Source: UNIVERSITY OF ARKANSAS submitted to NRP
OUTSMARTING FOOD AND WATERBORNE PATHOGENS FOR THE SAFE PRODUCTION OF LEAFY GREENS IN RECIRCULATING HYDROPONIC SYSTEMS
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
COMPLETE
Funding Source
AFRI COMPETITIVE GRANT
Reporting Frequency
Annual
Accession No.
1022864
Grant No.
2020-68008-31559
Cumulative Award Amt.
$299,608.00
Proposal No.
2019-07056
Multistate No.
(N/A)
Project Start Date
Jun 1, 2020
Project End Date
Dec 31, 2024
Grant Year
2020
Program Code
[A1701]- Critical Agricultural Research and Extension: CARE
Recipient Organization
UNIVERSITY OF ARKANSAS
(N/A)
FAYETTEVILLE,AR 72703
Performing Department
Horticulture
Non Technical Summary
Leafy greens are increasingly grown in greenhouses and vertical farming operations using recirculating hydroponic systems. Because all plants essentially share the same hydroponic nutrient solution, the spread of food and waterborne pathogens is a major concern and current problem. The potential for foodborne pathogens to survive in recirculating hydroponic systems and become internalized by plants is not fully understood. Pythium is one of the dominant waterborne plant pathogens reducing crop yields in hydroponics. We propose an integrated research and extension project to develop novel controlled-environment agriculture (CEA) and outreach techniques to "outsmart" food and waterborne pathogens for the safe production of hydroponic leafy greens. Objectives 1 and 2 aim to develop new knowledge regarding food safety and plant disease risks for CEA. Objective 3 evaluates practical and low-cost strategies that allow CEA producers to decrease food and waterborne pathogen risks. Objective 4 focuses on rapid--years 1 to 3 of this project--and long-term application of research results targeting small and large operations. Controlled-environment agriculture production is common in urban and rural areas, and increasing the safety of hydroponically-grown food will provide positive benefits for consumers and public health across a range of communities. Implementing hydroponic management practices that reduce yield losses from Pythium will also increase the profitability and success of CEA producers, benefitting local and regional economies. Considering the national scope and increasing trend for CEA, we anticipate the research and outreach from this project will be critical in progressing the industry forward and building food system resilience.
Animal Health Component
100%
Research Effort Categories
Basic
0%
Applied
100%
Developmental
0%
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
2051430116050%
2051430110150%
Knowledge Area
205 - Plant Management Systems;

Subject Of Investigation
1430 - Greens and leafy vegetables;

