Floriculture crops in the United States have an estimated wholesale value of $4.4 billion and include a diverse assortment of bedding plants, potted flowers, and cut flowers. Poinsettia contributed a wholesale value of $140 million in 2015 (U.S. Department of Agriculture, 2016) and are one of the top potted flowering plants in the United States (Dole and Wilkins, 2005). In the greenhouse production of floriculture crops, recirculating irrigation systems have been widely adopted to lower water usage and conserve fertilizers that can otherwise be lost via discharge runoff (Bush et al., 2003; Hong et al., 2003; MacDonald et al., 1994; Sanogo and Moorman, 1993). This is especially true for greenhouses with large water use [1.9–3.8 million liters per day (Meador et al., 2012)]. Ebb-and-flow and flood-floor irrigation systems typically recirculate the irrigation water and are used to maximize production area and decrease labor costs (Ehret et al., 2001; van Der Gaag et al., 2001). In this type of system, irrigation water is pumped from a water reservoir to flood the floor or bench at a specified water level for a desired duration, and then drained back (often by gravity flow) to the reservoir for recycling in the next irrigation event. Although recycling irrigation water offers many benefits to greenhouse growers, plant pathogens can also be disseminated in the recycled irrigation water (Ehret et al., 2001). Thus, limiting pathogen transmission in the recirculating irrigation systems is critical to the floriculture industry.
Pythium species and other water molds can be highly destructive to floriculture crops, and spread readily in irrigation water (Goldberg et al., 1992; Hong and Moorman, 2005; Lewis Ivey and Miller, 2013; Stanghellini et al., 1996a, 1996b). Pythium root rot causes plant stunting, wilt, and death and can also reduce horticultural quality of infected crops (Tompkins and Middleton, 1950). The pathogen can become established in a greenhouse through infested soil and dust (Stephens et al., 1983), contaminated seedlings, cuttings, or other plant material from propagation greenhouses (van Der Gaag et al., 2001). Surface water used for irrigation can also be infested with Pythium and Phytophthora species (Bush et al., 2003). Management of Pythium species is particularly challenging for potted plants; frequent irrigation and high moisture levels are ideal for the reproduction and transmission of this pathogen (Elmer et al., 2012). The high porosity of peat potting media may also facilitate the movement of zoospores, which are an important type of Pythium species inoculum (Oh and Son, 2008). Thus, Pythium species could be rapidly disseminated in a greenhouse via ebb-and-flow and flood-floor production systems (Hoitink, 1991), leading to crop damage and loss and requiring proactive management strategies.
Fungicide application is a common and important strategy to limit pythium root rot in greenhouse production (Moorman and Kim, 2004). Currently, two of the main fungicides used for pythium root rot control are etridiazole (Terrazole; OHP, Mainland, PA) and mefenoxam (Subdue Maxx; Syngenta Crop Protection, Greensborough, NC) (Moorman and Kim, 2004; Raabe et al., 1981). Etridiazole effectively reduced pythium root rot in poinsettia and easter lily (Lilium longiflorum) when applied as a soil drench (Ascerno et al., 1981; Hausbeck and Harlan, 2013b; Raabe et al., 1981). Also, etridiazole is one of the few commercial fungicides that are labeled for chemigation in ebb-and-flow and flood-floor irrigation systems. Mefenoxam can also limit crop loss from pythium root rot (Moorman et al., 2002). However, resistance to mefenoxam has developed in greenhouse populations of Pythium species, partly due to repeated fungicide use (Lookabaugh et al., 2015; Moorman and Kim, 2004; Moorman et al., 2002). Failure to control Pythium diseases using mefenoxam has been reported in greenhouses (Hausbeck and Harlan, 2013a; Moorman and Kim, 2004), and resistant isolates were detected in surface water used for irrigation (Carlson et al., 2004). Fungicide resistance has become a limiting factor in the control of pythium crown and root rot; alternative strategies (e.g., filtration) for pathogen control in irrigation water are needed (Hausbeck and Zhang, 2016).
