparing these non-solarized plots between open and closed tunnels, we can quantify the effect of sealing the tunnel but not placing tarps on plots. Individual bed solarization would increase rotation and cropping options throughout the year and may appeal to more growers. The high tunnel air temperatures will be managed normally in the open tunnel. We will repeat these methods in July and September of Year 1 and in April, July, and September of Year 2.

Objective 2

Within the experiments described for Objective 1, located at each university research farm, survivability of five economically important disease agents will be screened. Pathogens will include Sclerotinia sclerotiorum (timber rot, lettuce drop), Fusarium solani (Fusarium crown and foot rot), Athelia rolfsii (formerly Sclerotium, Southern blight), Pythium ultimum (Pythium root rot and damping off), and Rhizoctonia solani (Rhizoctonia crown and root rot). The five soilborne pathogens will be prepared by one of two methods. For fungi that produce sclerotia (Athelia rolfsii, Sclerotinia sclerotiorum, and Rhizoctonia solani), 25 sclerotia will be placed into a mesh packet and buried directly. For fungi that do not produce sclerotia (Fusarium solani and Pythium ultimum), inoculum will be 20 g of infected grain buried in mesh packets. Three replications of each pathogen will be buried in each plot and at each depth. Upon termination of each treatment, pathogens will be analyzed for survival by plating onto culture media in the lab. Sclerotia will be recovered from packets, rinsed in sterile water, and plated onto ¼ acidified PDA plates. Grain-based inoculum will be rinsed and quantified by plating onto selective media and using a predetermined formula based on dry weight of organic matter. Viability of each pathogen will be documented and analyzed against soil temperature and experimental variables.

Objective 3

For the university research farm trials in Years 1 and 2, dominant weed species will be identified at each site before rototilling, cultivating, and solarizing at each timepoint (April, July, and September). Immediately after each solarization treatment, surviving weed density and coverage will be evaluated separately by species. This measurement will allow us to determine if the solarization events kill all weeds present. Weed emergence will be quantified weekly for eight weeks post-solarization (the approximate length of time that lettuce will be grown between ending one solarization season or event and beginning the next) to evaluate the efficacy of the different solarization treatments over the duration of the subsequent lettuce crop. This weekly data collection will involve counting emerged weeds in two 0.25 m² quadrats in each plot, and identifying seedlings to species. This will allow us to determine how long solarization events contribute to weed control and how the timing of weed emergence after solarization may vary by species. Lastly, we will also quantify weeds encroaching from plot edges (i.e., creeping perennials, such as field bindweed, and re-rooting grasses, such as large crabgrass). This data collection will occur in conjunction with insect sampling for Objective 4. Dr. Haramoto will travel to Knoxville annually to assist with weed seedling identification. In year 2, dominant weed species will also be determined at each on-farm site prior to solarization. Immediately after each treatment, surviving weed density and coverage will be determined as at the university research farm trials. Plot photos will be used to document weed establishment after the solarization events.

Objective 4

This work will also be conducted in the university research farm experiments in Years 1 and 2. As in previous objectives, pest populations will be directly and passively monitored before and after solarization. Patches of weeds that recolonize the plots will be sampled for arthropod pests, chiefly aphids, though thrips, whiteflies, and others may be collected. Direct sampling will involve collecting weekly vacuum samples, collecting on two passes each, for the length of the plot. This will be done for 4 weeks following plastic removal in the 2 and 4 week treated areas for a total of 6 weeks of sampling. Vacuum samplers will be adapted from leaf blowers with a reverse suction setting, with fine mesh paint strainers used as collection bags. Bags will be inserted into the vacuum tube attachment and rubber banded in place to secure them and prevent their being sucked into the vacuum unit. Collected arthropod specimens will be preserved in 70% alcohol and identified to family and assessed by time of arrival and population numbers. Passive monitoring will involve weekly collections of three pairs of yellow and blue sticky cards. The card pairs will be placed at each end and in the center of the plots. These will also be deployed for 4 weeks after the removal of plastic in the 2 and 4 week treated areas, for a total of 6 weeks of monitoring. Non-solarized plots will be assessed over the same 6-week period with the same methods. The onset of observable pest symptoms will be noted across the six-week period. The goal of this assessment is to establish if solarization is a mechanical management method that may delay pest colonization of high tunnels and help to eliminate populations of “banker plants and aphids” that may continually re-infest crops. This would guide Extension efforts focused on pest monitoring and diagnostics that are the foundation to sustainable arthropod pest management.

Objective 5

This work will also occur in Years 1 and 2 and the university research farms. After each four-week solarization treatment is completed, we will transplant four to six week old butterhead lettuce in all plots and in both tunnels. The closed tunnel will be opened as it normally would for lettuce production. The lettuce will be evaluated for marketable yield and quality as well as any pest or disease signs and symptoms, comparing production in solarized plots (open and closed tunnel plots) to non-solarized treatment plots. Monitoring of arthropod pests will be accomplished in the same fashion described in Objective 4, with direct sampling using weekly vacuum samples and blue and yellow sticky cards. Onset of pest damage will also be tracked across time. We have chosen to work with lettuce because of its short days to maturity (50-60 days). This will allow us to grow a full crop before the next solarization event when one high tunnel must be closed again. We are using lettuce as an indicator/proof-of-concept crop and understand that many growers would not necessarily choose to grow lettuce in a high tunnel in May or August in our region. This is an artifact of evaluating multiple solarization windows across the seasons in a research setting versus a grower choosing the best time to solarize for their cropping schedule.