Field Of Science
1101 - Virology; 1160 - Pathology;
Goals / Objectives
The long-term goal of this Critical Agricultural Research and Extension project is to decrease human food safety risks and plant disease issues with leafy greens in recirculating hydroponic and controlled-environment agriculture (CEA) systems. The context of this goal is based on (i) increasing consumer demand for safe food products, (ii) recent foodborne disease outbreaks in field-grown leafy greens and vegetables, (iii) recurrent crop failures and nationwide economic losses caused by plant pathogens, (iv) increasing use of CEA and hydroponics for food production, and (v) limited published information and available resources regarding the mitigation of pathogen risks in recirculating hydroponic systems. Objectives for reaching our goal include (i) characterizing food safety risks during production in recirculating hydroponic systems, (ii) evaluating hydroponic nutrient effects on plant disease risk, (iii) evaluating soilless substrates and root zone pH effects on food and waterborne pathogens, and (iv) developing and delivering novel extension outreach materials targeting CEA producers.
Project Methods
PD Dickson's team will design and construct research-scale deep water culture (DWC) hydroponic systems for replicated studies. Systems will be made from PVC and plastic materials. Systems will be recirculating, where a submersible pump will continually pump nutrient solution from the reservoir to the plants with excess solution draining back into the reservoir.Appropriate surrogate (i.e. avirulent) forms of viral and bacterial pathogens (norovirus and L. monocytogenes) will be identified, cultured, and maintained under sterile and controlled-environment laboratory conditions by Co-PD Gibson. These viral and bacterial surrogates include Tulane virus (TuV) and L. innocua (FSL C2-008), respectively. Co-PD Gibson will conduct preliminary work establishing methods to inoculate hydroponic nutrient solutions as well as quantify populations in liquid and leaf tissue samples using both cultural methods and digital droplet polymerase chain reaction (ddPCR) for low concentration experiments.Co-PD Rojas has Pythium aphanidermatum in Soilborne Pathology and Ecology Laboratory (SPEL) culture collection and will obtain new strains of Pythium (P. aphanidermatum) by sampling hydroponic nutrient solutions collected from local hydroponic operations in Arkansas. Pythium will be isolated on semi-selective culture media Corn Meal Agar (CMA) amended with antibiotics. Preliminary work will consist of establishing methods to inoculate hydroponic nutrient solutions and quantify Pythium inoculum using hemocytometer and/or CFUs per milliliter. Preliminary work will also consist of establishing protocols to identify Pythium infection and life-cycle components at different stages of the production.Lettuce (Lactuca sativa L.) will be used as a model crop species, and green- and red-leaf lettuce cultivars will be evaluated since both are used in commercial hydroponic head and baby leaf production. Cultivars will be selected based on recommendations from the stakeholder advisory group.Objective 1: Characterize food safety risks during production in recirculating hydroponic systems (PD Dickson and Co-PD Gibson). Characterization of food safety risks will consist of conducting replicated greenhouse experiments to measure TuV and L. innocua--surrogates for hNoV and L. monocytogenes, respectively--internalization by plants in DWC systems. As proof-of-concept with green-leaf lettuce, the first experiment will consist of pathogen concentrations in the hydroponic nutrient solution initially set at 105 plaque forming units (PFU)/mL and 107 colony forming units (CFU)/mL for TuV and L. innocua, respectively. Following these initial levels, a range of concentrations for TuV and L. innocua will be investigated to represent sequentially lower contamination levels based on the peer-reviewed literature and detection limits of the assays. Additional follow-up experiments will evaluate the effects of a "contamination event" as well as different lettuce cultivars.Objective 2: Evaluate hydroponic nutrient effects on plant disease risk (PD Dickson and Co-PD Rojas). Growth chamber experiments will be conducted to evaluate the effects of applied micronutrient concentration on infection of green-leaf lettuce roots with Pythium (P. aphanidermatum). Metal micronutrients iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) as well as boron (B) will be evaluated because of previously reported involvement in disease suppression for agronomic plant species. Individual micronutrients will be adjusted to 0.5×, 1×, 2×, and 4× the recommended concentration in a modified Hoagland's nutrient solution--a standard hydroponic nutrient solution recipe--plus a zero nutrient control. Non-destructive plant growth measurements will consist of measuring canopy height and width, leaf chlorophyll concentration using a Minolta leaf SPAD meter, and root area, root mass, and lesions (necrosis) from Assess (APS, St. Paul Minnesota, USA) root image analysis. Response curve, time course, and survival analyses will be used to evaluate individual nutrient concentration effects on Pythium infection and the amount of time required until infection reduces crop yield. In a follow-up experiment, silicon (Si) will be added to the nutrient solution at 0, 0.75, 1.5, 3, 6 mg/L Si (0, 0.25×, 0.5×, 1×, and 2×).Objective 3: Evaluate soilless substrate and pH effects on food and waterborne pathogens (PD Dickson, Co-PDs Gibson and Rojas). Greenhouse experiments will evaluate different types of soilless substrate and nutrient solution pH for effects on plant internalization of the same foodborne pathogens evaluated in Objective 1 and infection from the same Pythium species used in Objective 2. For the substrate experiments, green-leaf lettuce will be grown in potted containers holding different soilless substrate types as proof-of-concept to determine if certain organic substrates minimize pathogen risk. Soilless substrate treatments will consist of a coarse Canadian sphagnum peat with long fibers and little dust (2-3 on the Von Post scale), coconut coir fiber from Sri Lanka, loblolly pine (Pinus taeda L.) wood fiber, and sterilized mineral rockwool (Grodan, The Netherlands). Separate greenhouse experiments will evaluate the effects of hydroponic nutrient solution pH on plant internalization of TuV and L. innocua and pathogenicity from Pythium.Objective 4: Develop and deliver novel extension outreach materials targeting CEA producers (PD Dickson and Co-PDs Gibson and Rojas). We plan to rapidly deliver research findings from objectives 1-3 in years 1 through 3 of this project as well as promote long-term and much needed extension support to meet critical CEA needs in the future.Outreach efforts will start in year 1 with development of a novel program, called Safe Grow, designed to (i) provide testing services and educational resources to individual CEA producers nationwide and (ii) establish critical food safety and disease risk benchmarks for the CEA industry. The Safe Grow program will allow CEA producers to submit samples of plant tissue, hydroponic nutrient solutions, and raw irrigation water to the University of Arkansas to screen for food and waterborne pathogens and test for nutrient levels and water quality. Evaluating nutrient solution and water quality issues has not yet been a focus of this project, but are still common problems in hydroponic production. Each participant will receive an individualized food safety/disease risk/water quality and nutrient report based the samples they submitted, which will also include research updates from Objectives 1-3 and a management decision-support tool with recommended strategies to minimize problems. Results from all participants will be pooled in a blind survey and analyzed, and each participant will receive a CEA benchmark report where they can compare their results with other anonymous participants. Safe Grow methods and protocols will be submitted for approval by the Internal Review Board (IRB) prior to conducting the program. The Safe Grow program will be marketed using e-GRO and via social media, advertisements in industry trade magazines, and with the help of extension colleagues at the University of Arkansas and other universities. A website and link will be set up through the University of Arkansas for participant registration and processing of information.We will utilize the resources available through eXtension to form a new CoP specific to CEA and Specialty Crop Production. We will establish the new CoP and WordPress website and recruit members of the extension community. Recruitment of members will be both targeted and open, meaning we will specifically reach out to extension specialists and researchers with established expertise in CEA as well as promote the CoP via the four regional centers for food safety training, outreach, and technical assistance.

