Correction: Son et al. Variation in Pathogenic Organisms as Affected by Using Hydroponic Nutrient Wastewater in Horticultural Facilities. Agriculture 2022, 12, 1340

1. Missing Citation

The original article [1] has been amended to increase the clarity of citation sources within the text.

The authors included additional references: paper by Ahn, et al. (2011) “Comparison of Nutrient Replenishing Effect under Different Mixing Methods in a Closed-loop Soilless Culture using Solar Radiation-based Irrigation”, published in the Journal of Bio-Environment Control; book by Kim & Park (1995) “Hydroponics”; and “Development of Closed Hydroponic Technologies with an Environment-Friendly Substrate in Cultivation of Export Fruit Vegetable” published by RDA (2017). The citations have now been inserted in the Section 1, Paragraph 1 and should read:

The facility gardening industry in Korea has been referred to as the ‘white revolution’ and is becoming a major agricultural technology by producing high profits and adding value to various agricultural products through year-round cultivation. This industry has become crucial for farmers who face difficulties such as falling agricultural prices [1–5]. Since the 1990s, the facility horticulture industry has accounted for more than 40% of the total horticulture industry, driven by the increasing demand for productivity, high quality, and labor savings through automation [1–4,6–9]. Hydroponics is a cultivation method that utilizes a solid medium to provide crops with the necessary nutrients and moisture [1,2,4,10]. In Korea, hydroponics was introduced in 1954 and has gained prominence due to its ability to produce high-quality crops for both domestic consumption and exportation [1,2,4,11,12]. The costs associated with hydroponic cultivation are lower than those associated with traditional soil cultivation, and the former is also advantageous as it allows growing crops in different ways [1,2,4,13,14]. In addition, hydroponic cultivation is an easy method for managing temperature and humidity, pest control, disease prevention, and nutrient supply [1,2,4,15–18]. As a result, the proportion of the cultivation area has been increasing [1,2,4,13,14]. In developed European countries, such as the Netherlands, the percentage of hydroponics farms that reuse nutrient solutions is 95%, while in Japan, it is 15%. In contrast, South Korea has a much lower percentage, less than 5% [1,2,4,19–26]. The Netherlands implemented a mandate in 2004 requiring all greenhouses to be converted to circular systems. Additionally, by 2024, hydroponic farms in the Netherlands are required to adopt recycling methods to prevent the pollution of soil, nearby rivers, and groundwater caused by the discharge of wastewater from greenhouses [1,2,4,20]. The area dedicated to hydroponics in South Korea has experienced significant growth, expanding nearly fivefold from 811 hectares in 2003 to 4224 hectares in 2018 [1,2,4,15]. Greenhouses in Korea have been built up for environmental friendly facilities with a recycling and reprocessing system for the reuse of nutrients and drainage [1,2,4,24,25]. However, the majority of farmers—approximately 95%—in South Korea use non-circulating hydroponic systems with low facility costs [1,2,20,26]. This indicates an urgent requirement to establish agricultural recycling technologies to reduce the environmental pollution caused by the discharge of waste nutrients from non-circulating hydroponic systems [1,2,4,27].

The authors unintentionally omitted the paper by Son, et al. (2022): “Identification of major pathogenic fungi species for the reuse of drainage water in horticulture hydroponic system in Korea” published in the Korean Society of Biological Engineering Spring Conference and International Symposium. The citation has now been inserted in the Section 3.1, Paragraph 1:

Twenty-four fungal species from four phyla, six classes, eight orders, ten families, and ten genera were observed in the thirty six hydroponic greenhouses; the observed density was 399 (Table 2, Appendices A and B), which was similarly reported in the previous study [20,68]. Approximately 3.33 species were observed at a study site with a density of approximately 11.08. Fifteen species from three phyla, four classes, six orders, seven families, and seven genera were detected in paprika among the crop types with 105 densities; in tomatoes, a density of 91 was observed for thirteen species from four phyla, five classes, five orders, seven families, and seven genera. In strawberries, we observed a density of 203 for sixteen species from three phyla, five classes, seven orders, eight families, and eight genera [68].

