Reusing nutrient solution provides a unique, but challenging prospect within organic greenhouse production due to the restricted number of available pathogen control products. Oxidizing agents, such as ozone, peracetic acid, or hydrogen peroxide; organic acids such as citric or lactic acids and chlorine dioxide are approved for Canadian greenhouse production systems (Government of Canada, 2018). Although the mode of action varies among products, all provide varying degrees of pathogen control based on concentration, stability, and water quality of the nutrient solution (Raudales et al., 2014a). When choosing a disinfectant, the cost of H2O2 is significantly less than that of ozone, or chlorine dioxide due to the lack of specialized equipment required for use (Raudales et al., 2014b).
In solution, H2O2 readily breaks down to hydroxyl radicals (OH-) and oxygen, making it an ideal component of any “green” chemistry program (Carrasco and Urrestarazu, 2010). The generation of OH- from H2O2 provides direct control over pathogens and algae within irrigation water, although applied concentrations and contact periods varied between trials (Baldry, 1983; Bosmans et al., 2016; Raudales et al., 2014a; Runia, 1995; Vanninen and Koskula, 1998; Van Wyk et al., 2012). H2O2 concentrations as low as 37 mg·L−1 for 15 min (Elmer, 2008) and as high as 200 mg·L−1 for 24 h (Ehret et al., 2001) have been found to provide similar levels of control for Fusarium sp. in deionized water. Exposure to 400 ppm of H2O2 for 60 min removed 99.97% of tomato mosaic virus (Tobamovirus) cells, whereas exposure to 100 ppm for 5 min eliminated Fusarium oxysporum conidia (Runia, 1995). Within a greenhouse irrigation system, 100 ppm of H2O2 reduced free-living and biofilm-associated rhizogenic Agrobacterium by 3.7 and 3.5 log cfu/mL, respectively, after 72-h exposure (Bosmans et al., 2016). Against nematodes, application of 400 ppm for 24 h eliminated burrowing nematode (Radopholus similis) within recirculated irrigation water (Runia and Amsing, 1996). These rates support manufacturer-recommended concentrations of either stabilized or pure, H2O2 products for disinfecting plant pathogens found in irrigation water (Raudales et al., 2014a). Improving pathogen control at recommended rates requires the maintenance of relatively stable concentrations of H2O2, which is better achieved through repeated dosing of irrigation water, rather than an increase in disinfectant concentration (Copes, 2009). Once-daily injections of 30 ppm H2O2 decreased the number of hairy root disease (Agrobacterium) infested ‘Kanavaro’ tomato (Solanum lycopersicum) plants by 20% within a commercial irrigation circuit after 12 weeks (Bosmans et al., 2016). Similar effects were observed when H2O2 foliar spray applications increased from one to five times per week, with both severity and incidence of Puccinia hemerocallidis reduced on ‘Pardon Me’ daylily (Hemerocallis) leaves (Copes, 2009). Consistent dosing of water with H2O2 leads to the exposure of crops to H2O2 during irrigation events and brings the potential for a phytotoxic response if excessive concentrations are circulated.
Exposure of crops to H2O2 is directly based on the irrigation method used. Overhead irrigation, using spray nozzles attached to automated booms, is employed for production of microgreens, and lettuce in southwestern Ontario, Canada. Microgreens, seeded in shallow trays onto the growing substrate, use overhead irrigation to achieve even watering across the tray surface. These types of irrigation techniques result in exposure to H2O2 as water is sprayed directly onto the foliage of the crop during each irrigation event. With microgreens being harvested anywhere from 6 to 21 d after seeding, any phytotoxic symptoms from H2O2 use would manifest rapidly because these young crops have not hardened against environmental stressors (Viršilė and Sirtautas, 2013).
Evaluating the phytotoxic potential of H2O2 yielded surprisingly limited research concerning damage from foliar spray applications. Alfalfa (Medicago sativa) sprouts, foliar sprayed with H2O2 concentrations from 200 to 1000 mg·L−1 via a misting head, experienced no leaf damage or decrease in growth (Fett, 2002). In contrast, H2O2 phytotoxic effects were reported at concentrations as low as 9 and 12 mg·L−1, when applied every 6 h, resulting in the yellowing of radish and garden cress (Lepidium sativum) (Coosemans, 1995). Lettuce seedlings exposed to 8 mg·L−1 H2O2 for 24 h experienced a decrease in growth, whereas application of 85 mg·L−1 over 24 h resulted in seedling death (Nederhoff, 2000). Exposure to 500 mg·L−1 was reportedly harmful to plant roots, although application methodology and species information were withheld (Van OS, 1999). Within the listed studies, neither the volumes of applied irrigation water nor the degree of phytotoxicity respective to the given plant species were reported. However, phytotoxicity in the form of leaf senescence has been described in nursery crops exposed to 3.4 to 10 g·L−1 after one to three foliar applications of H2O2 (Copes et al., 2003). These studies highlight the requirement of research to accurately characterize the degree of H2O2 phytotoxicity displayed by plants after repeated foliar applications of irrigation water augmented with H2O2. Further, they provide a framework for concentrations at which H2O2 may cause phytotoxic effects on young crops grown under greenhouse conditions.
This study was conducted to identify how organic microgreens and lettuce plug (21 d) crops respond to daily foliar spray with H2O2 at concentrations commonly used to control plant pathogens within irrigation water. The results were used to assess an upper threshold at which H2O2 can be applied to these crops without a reduction in market quality and physiological growth.
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