Progress 06/01/23 to 05/31/24

Outputs
Target Audience:The primary target audience is hydroponic and soilless greenhouse growers producing leafy greens and herbs crops. Changes/Problems:This project received a no cost extension to provide more time to meet the outreach and delivery objectives. The next report will be the final report. What opportunities for training and professional development has the project provided? Nothing Reported How have the results been disseminated to communities of interest? Nothing Reported What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? Novel outreach and deliverables were provided at an annual Root Alliance (rootalliance.org) stakeholder meeting in Oceanside, CA. Audience members included professional hydroponic growers, soilless substrate and fertilizer companies, biocontrol and pest management companies, researchers (university, USDA, private sector), and university students (undergraduate and graduate).

Publications


    Progress 06/01/22 to 05/31/23

    Outputs
    Target Audience:The target audience for this research is primarily producers of hydroponic leafy greens in greenhouse and indoor/vertical farming systems as well as allied horticultural companies. Allied horticultural companies include but are not limited to distributors of hydroponic supplies and equipment, soilless substrate manufacturers, fertilizer companies, plant breeders, consultants working in hydroponics, and companies producing horticultural softwares and related technologies. Other intended audiences include students, researchers, and extension specialists working in hydroponics, soilless culture, and controlled-environment agriculture. Changes/Problems:We have been delayed on implementing the Safe Gro survey, collecting samples, and analyzing these samples. In addition, we have minor work to still complete related to the investigation of human pathogen risk and risks based on substrate type. Initial delays in starting and gaining momentum were a result of this grant being awarded in the peak of the COVID-19 pandemic, and although we have overcome these and some challenges in the research, more recent delays have been caused by personnel hiring challenges, supply chain issues, and unexpected turnovers in personnel and students (PD Dickson) within the last year. In addition, Co-PD Rojas left the University of Arkansas in 2023 for employment at Michigan State University, requiring PD Dickson's lab to take on additional responsibilities under short staffing, which created challenges in advancing the project to completion. We are requesting a second NCE for 6 months (until December 31, 2024) to complete our ongoing work, and continued access to the remaining funds will enable completion of this work. In the request we have outlined a new timetable to completion over the next six months. The Safe Gro Survey and remaining projects are near ready to implement and execute, and the staffing issues that prevented progress have been resolved as April 2024. We are confident we can acheive completion within the next 6 months. What opportunities for training and professional development has the project provided? Nothing Reported How have the results been disseminated to communities of interest?Research results have been presented at scientific meetings for the American Society for Horticultural Sciences, American Pathological Society, and International Association for Food Protection. They have also been presented through grower meetings, articles, and field days including the Horticulture Industry Show (2022), e-GRO (2021 through 2023), and the Root Health Symposium (Gainesville, FL 2024). What do you plan to do during the next reporting period to accomplish the goals?We are currently requesting a second NCE for 6 months (until December 31, 2024) to complete our ongoing work. Specifically, we are still aiming to implement the Safe Gro survey, collect samples, and analyze these samples. In addition, work related to the investigation of human pathogen risk in recirculating DWC systems as well as risk based on substrate type will be fully completed by December 2024. Continued access to the remaining funds will enable completion of this work. For context, we were awarded this grant in June 2020 in the peak of the COVID-19 pandemic and were delayed in starting and gaining momentum on numerous aspects of this project due to personnel hiring challenges, supply chain issues, etc. Co-PD Rojas moved from the University of Arkansas to Michigan State University in 2023. In addition, PD Dickson has had unexpected turnovers in personnel within the last year which has also created challenges in advancing the project to completion.

