Date of Graduation

12-2022

Document Type

Thesis

Degree Name

Master of Science in Horticulture (MS)

Degree Level

Graduate

Department

Horticulture

Advisor/Mentor

Dickson, Ryan W.

Committee Member

Rojas, Alejandro

Second Committee Member

Gibson, Kristen E.

Third Committee Member

Bertucci, Matthew B.

Keywords

Horticulture; Hydroponics; Lettuce; Micronutrient; Plant pathology; Pythium

Abstract

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.

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