Date of Award

8-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biosystems Engineering

Committee Member

Daniel R Hitchcock, Committee Chair

Committee Member

Sarah A White, Committee Co-Chair

Committee Member

Christophe J G Darnault

Committee Member

Steven N Jeffers

Committee Member

Tom O Owino

Abstract

Increased incidences and severity of drought have reduced reliable access to freshwater sources for irrigation purposes by nursery and greenhouse plant producers. Many plant producers are now considering onsite remediation and reuse of water captured from irrigation runoff. However, potential contamination of recycled water with plant pathogens, primarily species of Phytophthora, is the primary concern preventing many growers from reusing their water. Species of Phytophthora are capable of infecting thousands of host plants and cause some of the most economically important diseases of nursery and greenhouse crops worldwide. Phytophthora spp. produce motile, swimming zoospores that often serve as propagules of dispersal and often are the primary infective propagules that initial infections on many plants. While many chemical and physical treatment methods are currently used to disinfest recycled irrigation water, there are many drawbacks to using these technologies. Biological methods for managing Phytophthora spp. in waterways, including bioreactors and constructed wetlands, are not as widely implemented and are not well understood.

The overall goal of this dissertation was to assess the potential of passive biological and ecological treatment technologies to remediate Phytophthora spp. from irrigation runoff at nurseries and greenhouses, so treated irrigation runoff may be reused on site. Through a series of greenhouse experiments, we determined that the following plant species may be susceptible to the species of Phytophthora indicated: Carex stricta (P. cinnamomi and P. cryptogea), Panicum virgatum (P. nicotianae), and Typha latifolia (P. cinnamomi, P. cryptogea, and P. nicotianae). Agrostis alba, Iris ensata 'Rising Sun', and Pontederia cordata plants did not appear to be susceptible to the species of Phytophthora tested during this study; therefore, they may be suitable for use in constructed wetland systems. Using a controlled model floating treatment wetland (FTW) system, we determined that FTWs established with Pontederia cordata plants reduced the flow-through of viable Phytophthora nicotianae zoospores as compared to control units containing no FTW at a target hydraulic retention time (HRT) of 4 h. Finally, we determined that laboratory-scale bioreactors containing fir bark reduced flow-through of P. nicotianae viable zoospores as compared to control units that did not contain any substrate, during low and high input nitrogen concentration conditions (11.6 ± 0.3 mg/L N and 72.0 ± 3.7 mg/L N, respectively) and at flowrates equivalent to a target 2 h and 8 h HRT.

These are the first studies to evaluate the efficacy of small-scale FTWs and agricultural bioreactors to manage Phytophthora species in water and some of the only studies to evaluate ecological technologies for plant pathogen remediation at representative field hydraulic conditions. Future studies should investigate the biogeochemical transformations of nutrients and associated microbial communities within ecological remediation systems to gain further insight into the potential of microbiologically aided removal mechanisms. Interdisciplinary approaches such as this one—which involve teams of agricultural engineers, plant pathologists, plant scientists, and hydrologists—will be crucial for future studies seeking to understand the aquatic ecology of plant pathogens and potentially novel ecological methods for remediation. Increased confidence in and implementation of ecological treatment technologies will enable producers of greenhouse and nursery crops to safely, economically, and sustainably remediate runoff and drainage waters onsite so that they are able reuse this water for irrigation purposes. Recycling water will help agricultural producers gain access to a reliable water source at a time when access to surface and ground waters is becoming increasingly scarce and contentious due to overuse and increased incidence and severity of droughts.

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