Micronutrient Concentration Effects on Lettuce Growth and Susceptibility to Pythium

In hydroponic production waterborne pathogens such as Pythium are ubiquitous and continually threaten a wide range of Controlled Environment Agriculture (CEA) crops in hydroponic production, including but not limited to: lettuce, spinach, basil, arugula, cucumber, tomato, sweet pepper, roses, chrysanthemums, and cannabis (Sutton et al., 2006; Gull, 2002; McGehee and Raudales, 2021; Gillespie, 2020). Despite extensive sanitation measures, disease control in hydroponics is fallible and requires constant surveillance and management to minimize outbreaks (Sutton et al., 2006). A potential disease suppression strategy is to increase micronutrient concentrations within hydroponic systems to naturally strengthen plant defenses against pathogens such as Pythium. This thesis combines previous literature and research that looks at the effects of nutrient solution management and Pythium root rot disease on hydroponic lettuce. A series of preliminary studies were conducted to determine the correct Pythium species and strain, environmental parameters, and dosing methods in order to induce disease in hydroponic ‘Rex’ lettuce. These studies found that the Pythium strain and species P. myriotylum ‘PM1` had increased pathogenicity to cultivar ‘Rex’ lettuce and effectively caused root browning at concentrations of 1.80 × 104 oospores per L of solution or greater. Experiments in which preliminary findings were implemented looked at increasing Si and metal micronutrients iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) concentrations above standard hydroponic nutrient formulations for the effects on plant growth and susceptibility to P. myriotylum with hydroponic ‘Rex’ lettuce. Metal micronutrient/Si concentration and Pythium effects were measured with leaf SPAD chlorophyll content, shoot height and width, total plant fresh mass, percent reduction in lettuce growth, and root disease severity. It was found that increasing metal micronutrient and Si concentrations above standard hydroponic formulations resulted in decreased plant growth and yield. Overall, Pythium reduced plant growth and yield, however, increasing metal micronutrient and Si concentrations did not reduce Pythium disease severity compared to the standard solution, except for Cu at 10 mg·L-1. High concentrations of Cu have known fungicide and algaecide effects, however, can also be phytotoxic and reduce plant yield. Ultimately, combining proper sanitation, best management and cultural practices, appropriate hydroponic system design, and implementation of water treatment technologies will be the most effective strategy in controlling waterborne pathogens for hydroponic growers

Micronutrient Concentration Effects on Lettuce Growth and Susceptibility to Pythium

In hydroponic production waterborne pathogens such as Pythium are ubiquitous and continually threaten a wide range of Controlled Environment Agriculture (CEA) crops in hydroponic production, including but not limited to: lettuce, spinach, basil, arugula, cucumber, tomato, sweet pepper, roses, chrysanthemums, and cannabis (Sutton et al., 2006; Gull, 2002; McGehee and Raudales, 2021; Gillespie, 2020). Despite extensive sanitation measures, disease control in hydroponics is fallible and requires constant surveillance and management to minimize outbreaks (Sutton et al., 2006). A potential disease suppression strategy is to increase micronutrient concentrations within hydroponic systems to naturally strengthen plant defenses against pathogens such as Pythium. This thesis combines previous literature and research that looks at the effects of nutrient solution management and Pythium root rot disease on hydroponic lettuce. A series of preliminary studies were conducted to determine the correct Pythium species and strain, environmental parameters, and dosing methods in order to induce disease in hydroponic ‘Rex’ lettuce. These studies found that the Pythium strain and species P. myriotylum ‘PM1` had increased pathogenicity to cultivar ‘Rex’ lettuce and effectively caused root browning at concentrations of 1.80 × 104 oospores per L of solution or greater. Experiments in which preliminary findings were implemented looked at increasing Si and metal micronutrients iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) concentrations above standard hydroponic nutrient formulations for the effects on plant growth and susceptibility to P. myriotylum with hydroponic ‘Rex’ lettuce. Metal micronutrient/Si concentration and Pythium effects were measured with leaf SPAD chlorophyll content, shoot height and width, total plant fresh mass, percent reduction in lettuce growth, and root disease severity. It was found that increasing metal micronutrient and Si concentrations above standard hydroponic formulations resulted in decreased plant growth and yield. Overall, Pythium reduced plant growth and yield, however, increasing metal micronutrient and Si concentrations did not reduce Pythium disease severity compared to the standard solution, except for Cu at 10 mg·L-1. High concentrations of Cu have known fungicide and algaecide effects, however, can also be phytotoxic and reduce plant yield. Ultimately, combining proper sanitation, best management and cultural practices, appropriate hydroponic system design, and implementation of water treatment technologies will be the most effective strategy in controlling waterborne pathogens for hydroponic growers

Metal Micronutrient and Silicon Concentration Effects on Growth and Susceptibility to Pythium Root Rot for Hydroponic Lettuce (<i>Lactuca sativa</i>)

The objectives were 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 experiment, lettuce was grown in hydroponic solutions with metal micronutrients iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) supplied at either 0, 2.5, 5, or 10 mg∙L−1. A standard commercial hydroponic solution was also included as a control, with metal micronutrients supplied at 2 Fe, 1 Mn, 0.5 Cu, and 0.5 Zn mg∙L−1. In the second experiment, hydroponic lettuce was grown with Si at 0, 7, 14, 28, and 56 mg∙L−1. Hydroponic treatment solutions for both experiments were either dosed with Pythium 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 of this study 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. 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