Objective 6

Based on the results from the first year of the trials at the university research farms, we will work with the grower cooperators to identify the best option and timeframe to conduct the trials in their high tunnels. Growers will be provided with recommendations on how and when to implement solarization. We will also assist the grower with fully executing the trial. This would include sampling beforehand to identify and quantify the targeted issue (weeds, pests, plant-parasitic nematodes, and/or pathogens), helping install solarization, and sampling at different time points afterward to determine efficacy. The sampling and data collection at these farms will depend on what the growers are dealing with or what issue they specifically are trying to target and what approach they decide to use (whole tunnel or individual beds). Sampling methods will be as similar as possible to the methods described in the research farm trials. Since mesh bags containing pathogens will not be buried in the on-farm plots, pathogens will be quantified pre- and post-solarization using soil recovery and plating methods, or through direct symptom observation, depending on the diseases targeted. Results and outcomes of the on-farm trials will be discussed and shared with growers at the Kentucky Annual Fruit and Vegetable Conference and the Pick TN Conference, as well as field days (UT Steak and Potatoes Field Day and UK Horticulture Research Farm Field Day).

Objective 7

We will perform an overall cost analysis for solarization in HT lettuce production. Because lettuce is a high-value crop, it is a good candidate for soil solarization (Hasing, Motsenbocker, and Monlezun, 2004). We will compare solarization scenarios (2 to 4 weeks, open and closed tunnels) to a no-solarization scenario (baseline scenario) that will be aligned with practices commonly used by TN and KY HT producers for weed and disease control management. Similar to Hasing, Motsenbocker, and Monlezun (2004), all production costs associated with lettuce production will be estimated, including the opportunity cost associated with keeping the tunnel or bed unproductive during the solarization period (2 and 4 weeks). We will perform this same analysis for all farms involved in objective 4, but the baseline scenario would be related to the practices currently used by growers to control weed and disease pressure and the opportunity cost associated with keeping the field unproductive during the solarization period will be estimated with growers depending on the crops on specific time periods. Similar to Hasing, Motsenbocker, and Monlezun (2004), we will estimate break-even yields or the yields required to cover production costs for the no solarization and solarization scenarios. Also, similar to Rysin and Louws (2015), we will estimate the marketable yield required when using solarization to make these treatments as profitable as the baseline scenarios. We will conduct a sensitivity analysis to assess how input and crop prices could impact costs and break-even yields.

Costs will be estimated using information from field trials, a farmer survey, cooperating farmers’ interviews, university budgets, input suppliers, and other secondary information. We will validate cost estimates using grower interviews and expert focus groups. We will conduct a mixed-mode survey (i.e., paper/mail and Web) of KY and TN vegetable producers to gather general production and economics information from their farm businesses. Questions will be formulated in a way that could guarantee farmers’ privacy when summarizing survey results.

The baseline costs estimated using survey data will be validated through farmer interviews and online focus groups with Extension personnel familiar with high tunnel vegetable production. We will conduct in-person interviews with at least three farmers per state. We will use a focus group approach similar to the one used by the Agri Benchmark Network, (Chibanda et al., 2020). We will have a set of four to six participants per focus group. Two focus groups, one per state, will be conducted via Zoom. A standard questionnaire will be used and filled in jointly with the focus group members. A moderator will direct the discussion around the typical farming situation in a typical year. The discussion aims at achieving a consensus for enterprise baseline budgets taking out extreme figures or particularities of these budgets (Chibanda et al., 2020). 

In addition to the proposed analysis described above, similar to Rysin and Louws (2015), we will provide real-life examples of the economic feasibility of adopting solarization in high tunnel production by using a partial budget analysis (Velandia, Galinato, and Wszelaki, 2020) to evaluate changes in costs and revenue, and therefore, net returns when transitioning from the current strategy used by farmers involved in on-farm trials to solarization. A sensitivity analysis will be performed to evaluate how changes in yield, prices, and labor costs could impact the economic feasibility of adopting solarization.

Objective 8

In Year 3, we will host workshops for growers and county Extension agents that will provide hands-on demonstrations on how-to implement solarization. We will discuss the logistics of irrigation and burying the tarp. Trainings will be recorded and posted to YouTube for people who cannot attend or who would like to re-watch. We will also create short step-by-step clips that provide abbreviated instructions on how to implement solarization. This will include tips on solarizing an entire tunnel compared to solarizing individual beds. Factsheets will be created on solarization methods, disease management efficacy, weed management efficacy (which weeds were managed and for how long), and arthropod pest management efficacy. Some of this information will also be included in updates to factsheets that already exist on certain pathogens, pests, and weeds. For example, the University of Kentucky already has a factsheet titled Sclerotinia Diseases of Vegetable Crops (PPFS-VG-29). Details on the efficacy of solarization will be added to the management section of this factsheet. This will help increase the dissemination of our results.