    Impacts
    What was accomplished under these goals? Hydroponic systems are "closed loop" systems, meaning the hydroponic solution (fertilizer nutrients + water) is reused and recirculated continuously among all plants. The problem being addressed is the recirculated solution poses a contamination risk for human and plant diseases. Co-PD Gibson has investigated the persistence of a human norovirus surrogate, (Tulane virus, TV), Salmonella Javiana, Listeria monocytogenes, and Listeria innocua in recirculating hydroponic solutions as a function of temperature (15°C, 25°C, 30°C, and 37°C) and time (over 21 day period, typical crop time). In completing this work, the internalization of these pathogens into the root and shoot tissues of lettuce was measured. Mixed models were used to determine the effects of temperature and time (day) on pathogen concentration in solution and tissues. In addition, decimal reduction values (D-values) were calculated using the slope from linear regressions by plotting each data point from each temperature. D-values indicate the time needed to reach 90% reduction in virus or bacterial loads (in days) in solution for each temperature. Infectious TV persisted in nutrient solution throughout the 21-day study period at 15°C, 25°C, and 30°C. Meanwhile, TV was not detected after day 5 in nutrient solution at 37°C. Similarly, decimal reduction values (D-values) of TV at 15°C, 25°C, 30°C, and 37°C were 48.0, 11.3, 8.57, and 7.02 days, respectively. At all temperatures, S. Javiana persisted in solution throughout the 21-day study period. However, L. monocytogenes and L. innocua persisted for only 3 to 4 d and 7 to 8 d, respectively. Similarly, decimal reduction values (D-values) of S. Javiana indicate longer persistence than L. innocua and L. monocytogenes at all temperatures. For instance, at 15°C and 25°C, D-values for S. Javiana were estimated at 82 and 26 d, respectively, compared to D-values of 3.6 and ~3 d for L. monocytogenes. The initial concentrations of S. Javiana and L. monocytogenes were 4.10 and 4.62 log CFU/g in roots, 3.15 and 4.19 log CFU/g in rockwool, and 5.86 and 6.66 CFU/ml in solution, respectively. S. Javiana persisted in roots, rockwool, and NS throughout the 21 sampling days with a log reduction of 2.85 log CFU/g, 3.66 log CFU/g, and ≥5.50 CFU/ml, respectively. L. monocytogenes population declined to <1 log CFU/g in roots, rockwool, and NS between 7 to 14 days. No pathogens were recovered from the edible portion of the lettuce. Pathogen populations in rockwool, roots, and solution were positively associated across sample type (R=0.78 to 0.96). Weak to moderate (R=-0.18 to -0.46) and weak (R=-0.02 to -0.12) associations with pH and temperature, respectively, were calculated for pathogen populations. PD Dickson and Co-PD Rojas investigated the virulence of different species and strains of Pythium (water-borne plant pathogen) in the hydroponic solution and with lettuce. Eighteen isolates of P. aphanidermatum, P. dissoticum, and P. myriotylum were screened for pathogenicity with 'Rex' lettuce (butterhead variety) and tested at concentrations ranging up to 105 oospores per L of hydroponic solution. Overall, 'Rex' lettuce showed tolerance to most P. aphanidermatum and P. dissoticum isolates but was susceptible to P. myriotyllum isolated from commercial hydroponic lettuce by researchers at the University of Connecticut, and therefore P. myriotylum was used for subsequent testing. Studies were then conducted to evaluate the effects of increasing metal micronutrient concentrations and silicon (Si) concentrations on plant growth and susceptibility to Pythium root rot with hydroponically grown lettuce (Lactuca sativa). In the first study, 'Rex' lettuce was grown in recirculating hydroponic solutions with metal micronutrients iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) supplied at concentrations ranging from 0 to 10 mg?L−1. A standard and commonly used commercial hydroponic solution was included as a control in all experiments, with metal micronutrients supplied at 2 Fe, 1 Mn, 0.5 Cu, and 0.5 Zn mg?L−1. In the second study, hydroponic 'Rex' lettuce was grown with Si at 0, 7, 14, 28, and 56 mg?L−1. Hydroponic treatment solutions for replicate systems in both studies were either dosed with P. myriotylum (Pythium treatment) at 1.80 × 104 oospores per L or deionized water as a non-Pythium control. Data were collected on leaf SPAD chlorophyll content, shoot height and width, total plant fresh mass, and root disease severity. Increasing the Cu concentration in solution decreased Pythium disease severity but reduced lettuce growth and yield. Increasing the concentration of the other metal micronutrients also tended to reduce lettuce growth but had no significant influence on root disease. Supplementing the hydroponic solution with Si had no effect on Pythium root disease severity and slightly decreased lettuce growth at 56 mg?L−1 Si. Results suggest that the management of micronutrients and Si nutrition is not an effective strategy and, at best, a risky strategy for controlling Pythium in hydroponic lettuce. Based on these results and those from other researchers, we concluded that growers would likely benefit from maintaining metal micronutrient and Si concentrations within the ranges of (in mg?L−1) 0.5 to 5.5 for Fe, 0.1 to 2.0 for Mn, 0.1 to 0.6 for Cu, 0.1 to 0.6 for Zn, and 0 to 28 for Si for many hydroponic crops. Supplementing Si has the potential to negatively influence plant growth and quality for certain plant species, and testing is necessary to evaluate phytotoxicity risks prior to implementing in commercial practice. Overall, successful mitigation of root rot pathogens in commercial hydroponic production requires the combination of proper sanitation, best management and cultural practices, appropriate hydroponic system design, and the implementation of a water treatment system with proper design and a multi-barrier approach.

