Culinary fresh herb production in the United States has grown rapidly over the past decade. For example, U.S. Department of Agriculture (USDA) census data show a 96% increase in fresh herb production from 2012 to 2017 (USDA NASS, 2014, 2019) with 74% of fresh herbs produced in California, New Jersey, and Texas in 2017 (USDA NASS, 2019). This rather centralized production in limited areas necessitates long-distance transportation of this highly perishable produce to reach consumers across the country. Hydroponic production in controlled environments allows year-round local production of perishable fresh produce in densely populated areas, closer to the point of consumption. As a result, hydroponic basil quickly became a major product out of local greenhouses throughout the United States. In this report, we refer to “hydroponics” as liquid culture (i.e., growing plants without major use of aggregate medium). Leafy greens and herbs are typically grown hydroponically instead of soilless substrate culture. In contrast, fruiting vegetables are grown in soilless substrate culture with large amounts of aggregate media (such as rockwool) to support plants.
Although hydroponic production greatly reduces the incidence of soilborne root diseases, pathogens can still be introduced into production facilities through air, sand, soil, peat, source water, seeds, or insects (Stanghellini, 1996; Stanghellini and Rasmussen, 1994). Once a pathogen, especially a fast-spreading oomycete species, is introduced into a hydroponic system, dispersal can occur quite rapidly due to genetically uniform hosts and consistent environmental conditions (Stanghellini and Rasmussen, 1994; Wohanka, 2002). Prevention of pathogen introduction in hydroponic systems is critical, as effective chemical control agents for root diseases of edible crops are limited and may not be registered for use in greenhouses or indoors (Jensen and Collins, 2011; Stanghellini, 1996). Various disinfection systems (e.g., ultraviolet irradiation) have been introduced to commercial hydroponic systems to mitigate the risk of disease introduction and spread through the recirculation system for the nutrient solution (Wohanka, 2002). However, once a disease outbreak occurs, growers are often forced to suspend production and disinfect growing systems, leading to decreased yields and profit, changes to crop schedules, and increased labor (Stanghellini, 1996).
Among the most common oomycete pathogens experienced in hydroponic crop production are Pythium and Phytophthora spp. (Stanghellini and Rasmussen, 1994). Although these oomycete pathogens can infest roots of virtually all crop species grown hydroponically, basil and spinach (Spinacia oleracea) are particularly susceptible to infection by oomycete pathogens (Mattson, 2018). Several preventive measures, such as reducing the temperature of the hydroponic nutrient solution (e.g., Albright et al., 2007) or adding biofungicides (e.g., Utkhede et al., 2009), have been studied to minimize the risk of disease. This study focuses on a low-cost approach of reducing pH below the conventional range (5.5–6.5), originally suggested as a grower practice for hydroponic spinach in Japan (S. Tsukagoshi, personal communication). However, information on plant responses to specific pH levels is not available.
pH is among the most important parameters affecting sporangium production, germination, and mycelial growth of Pythium and Phytophthora spp. (El-Sharouny, 1983; Ho and Hickman, 1967; Kong et al., 2009). Ho and Hickman (1967) reported that the average period of motility of Phytophthora megasperma var. sojae zoospores was more than 20 h at pH 6.25 but was reduced to 1 h at pH 4.85. In addition, in a study conducted with Rhododendron macrophyllum grown in a peat/sand substrate inoculated with Phytophthora cinnamomi, plants grown in the substrate adjusted to pH 3.4 to 3.7 displayed no symptoms of disease, whereas those grown at pH 5.8 had an average disease symptom rating of 3.6 of 5.0 (Blaker and MacDonald, 1983). However, lower pH (<5.5) is typically avoided for hydroponic nutrient solution, as specific nutrient disorders and growth inhibition are commonly experienced outside the conventional range (Sambo et al., 2019; Savvas and Gruda, 2018; Sonnevelt, 2002). Of interest, numerous studies suggest that hydronium and hydroxide ion toxicity are found only at the extreme ends of acidity and alkalinity (Arnon and Johnson, 1942; Islam et al., 1980; Vlamis, 1953), and growth inhibition can usually be attributed to one or more pH-dependent factors including nutrient availability, ion antagonism, and precipitation of fertilizer salts (Bugbee, 2004; Fageria, 1983; Hawf and Schmid, 1967; Mengel et al., 2001; Peterson et al., 1984; Sambo et al., 2019).
It has been reported that plants can grow in substrates with a relatively wide range of pH with some necessary nutrient adjustments. For example, tomato (Solanum lycopersicum), lettuce (Lactuca sativa), and bermudagrass (Cynodon dactylon) grew in nutrient solution ranging from pH 4.0 to 8.0, and nutrient solution pH did not affect the pH of the shoot and root sap (Arnon and Johnson, 1942). In addition, this same experiment showed that at pH 4.0, tomato and lettuce shoot and root growth were less than at the more optimum pH of 6.0, but that growth was improved by increasing Ca concentration in the nutrient solution.
Commonly referenced charts showing nutrient availability at different pH values (e.g., Peterson, 1982) typically indicate that availability of micronutrients such as Cu, Zn, Mn, and B is increased with decreasing pH, while Mo availability decreases. This suggests that the likelihood of Cu, Zn, Mn, and B toxicity and Mo deficiency increases with decreasing pH. Nevertheless, specific responses of leafy greens and herbs to lower-than-conventional pH and possible mitigation of nutrient disorders under low pH by adjusting nutrients are not known.
If leafy greens and herbs can exhibit normal growth in highly acidic conditions (e.g., pH below 5.0), the risk of crop failure due to oomycete pathogens might be reduced. Therefore, the objectives of the present study were primarily to examine the influence of low nutrient solution pH on ‘Nufar’ and ‘Dolce Fresca’ basil growth and nutrient concentration in leaf tissue, and secondarily, to determine the efficacy of adjusting micronutrient concentrations in the hydroponic solution, aiming to mitigate possible nutrient disorders at low pH. Our first hypothesis was that adjusting micronutrient concentrations to account for decreased/increased availability of specific nutrients allows basil plants to exhibit normal growth without nutrient disorders in lower-than-conventional pH (Expt. 1). Our second hypothesis was that low pH suppresses pythium root rot initiated by zoospore inoculum (Expt. 2).
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