This reference has also been added at the end of the Section 3.1, Paragraph 3, without any changes in the text.

2. Text Correction

Text corrections to clarify citation sources and revised text have been made in the following sections:

1. Section 1, Paragraphs 2–4:

Due to the growing demand for high-quality agricultural products, there has been an adoption of large-scale hydroponic systems, leading to an increasing proportion of institutional horticulture [1,2,4,28–32]. However, this trend has resulted in various environmental issues being reported, including agricultural water depletion, non-point source pollution, and loss of fertilizer components [1,2,4,28–32]. Nonetheless, hydroponics offers the advantage of a highly controllable nutrient supply, facilitating plant growth, increased quantity, and improved quality [1,2,4,33,34]. However, pathogenic microorganisms may spread rapidly within the flowing medium because of a higher chance of contact with the crop’s root, which is a disadvantage of hydroponics [1,2,4,33,34]. In non-circulating hydroponics, if the nutrient solution is drained after being supplied once, and the waste nutrient solution is discharged directly into rivers, it can result in significant adverse effects, such as groundwater and soil pollution, as well as an environmental burden due to the presence of excessive nitrogen, phosphorus, pathogens, and other contaminants [1,2,20,35]. To effectively implement a circular hydroponic system, it is essential to carefully monitor and prevent the spread of pathogens, including fungi and bacteria, present in the medium and drainage, as they can cause diseases when reused. Additionally, proper treatment of the drainage volume should be conducted during reuse [1,2,4]. By employing systematic circulating hydroponic facilities, environmentally friendly cultivation techniques such as fertilizer conservation and the prevention of water pollution can be achieved [1,2,4,36].

Major bacterial pathogens found in horticulture facilities include Agrobacterium tumefaciens, Erwinia carotovora subsp. carotovora, and Pseudomonas spp. [2,4]. Such fungal pathogens include Botryosphaeria spp., Colletotrichum spp., Fusarium spp., Penicillium spp., Phytophthora spp., Pythium spp., Leveillula taurica, Cladosporium spp., and Alternaria solani [2,4]. In previous studies, most of the samples analyzed were collected from the crop roots and hydroponic media. The primary pathogens detected in previous studies were Agrobacterium tumefaciens [37,38], Erwinia carotovora subsp. carotovora [39,40], Pseudomonas spp. [2], Pseudomonas syringae [41,42], Pseudomonas marginalis [43–45], Pseudomonas ciridiflava [43–45], Botrytis cinerea [46], Colletotrichum spp. [47], Fusarium spp. [48–51], Phytophthora spp. [52–54], and Leveillula taurica [55,56]. Lee et al. [2] reported that fungi and bacteria detected in the media and roots moved to the drainage through the supplied nutrient solution; therefore, the present study evaluated the degree of the detection of fungi and bacteria in the drainage for each crop.

In this study, we conducted the identification, quantification, and evaluation of harmful fungal and bacterial species present in drainage water and the growing medium used for paprika, tomato, and strawberry crops, which are commonly cultivated in hydroponic greenhouses in Korea [1,2,4]. Based on the results of our analysis, we emphasize the need for effective management measures in circulating hydroponic systems to ensure the safe reuse of discharged drainage. The questions established for the study are: is sterilization necessary to reuse, and what are the main species of drainage wastewater [1,2,4]? Is there a difference in concentration from the detected species according to the type of facility and discharge type [1,2,4]? In conclusion, the main objective of this study is to provide fundamental data on the importance and viability of sterilization facilities for achieving sustainable agriculture and establishing eco-friendly horticultural facility complexes. Additionally, the study aims to contribute to the stable cultivation of circular hydroponics. By offering this valuable information, we aim to support the implementation of environmentally conscious and sustainable practices in the field of hydroponics.