    Publications

    • Type: Journal Articles Status: Under Review Year Published: 2024 Citation: Dickson, R.W., M.E. Humphrey, S. Padhye, J.B. Tebow, and L.M. Machesney. Quantifying temperature effects on crop timing and quality for compact container-grown pepper. Submitted to HortScience, in review.
    • Type: Journal Articles Status: Published Year Published: 2023 Citation: Dhulappanavar, G. R., & Gibson, K. E. (2023). Persistence of Salmonella enterica subsp. enterica ser. Javiana, Listeria monocytogenes, and Listeria innocua in hydroponic nutrient solution. Journal of Food Protection, 86(10), 100154.
    • Type: Journal Articles Status: Published Year Published: 2024 Citation: Dhulappanavar, G. R., & Gibson, K. E. (2024). Hydroponic nutrient solution temperature impacts Tulane virus persistence over time. Food and Environmental Virology, 1-8.


    Progress 06/01/21 to 05/31/22

    Outputs
    Target Audience:The target audience for this research is primarily producers of hydroponic leafy greens in greenhouse and indoor/vertical farming systems as well as allied horticultural companies. Allied horticultural companies include but are not limited to distributors of hydroponic supplies and equipment, soilless substrate manufacturers, fertilizer companies, plant breeders, consultants working in hydroponics, and companies producing horticultural softwares and related technologies. Other intended audiences include students, researchers, and extension specialists working in hydroponics, soilless culture, and controlled-environment agriculture. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?This project opened training and development opportunities for graduate students and staff within the research group. Students and staff participated at the Horticulture Industries Show in various educational sessions on plant disease, food safety, and horticultural topics. Students also enrolled in a specialty hydroponics course offered by the University of Florida Institute of Food and Agricultural Sciences and leading hydroponic research and extension specialists. How have the results been disseminated to communities of interest?Results have been disseminated to hydroponic growers, representatives from allied horticultural companies, researchers, extension specialists, and students at the Horticulture Industries Show in Fayetteville, AR, in 2022. In addition, results have been disseminated to research scientists and extension specialists through peer-reviewed scientific publications and also to industry stakeholders through e-GRO Alerts and trade publications. What do you plan to do during the next reporting period to accomplish the goals? Our research and dissemination plants for the next reporting period are summarized as follows: Evaluate effects of soilless substrate on mitigation of Pythium and various foodborne pathogens Evaluate the effects of hydroponic solution temperature, pH, and electrical conductivity on foodborne pathogen persistence in hydroponic systems. Finish the SafeGrow Survey of water quality at commercial grower locations Planning a major field day plus a webinar for CEA stakeholders Finish publications and outreach objectives, including the SafeGrow final report.

