A Model System to Evaluate the Potential of Floating Treatment Wetlands to Manage Phytophthora species in Recycled Irrigation Water

Abstract

Highlights A floating treatment wetland (FTW) planted with Pontederia cordata reduced the flow-through of viable zoospores. Bacterial communities associated with FTWs that differed in remediation efficacy were characterized. FTWs have the potential to reduce plant pathogen load in water recycled for agricultural irrigation. Abstract. The increased incidence and severity of drought have reduced reliable access to freshwater irrigation sources for nursery and greenhouse plant producers. Many plant producers are now considering onsite remediation and reuse of water captured from irrigation return flow. However, potential contamination of recycled water with plant pathogens, primarily species of Phytophthora, is the primary concern preventing many growers from reusing their water. Floating treatment wetlands (FTWs) consist of plants established on a buoyant structure that floats on the surface of a water body with roots extended down into the water column. FTWs effectively remediate nutrients in agricultural runoff, but their potential to manage plant pathogens was previously unknown. Therefore, our objective was to investigate the potential efficacy of FTWs to manage species of Phytophthora in recycled irrigation water. Amplicon sequencing was performed to observe how the bacterial communities associated with the roots of plants in the various FTWs differed. The research was conducted using a controlled model system. Each model FTW consisted of a 3-m-long plastic trough that contained no plants, Agrostis alba plants, or Pontederia cordata plants. Nutrient-amended pond water continuously flowed through each trough at a set hydraulic retention time (HRT) of 1 h or 4 h. A standard inoculum concentration of Phytophthora nicotianae zoospores in aqueous suspension was added to influent water entering each trough, and zoospore activity in effluent water leaving each trough at predetermined time intervals was monitored with a baiting bioassay. Results from this study demonstrated that controlled model FTWs containing P. cordata plants effectively reduced the movement of viable P. nicotianae zoospores through a channel of nutrient-amended water at a target HRT of approximately 4 h. Movement of viable zoospores through the troughs was not reduced at the higher flow rate of 1 h HRT or for FTWs containing A. alba plants. Furthermore, the roots of A. alba and P. cordata had enriched bacterial communities that differed from one another. The mechanism by which FTWs containing P. cordata plants reduced zoospore activity was not determined; however, it may be due to interception of zoospores by plant roots, effects from plant root exudates, interactions with the root microbiome, or differences in water quality parameters. This is the first published study evaluating the potential efficacy of floating treatment wetlands to manage plant pathogens in irrigation water. Keywords: Ecological remediation, Horticulture, Microbial community analyses, Pathogens, Water reuse.