2. Section 2.1, Paragraph 1:

The experimental plots were selected for greenhouses cultivating paprika, tomatoes, and strawberries, the most frequently grown crops in hydroponic greenhouses in South Korea [1,2,4,20,57]. Study sites were chosen for thirty-six different areas, with three each per crop (the only selection criterion was crop type), and the drainage from each site was sampled. Korea’s hydroponic greenhouses usually grow from September to May [4,58–60]. In general, the crops are planted in September, ending a one-time cycle. In Korea, the maximum temperature rises to about 35 degrees from June to August, with a high humidity, making it difficult for crops to grow inside the greenhouse [5,61–63]. In some areas, crops are produced in the summer, but temperature control is difficult [64]. Therefore, the sample for the study was a place where the crops were sufficiently grown for more than 6 months (sample collection was a similar age to the crops). The samples were collected between March and May 2020, and the physicochemical properties and concentrations of nutrient solution were similar for each crop. The samples were taken once for uniformity (outside, crop age, time) in the discharge area shown in Figure 1 and Video S1.

3. Section 2.2, Paragraphs 1–8:

Paprika, tomato, and strawberry, which are the most commonly grown crops in hydroponics in Korea, were selected as study crops based on the findings of the Ministry of Agriculture, Food and Rural Affairs (MAFRA) [1,2,4,6,7,9]. By analyzing the harmful fungi and bacteria most commonly found in Korean hydroponics, 57 fungi and 11 bacteria were detected [1,2,4,39,67]. The most common fungal and bacterial pathogens in Korea are Cladosporium spp., Botrytis cinerea, Pythium spp., Fusarium spp., Colletotrichum loeosporioides, Pseudomonas spp., and Erwinia carotovora subsp. [1,2,4,67]. These harmful fungi and bacteria are known to be typical species that require management in horticultural practices [1,2,64].

To detect the major fungal and bacterial species, DNA multi-scan analysis was performed (Eurofins Agro; Eurofins Scientific, Agro, LLC, Wageningen, The Netherlands) using the following steps [1,2,4].

First, the analysis samples were prepared (drainage samples were collected in 1 L sterile collection bottles).

Second, the sample were put into contact with a sterile liquid medium (for the release of fungal and bacterial isolates).

Third, DNA amplification was performed using the PCR process (low-level DNA detection).

Fourth, DNA was put into contact with a specific membrane.

Fifth, the visualization of attached fungi and bacteria was performed (detected pathogens were classified into six categories according to European standards: <25 colony-forming unit (CFU)/m³ = very low (1), <100 CFU/m³ = low (2), <500 CFU/m³ = moderate (3), <1000 CFU/m³ = moderate-to-high (4), <2000 CFU/m³ = high (5), and >2000 CFU/m³ = very high (6)).

By following these steps, researchers can identify and classify the harmful fungi and bacteria commonly found in hydroponics in Korea. This information can be used to develop appropriate management strategies to control and mitigate the effects of these pathogens in captive horticultural practices [1,2,4].

3. References Correction

Additional references were included:

10. Ahn, T.I.; Shin, J.H.; Noh, E.H.; Son, J.E.; Comparison of Nutrient Replenishing Effect under Different Mixing Methods in a Closed-loop Soilless Culture using Solar Radiation-based Irrigation. J. Bio-Environ. Control 2011, 20, 247–252.

11. Kim, K.Y.; Park, S.G. Hydroponics; Ohsung Publishing: Seoul, Republic of Korea, 1995; pp. 43–127.

19. RDA. Development of Closed Hydroponic Technologies with an Environment-Friendly Substrate in Cultivation of Export Fruit Vegetable; RDA: Jeonju, Republic of Korea, 2017.

68. Son, J.K.; Kang, T.K.; Park, M.J. Identification of major pathogenic fungi species for the reuse of drainage water in horticulture hydroponic system in Korea. In Proceedings of the 2022 Korean Society of Biological Engineering Spring Conference and International Symposium (Collection of Abstracts), Daejeon, Republic of Korea, 13–15 April 2022; p. 474.

The following reference was removed:

2. Nam, Y.I. Present status and developmental strategy of protected horticulture industry in Korea. KCID J. 2003, 10, 15–23. Available online: https://www.dbpia.co.kr/Journal/articleDetail?nodeId=NODE09657133 (accessed on 15 September 2021).

With this correction, the order of some references has been adjusted accordingly. The authors state that the scientific conclusions are unaffected. This correction was approved by the Academic Editor. The original publication has also been updated.