Ecological disinfestation: Physical and biological characterization of filtration substrates for removing zoospores of Phytophthora nicotianae from water.

摘要

Although recycling irrigation water is beneficial to the nursery industry as it serves to decrease potable water usage and reduces the release of excess agrichemicals into the environment, it also poses the risk of becoming the source for and means of dispersing water-borne plant pathogens. Plant pathogens of major concern in irrigation water are species of Phytophthora. These pathogens are a significant threat because they can attack a vast number of plants, many of which are ornamental crops grown in nurseries, by releasing motile zoospores directly into irrigation water. Several treatment methods are available to remove plant pathogens from irrigation water; however, many of these methods are expensive, involve chemicals that are potentially toxic to plants, humans, and environment, and require frequent and time-consuming maintenance. Slow sand filtration is an ecologically based treatment system that only recently has been investigated for treatment of irrigation water at nurseries. Slow sand filtration can be fairly effective, but its application is limited because of the large surface area required and relatively slow flow rates that limit the volume of water treated in a set time period.;The overall goal of this research was to examine four novel substrates, in addition to sand, to determine if any could effectively filter zoospores of Phytophthora nicotianae from waters, with the potential to process large volumes of water, decrease maintenance required for slow sand filtration, or utilize faster flow rates while retaining filtration efficiency. The substrates examined were sand, crushed brick, calcined clay, Kaldnes RTM medium, and polyethylene beads. My objectives were to characterize substrate physical parameters (uniformity coefficient and effective grain size), to determine substrate physical filtration capacity at six substrate depths (0, 5, 10, 20, 40, and 60 cm), to characterize the contribution of microbial components on filtration capacity---including the density of bacteria and dominant species of bacteria colonizing each substrate.;Sand was the most effective physical filter and completely removed zoospores from water at depths of 40 and 60 cm. Sand also was the most uniform substrate and had the smallest effective grain size. KaldnesRTM medium and polyethylene beads were the least effective substrates at physically removing zoospores from water. When a microbial load was allowed to develop in the substrates for 21 days, all substrates removed significantly more zoospores than without a microbial load. Sand and calcined clay stimulated the highest density of microbial growth in the top 5 cm of the column. Sand was the most effective substrate when considering both physical and biological mechanisms for zoospore removal.;Overall, sand remains the most effective filtration substrate for the removal of zoospores of P. nicotianae from water. Crushed brick and calcined clay were less effective but have the potential to be further optimized as alternative filter substrates if substrate depth were increased beyond 60 cm or the substrate were screened to smaller or more uniform grain sizes. KaldnesRTM medium and polyethylene beads were not effective as filtration substrates for removing zoospores of P. nicotianae even though polyethylene beads had the highest increase in removal efficiency with increasing biological load. Upon further optimization and testing with increased flow rates, crushed brick or calcined clay may have potential to be utilized as the foundation of a slow filtration system or constructed wetland module focused on removing plant pathogens from irrigation water.