Management of pathogens in recycled irrigation water has been a persistent challenge in greenhouse production. Ultraviolet radiation, heat treatment, chemical disinfection, ozonation, and filtration have been used to remove pathogens from irrigation water with varying degrees of success (Ehret et al., 2001; Hong and Moorman, 2005; Raudales et al., 2017). Many of these methods are cost-prohibitive to install and operate in commercial greenhouses. In contrast, filtration is a low-cost method that disinfests irrigation water by the physical removal of pathogens using granular porous media (e.g., sand) or membrane filters (Hong and Moorman, 2005). Membrane filtration can effectively remove zoospores if the membrane pore size is small enough to retain the motile zoospores that have a pleomorphic cell membrane (Schuerger and Hammer, 2009). Membrane filters with pore sizes of 1 and 5 µm were able to remove the Pythium zoospores effectively from recirculating irrigation water in laboratory tests (Tu and Harwood, 2005). However, it is unknown whether this could be transferable to greenhouse settings. Diplanetism (where a zoospore encysts and releases a smaller motile zoospore) could decrease the efficacy of membrane filters (Erwin et al., 1983), although the occurrence of diplanetism in commercial greenhouses is unknown. Additional challenges with membrane filters are frequent leakage and membrane clogging and fouling (Ehret et al., 2001; Tu and Harwood, 2005), resulting in increased maintenance cost and decreased performance over time.
In contrast, deep-bed filtration (e.g., sand filtration) is cost-effective in terms of construction, operation, and maintenance. Slow filtration with granular materials has been studied as a means to remove Pythium species from the greenhouse irrigation water since the 1970s (Darling, 1977). However, it is not widely used in commercial U.S. greenhouses due to the slow water flow rate [100–300 L·m−2 per hour (Ehret et al., 2001)] that prohibits the movement of large volumes of water to multiple greenhouse ranges in an acceptable time period (Hong and Moorman, 2005). Previous studies on the effectiveness of deep-bed filtration for removing plant pathogens from irrigation water have focused on slow sand filtration (Ehret et al., 2001; Hong and Moorman, 2005; Lee and Oki, 2013), whereas the effectiveness of rapid filtration on pathogen removal has not been well investigated. Additional data could help to determine whether this technique could be adopted to manage pathogens in irrigation water in greenhouses.
The objective of this study was to investigate the ability of rapid filtration systems to limit pythium root rot of potted poinsettia in greenhouses with ebb-and-flow and flood-floor irrigation systems. Six small-scale ebb-and-flow recirculating irrigation systems were constructed to simultaneously test the effect of filter media type (sand and activated carbon), fungicide application (etridiazole), and inoculum source mode (infested growth media vs. infested water) in two greenhouse experiments. Poinsettia was selected as a model crop because of its popularity as a potted flower, its economic importance, and the prevalence of pythium root rot outbreaks during its production. Pythium aphanidermatum was chosen because it is one of the most prevalent Pythium species in greenhouses and is more aggressive for developing disease symptoms on poinsettia than Pythium irregulare (Lookabaugh et al., 2015). This study was intended to show a proof-of-concept to use rapid filtration systems in removing Pythium propagules from recirculating irrigation water.
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To test the effectiveness of filtration units in controlling disease outbreaks in greenhouse-grown poinsettias, six self-contained ebb-and-flow irrigation systems (including optional filtration units) were constructed (Fig. 1). A typical irrigation system consisted of a 8 × 4 ft (2.4 × 1.2 m) black plastic ebb-and-flow bench (Hummert, St. Louis, MO), an optional filtration unit, two 130-L (34.3-gal) holding tanks, two 12V-centrifugal water pumps, two check valves, two auto valves, two water-level sensors, and a timer. Only one holding tank was included in the non-inoculated and inoculated control treatments in Expt. 1, and in the inoculated control and “diseased plant” treatments in Expt. 2. Otherwise, the prefilter tank and the holding tank were directly connected for the nonfiltration treatments. The irrigation water was drawn from the prefilter tank by one water pump, passed through a check valve, the filter unit, an auto valve [i.e., 0.5-inch (1.27 cm) motorized ball valve (model MV-2-20-12V-R01-1; MISOL, Jiaxing, China)], and stored in the holding tank until a prescheduled irrigation time controlled by a timer. At the time of irrigation, the irrigation water in the holding tank was pumped into the ebb-and-flow bench via a check valve until reaching a desired watering height [i.e., 1–1.5 inches (2.54–3.81 cm) or 10 min of pumping time]. The two check valves were installed to prevent backflow. One check valve was next to the pump connected to the prefilter tank, and the other next to the pump connected to the holding tank (Fig. 1). The irrigation water in the bench was kept for a desired irrigation period before being drained back to the prefilter tank by opening an auto valve [i.e., the 0.75-inch (1.905 cm) motorized ball valve]. Two magnetic float water-level sensors (model a11062100ux0008, Uxcell, Kwai Fong, Hong Kong) were installed in the prefilter tank and holding tank, respectively. The water-level sensor in the prefilter tank detects the irrigation water drained from the bench, and the water pump is automatically turned on to deliver the water to the inlet of the filter unit. The second water-level sensor turns off the water pump connected to the holding tank, when the water level reaches the minimum level so as to prevent air entry into the pump.