    Impacts
    What was accomplished under these goals? Several key research accomplishments were achieved during this period and are summarized. We found in hydroponic culture that increasing the concentrations of micronutrients supplied in solution did not increase plant resistance to Pythium root rot, and in some cases reduced plant growth and yield. The exception was with copper, where increasing copper concentration decreased Pythium disease severity and mitigated yield losses. However, the copper concentrations needed in solution to reduce root disease also significantly reduce lettuce yield, likely a result of copper toxicity. Omitting micronutrients from solution had either a negative effect or no effect on plant yield and resistance to Pythium depending on the micronutrient. In scenarios where omitting a micronutrient had no effect on yield, it was likely that enough leached into solution from the hydroponic equipment. Overall, we determined that the optimal concentrations of micronutrients were within many of the common recommended ranges for growers, ranging from about 0.15 to 2.5 ppm depending on micronutrient. In contrast to recently published research from other researchers, we do not recommend increasing micronutrient concentrations above 1 ppm for manganese, zinc, and copper and above 2.5 pom for iron; our results indicate this would cause minimal to no reduction in Pythium disease risks, may even increase plant susceptibility to Pythium, and would likely reduce yield. Supplementing the hydroponic solution with silicon (Si) did not increase lettuce resistance to Pythium root rot, and at the highest level of soluble Si (56 ppm Si) there was a slight reduction in growth compared to no Si. Lettuce was also found to be a Si "non-accumulator" species, which means little Si is taken up into shoot tissues (Si represents <1% of shoot dry tissue). Although lettuce a Si "non-accumulator" there is evidence and data to support Si alleviates metal micronutrient toxicity in lettuce, and therefore supplementing the hydroponic solution with up to 14 ppm Si is still recommended for lettuce if it is not cost prohibitive. Survival and persistence of foodborne pathogens varied in hydroponic systems and depended on pathogen. Salmonella javiana and Listeria innocua persisted for >21 days in hydroponic solutions kept at 25 degrees Celsius, whereas Listeria monocytogenes persisted for only 5 days. These results indicate a potentially greater food safety risk with the former pathogens since longevity in hydroponic solutions was longer following a contamination event.

    Publications

    • Type: Journal Articles Status: Accepted Year Published: 2021 Citation: Tebow, J.B., L.L. Houston, and R.W. Dickson. 2021. Silicon foliar spray and substrate drench effects on plant growth, morphology, and resistance to wilting in container-grown edible species. Horticulturae. 7 (9), 263.
    • Type: Journal Articles Status: Submitted Year Published: 2022 Citation: Houston, L.L., Dickson, R.W., Bertucci, M.B., Trenton, R.L. Evaluation of species-specific replenishment solutions for hydroponically grown arugula and basil. Horticulturae.
    • Type: Journal Articles Status: Submitted Year Published: 2022 Citation: Helms, K.M., Dickson, R.W., Rojas, A.J., Gibson, K.E. Micronutrient and silicon concentration effects on plant growth and resistance to Pythium with hydroponic lettuce. Horticulture.
    • Type: Journal Articles Status: Submitted Year Published: 2022 Citation: Dickson, R.W. Evaluating average daily temperature and daily light integral effects on plant growth rate and quality with container-grown fruiting vegetables.
    • Type: Journal Articles Status: Accepted Year Published: 2022 Citation: DeGenring, L., R.W. Dickson, and A. Poleatewich. Inhibition of Botrytis cinerea growth and suppression of grey mold on petunia leaves using chitosan. 2022. Plant Disease. Accepted, in press.
    • Type: Journal Articles Status: Accepted Year Published: 2022 Citation: Harrison, D., M. de Oliveira, C. Wu, L. Florez-Palacios, A. Acuna, M. da Silva, F. Ravelombola, J. Winter, K. Brye, R.W. Dickson, A. Rojas, P. Chen, H. Nguyen, and L. Mozzoni. 2022. Developing a high-throughput method to screen soybean germplasm for hypoxia tolerance in a hydroponic system. Crop Science. DOI: https://doi.org/10.1002/csc2.20674
    • Type: Journal Articles Status: Accepted Year Published: 2022 Citation: Estepp, C.M., R.W. Dickson, and D.M. Johnson. 2022. Rapport, course technology, and self-regulated learning as predictors of student satisfaction in an online horticulture program. NACTA. Volume 66; pg 9-17.
    • Type: Journal Articles Status: Accepted Year Published: 2022 Citation: Dickson, R.D., K.M. Helms, B.E. Jackson, L.M. Machesney, and J.A. Lee. 2022. Evaluation of peat blended with pine wood components for effects on substrate physical properties, nitrogen immobilization, and growth of petunia (Petunia x hybrid Vilm.-Andr.). HortScience. DOI: https://doi.org/10.21273/HORTSCI16177-21
    • Type: Journal Articles Status: Accepted Year Published: 2021 Citation: Dickson, R.W., J.B. Ebba, P.R. Fisher, C.H. Harris, and T.R. Guerdat. 2021. Fertilizer and plant growth regulator strategies to improve consumer success with containerized plants. HortTechnology. 31 (3), 304-314.


