Filtration to Eliminate Phytophthora spp. from Recirculating Water Systems in Commercial Nurseries

Three filtration systems using slow sand filtration (SSF) and one using lava grain filtration (LGF) were tested for their ability to eliminate Phytophthora spp. from recycled water in commercial ornamental nurseries over four years. Samples were taken in May, August, and October of each year. Phytophthora spp. could not be detected in any of the filter effluents when filters were operating normally. Water stored in clean water reservoirs that were filled only with filtered water and were well protected from contamination via soil and air remained free of Phytophthora. All four filtration systems produced sufficient quantities of water for nursery production. The maximum annual quantity of water demanded by the nurseries ranged from 30,000 to 163,000 m3. The cost for 1 m3 filtered water was lowest for slow sand filtration compared with lava grain filtration. Recommendations for designing systems suitable for large ornamental nurseries with open air recycling systems are discussed. Introduction Phytophthora species are one of the most important groups of plant pathogens able to be spread with contaminated water (1,20,21). Many Phytophthora species, e.g., P. cactorum, P. cinnamomi, and the quarantine organism, P. ramorum, have a wide host range while others like P. alni (alder decline) and the quarantine organism P. fragariae var. fragariae (red core of strawberries) are specialized on single-host plants. Phytophthora species need water for the development of sporangia and motile zoospores, which are of great importance for plant infection. Epidemiological studies in commercial nurseries showed that Phytophthora species can be present in the water recirculation systems during all seasons (5,13,17,19). Thus, effective methods to decontaminate irrigation water in commercial nurseries are essential. Chemical treatment of the water in open air recycling systems is not allowed in all countries. Furthermore, chemical treatment might kill beneficial organisms. Other methods, like UV radiation, may not be successful with water containing suspended particulates as is normally found in open air water recycling systems. In German nurseries, large water reservoirs very often provide habitat for water plants and animals like birds and fishes. Therefore, environmentally acceptable methods like filtration systems should be considered for use. Filtration systems have been tested successfully for water decontamination in horticultural glasshouse production (15,22). Information for container grown, woody ornamentals in outdoor production exists only from trials in small, pilot scale filtration systems (18). There is no scientific information on their efficiency with large water volumes open air water recirculation systems in commercial nurseries. Therefore, the aim of this study was to test the ability of two different filtration systems — slow 14 March 2008 Plant Health Progress sand filtration (SSF) and lava grain filtration (LGF) — to eliminate Phytophthora spp. from recycled water under commercial conditions. Nurseries and Water Recirculation Systems The four nurseries involved in the studies had container areas between 5 and 15 ha (Table 1). The range of plants produced at each nursery varied from having great diversity to a high degree of specialization. Rhododendron and ornamental conifers were the most important plants. All four nurseries produced container plants on open air growing areas. The surplus water was collected from container blocks by drainage systems that lead the water to retention basins (Fig. 1). In all nurseries, these retention basins were open air ponds of different storage capacities. Water was taken from these ponds for irrigation and also for frost protection by irrigation. If water became limited, the retention basins were supplemented with water from wells. In all four nurseries, the water from the retention basins was pumped directly onto the surface of the filters (Fig. 1). Table 1. Characterization of the production areas and the water recirculation systems of four commercial ornamental nurseries using either slow sand filtration or lava grain filtration used in this study. Nursery SSF 1 SSF 2 SSF 3 LGF Production area (ha) 11 5 5 15 Irrigation system sprinkler sprinkler booms sprinkler booms drip irrigation sprinkler Drainage system horizontal slope horizontal slope horizontal slope vertical Retention reservoir Construction in-ground in-ground in-ground above-ground Design earthen basin ground of dense clay earthen basin ground of dense clay earthen basin ground of dense clay earthen basin ground covered with geomembrane liner Size (m3) 3,000 30,000 1,500 (since 2005: 2,000) 25,000 Fig. 1. Diagram of a water recycling system in container nurseries. 14 March 2008 Plant Health Progress Filtration Systems Two filtration methods were tested: SSF (9) and LGF (11). SSF was tested in three nurseries designated SSF 1, SSF 2, and SSF 3. The SSF systems consisted of graded gravel at the bottom (under-drain) and a fine-grained sand layer as filter medium at the top (Fig. 2). The water flowed slowly (≤ 0.3 m/h) through the layers and was collected as soon as it passed through the bottom layer. A layer of beneficial microorganisms developed at the sand/water interface at the top of the filter system. SSF 1 and SSF 3 the filtration systems were constructed in the ground and were not covered. These two systems had filter surfaces of 324 and 100 m2 and a filtration capacity of 90 and 30 m3/h, respectively (Table 2). SSF 3 combined a part of a former retention reservoir that was modified in 2006 to have a new clean water reservoir. The SSF 2 filtration system was constructed using a covered steel tank. It had a filter surface of 260 m2 and a filter capacity of 70 m3/h. 14 March 2008 Plant Health Progress Fig. 2. Slow sand filtration (SSF) systems: (A) schematic diagram of a generic SSF system; (B) construction of the lower drainage system in an earthen basin; (C) completion of gravel layers prior to filling in an earthen basin with sand; (D) earthen basin filter during operation; (E) construction of the lower drainage system on the concrete slab floor of a steel tank; and (F) steel tank filter during operation. A