The filter unit design is described in detail next. Activated carbon (AC) and sand were used as filter media. Operating water pressure of the AC and sand filters was maintained at 6.9 ± 1.4 and 5.7 ± 1.0 kPa (1.00 ± 0.20 and 0.83 ± 0.15 psi), respectively. Water pressure was measured by a pressure transducer at the top of the filters and recorded in a data-logger (model MCR-4V; T&D Corp., Matsumoto, Japan). All of the irrigation systems were sterilized before each experiment with a solution of greater than 30% of household bleach [i.e., 6.15% sodium hypochlorite (NaClO) solution] was applied in the benches and tanks using sprayers. Any adhering grime or algae was removed using scrub brushes. Then 10 L (2.6 gal) of 5% bleach solution was added to the prefilter tank and allowed to recirculate a couple of times in the absence of a filter unit. After cleaning, the systems were thoroughly rinsed several times with tap water and air-dried for several days.
Low-pressure sand and activated carbon (AC) filters were constructed for the greenhouse experiments (Fig. 2). Each filter unit was made with a 6-inch (15.2 cm)-diameter PVC pipe of 50 cm (19.7 inches) in length. The bottom of each filter column was sealed with an end cap fitting, and the top of each filter was assembled with a coupling, an adapter fitting, and a plug fitting in order. Two types of filter media [i.e., sand (99.69% silica, Granusil; Unimin Corp., New Canaan, CT) and AC (Filtrasorb 300; CalgonCarbon, Moon Township, PA), were used. The particle size distribution of the sand was 5.1% 297 to 420 µm, 57.2% 420 to 595 µm, 36.1% 595 to 841 µm, and 1.2% ≥841 µm (1 µm = 1 micron). The effective size of AC particles was 0.8 to 1.0 mm (0.03–0.04 inch). A 3-cm (1.2 inch) layer of coarser sand (500–841 µm) was placed at the bottommost and uppermost layers in the sand filter and bottommost layer in the AC filter to filter out large debris and minimize clogging. The total depth of filter media was 50 cm including the coarser sand layers. All filter media were used directly without washing. To support the filter media and allow for free drainage of filtered water, two screens each with different size openings [i.e., 0.5 × 0.5 inch (1.27 cm) and 0.25 × 0.25 inch (0.64 cm)] were prepared and bent to be fixed onto about the 2-cm (0.8 inch) length of the 6-inch PVC pipe using 12 screws and then mounted inside the end of the bottom cap. A stainless steel screen with 100 × 100 µm opening size was placed on the screens. The top of the filter media was also covered with the 100 × 100-µm and 0.5 × 0.5-inch screens similar manner to the bottom part to filter large debris and allow for an even distribution of water flow (Fig. 2). The bottom end cap was drilled and fitted with a 0.5-inch polypropylene bulkhead tank fitting (TF050; Banjo, Crawfordsville, IN) to connect the outlet pipe with a union fitting (Mueller/B&K, Collierville, TN) and a 0.5-inch motorized ball valve (MISOL, Jiaxing, China). The motorized ball valve was open during the operation of the pump connected to the prefilter tank. The pressure sensors were installed at the inlet of each filter. All of the components were assembled and sealed to make watertight columns.
Nutrient concentrations in the irrigation water during Expts. 1 and 2.