    Progress 06/01/20 to 05/31/21

    Outputs
    Target Audience:The target audiences reached by efforts during this reporting period include but the following. Commercial producers of edible food crops in hydroponic and soilless substrate culture systems. Production can be in greenhouses, high tunnels, indoor vertical farms, and other types of protected and controlled-environment agriculture. Members of the scientific community conducting applied research for protected and controlled-environment agriculture. Members may include researchers, professors, technical staff, post-docs, graduate and undergraduate students, and other members of universities and/or research institutes. Allied horticulture companies associated with crop production in protected and controlled environments. These include manufacturers and suppliers of electric lighting technologies, soilless substrates, fertilizers, agrichemicals, various plastic products, greenhouses, high tunnels, vertical farming units, and plant breeders. University students enrolled in coursework related to horticulture and crop production in protected and controlled environments. Courses include Greenhouse Management and Operation as well as Introduction to Hydroponic and Soilless Food Crop Production. Changes/Problems:The most significant and disruptive problems this past period either resulted from or were related to COVID-19. Like all universities, the University of Arkansas implemented strict social distancing guidelines and restrictive access to campus during 2020. The PI and Co-PIs on this project all had to immediately transition from in-person to remote course lectures and labs with minimal notice, and increase teaching efforts significantly to learn new online platforms and accommodate students. In addition, we had reoccurring student worker absences due to COVID-19 exposures, COVID-19 positive tests, and quarantining. In addition, the lead graduate student on this project had a serious health issue and underwent brain surgery (complicated by COVID-19), and was out of office for parts of summer and fall 2020. In addition to health concerns, there were significant delays in shipping of plastics and materials needed to build hydroponic systems, which we were told by suppliers that shipping delays were also related to COVID-19 shutdowns. Despite some delays from COVID-19, the research team feels confident we can overcome any setbacks from 2020 during the next reporting period, and goals for future reporting periods should not be impacted negatively. Other challenges/problems with experimentation were briefly discussed in the previous section (i.e. managing solution temperature, plant age at inoculation, and proper identification of pathogens strains and surrogates), and have been overcome with additional engineering of hydroponic systems, pilot testing, and fine-tuning of the methods and experimental approach. New approaches include conducting experiments at warm temperatures (30C), inoculating hydroponic systems with pathogens at 14 days after germination, using appropriate surrogates and Pythium strains, and evaluating plants in new leak-free (sealant-free) hydroponic systems. While conducting preliminary studies during the last reporting period, the research team has identified a severe lack in standardized operating procedures (SOPs) on how to effectively screen and evaluate hydroponic leafy greens crops for pathogen resistance, which has been confirmed by some of our industry research partners. During the next reporting period, we will discuss the feasibility of adding a research output that consists of protocols and methodology to develop reliable screening SOPs for use by other university researchers, plant breeders, and hydroponic R&D companies. What opportunities for training and professional development has the project provided? Nothing Reported How have the results been disseminated to communities of interest? Nothing Reported What do you plan to do during the next reporting period to accomplish the goals? Conduct experiments focusing on nutrient concentrations (micronutrients and silicon) and substrate effects on Pythium root infection and uptake of food-borne pathogens. A major issue has been getting repeatable Pythium infection and identifying a virulent Pythium strain. We will therefore grow lettuce at high temperatures (30C), inoculate seedlings at 14 days, and use the most virulent strain we have been able to isolate. Another challenge during preliminary experiments was designing a reliably leak-free hydroponic system, which is also common in commercial production. We custom-ordered special rubber gaskets that would eliminate the need for sealants and glue, for which the degradation was a common cause of leaking over time. We will use our newly designed leak-free recirculating systems. PI Dickson and Co-PI Gibson will present on research from this past reporting period and the next reporting period at the Horticulture Industry Show in Fayetteville, AR. The whole research team will execute outreach activities including the SafeGrow survey, workshops, and initiation of the eXtension group. A challenge and opportunity identified during the last period is a lack of standardized and published protocols on screening hydroponic leafy greens for Pythium resistance and resistance to food safety risks. Therefore, the research team plans to address this problem by setting up standard operating procedures supported by data collected during this period for future publication. Considering the importance as well as the inherent difficulties in screening for Pythium and food-borne pathogen resistance, the research team believes this could have a large impact for both the scientific and commercial communities.

