Evaluation of Substrates to Filter Zoospores of Phytophthora nicotianae From Artificially Infested Water

Competition for fresh water resources will become more intense in coming years, forcing producers of agricultural commodities to find alternative sources of water—such as recycling irrigation runoff. Constructed wetlands have been shown to reduce or remove agrichemicals from runoff water; however, the ability to remove water-borne plant pathogens, particularly species of Phytophthora and Pythium, by constructed wetlands has not been studied. The goal of this project was to evaluate and characterize the effectiveness of five substrates (sand, crushed brick, calcined clay, Kaldnes medium, and polyethylene beads) to filter zoospores of Phytophthora nicotianae from water. To examine the physical filtration capability of each substrate, a suspension of zoospores (10 zoospores/ml) was pumped through PVC columns (5 cm in diameter); six replicate columns were filled with each substrate, and each substrate was tested at six depths: 0, 5, 10, 20, 40, and 60 cm. Samples of the suspension were collected from the inlet and outlet of each column; zoospore densities were quantified by filtering 200-ml aliquots through polycarbonate membranes (47 mm in diameter, 3-µm pores), inverting the membranes onto PARPH-V8 selective medium, and examining plates for colony-forming units 1-3 d after filtration.  Sand was the most effective substrate for removing zoospores; zoospores were not detected in the effluent of this substrate at depths of 40 and 60 cm.  The other four substrates were less effective; densities of zoospores were reduced significantly by some substrates, but zoospores were not eliminated at any depth tested. In the next phase of this project, irrigation runoff from an ornamental plant nursery will be pumped continuously through the columns until a natural film of microbial growth has been established within each substrate.  Zoospore removal will be tested before and after microbial growth to determine the effect of this biological addition on filtration effectiveness. One depth will be used for each substrate based on results from the physical filtration experiment: sand, crushed brick, and calcined clay will be examined at a 10-cm depth and Kaldnes medium and polyethylene beads will be examined at a 60-cm depth. In the future, these substrates will be evaluated for removal of pathogen propagules using subsurface-flow constructed wetlands at a production nursery. If effective, constructed wetlands could provide an ecologically-based pathogen remediation system to facilitate the use of alternative water sources by ornamental plant producers.