    Impacts
    What was accomplished under these goals? In order to meet our research objectives and goals, our first accomplishment was to effectively design and test research-scale hydroponic systems that are both recirculating and deep water culture, mimicking the larger analogs used by commercial hydroponic operations for growing leafy greens (PI Dickson). Our research partners collaborated and gave input to the PI and Co-PIs on the system design. In the meantime, strains of Pythium causing root disease were isolated from commercial operations and established in the laboratory (PI Dickson and Co-PI Rojas). Both graduate and undergraduate students have been trained in techniques related to Pythium strain isolation, deep freeze storage, initiating sporulation, and the development of concentrated spore solutions used to inoculate hydroponic systems during experiments. In addition, we have identified the proper food-borne pathogen surrogates (non-pathogenic to humans) for characterizing food safety risks in hydroponics (Co-PI Gibson), including surrogates for common viruses (human norovirus) and bacteria. For Pythium and water-borne pathogens, preliminary experiments were conducted to (i) identify the appropriate and most virulent isolate for experimentation, (ii) determine the proper plant age and spore concentration for inoculating hydroponic systems with Pythium, (iii) determine whether Pythium should be added to the substrate or directly to the hydroponic nutrient solution, (iv) determine the persistence of Pythium spores in hydroponic systems, (v) determine the effects of nutrient solution and air temperature on disease severity/pathogen risk, and (vi) practice measuring the severity of Pythium lesions using digital imaging software and visual analyses. For food-borne pathogens, preliminary experiments were conducted to achieve the same goals as for Pythium and water-borne pathogens, with the exception of measuring lesions on plant roots. Instead, protocols are currently being tested for the quantification of food borne pathogens in plant leaf tissues. We have learned that Pythium spores and food-borne pathogen surrogates can survive in hydroponic nutrient solutions; however, conditions must be optimal for the pathogen to infect the crop. For example, higher temperatures above 27 degrees Celsius favor Pythium infection of roots, particularly for warm-weather Pythium species, partially because Pythium performs well at higher temperatures and plants are under a greater degree of temperature stress. Young lettuce plants at the 2-4 leaf stage are more susceptible to Pythium infection compared to lettuce with established heads and large root systems. This is because (i) young plants are growing more slowly until more leaves develop, (ii) root systems are small and can be overwhelmed by high pathogen loads, and (iii) there is simply more time in the crop life cycle for Pythium to infect roots and cause yield and growth losses. In contrast, we have seen that larger lettuce plants are naturally more resistant to Pythium infection, likely because plants are growing at more rapid rates and simply "outgrow" the pathogen, particularly when spore concentrations are low in solution. We have found that seedlings between 7 and 14 days after germination are more susceptible to Pythium compared to seedlings that are 14 days old or greater. We have also found that lettuce is particularly sensitive to Pythium at greater than 27 degrees Celsius, and that disease occurrence is significantly reduced at lower temperatures despite high spore concentrations in solution. Our trials indicate that it is possible to dip seedling trays in Pythium spore solutions to provide inoculation as opposed to adding spore solution directly to the recirculating nutrient solution, which may serve as a more practical research method for future studies and experimentation. We are still analyzing similar trials for food borne pathogens, but preliminary results suggest higher temperatures may create more of a food safety risk for hydroponic leafy greens, because plants are stressed at higher temperatures and also transpiring nutrient solution at more rapid rates (one of the main ways plants internalize food-borne pathogens). A food-borne pathogen contamination event would likely create a higher food safety risk when plants are nearer to maturity, because of the greater leaf area per plant and higher demands for water/nutrient solution uptake. Most activities to meet these goals will be concluded during the next two reporting periods, and therefore more detailed accomplishments can be expected in future reports.

    Publications