In this study, we successfully isolated dominant strains from postharvest rot tangelo fruits. Through morphological characterization, molecular identification, and pathogenicity testing, it was determined that the pathogens causing postharvest diseases of tangelo are Penicillium citrinum and Aspergillus sydowii.
Penicillium citrinum is known for its optimal growth pH of 7–8 and its ability to thrive across a wide temperature range (Ouyang et al., 2014). It has a powerful enzyme synthesis and metabolic system, utilizing various nutrient substrates and producing abundant spores. Previous studies have linked Penicillium citrinum to infections affecting winter jujubes, grapes, peach-shaped plums, and other fruits during cultivation, leading to snowflake-like mold spots, fruit cracking, and sugar leakage (Guo & Hou, 2013; Tournas, 2005). However, this study reveals that Penicillium citrinum primarily infects tangelo fruits in the early storage period, easily invading through micro wounds. Initial symptoms include brown lesions on the fruit skin, followed by the appearance of white hyphae in the middle stage. As the disease progresses, lesions expand, accompanied by a white mold layer, resulting in festering, serous decay, and a moldy smell.
Aspergillus sydowii, with an optimal growth pH of 6–7 and a logarithmic growth period of 24–36 hours, thrives in high-temperature and humid environments and has minimal nutritional requirements. This study identifies Aspergillus sydowii as a cause of postharvest rot disease in tangelo, displaying symptoms such as diseased spots with a mold layer in the early stage, leading to gradual soft rot. This is consistent with the symptoms observed in mango and pomegranate experiments (Pisani et al., 2015; Guo et al., 2015; Chang et al., 2020). The prevalence of Aspergillus sydowii in tangelo cultivation regions, such as in Fujian, Zhejiang, and other places in China, could be facilitated by the subtropical monsoon climate, characterized by moderate temperature and abundant rainfall, especially during storage when the environment is suitable. Accurate identification of Penicillium citrinum and Aspergillus sydowii provides a theoretical foundation for the prevention and control of postharvest rot disease in tangelo.
Electrolyzed water is prepared by electrolysis of a low concentration of salt or acid solution with varying properties, such as pH, oxidation reduction potential (ORP), and available chlorine concentration (ACC). Electrolyzed water can be divided into three types, including acidic electrolyzed water (AEW) (pH ≤ 3, ORP > 1100 mV), micro-acid or acidic slightly electrolyzed water (saw) (pH 5.0-6.5, ORP > 900 mV), and alkaline electrolyzed water (Alew) (pH 10.0-11.5, ORP = -800-900 MV). Acidic electrolyzed water is widely used in food, agriculture, and medical fields because of its wide sterilization range, obvious effect, safety to human body and environment, convenient preparation and low cost. In 2002, Japan included acidic electrolyzed water in the list of legal food additives. In 1994, acidic electrolyzed water developed rapidly after it was introduced in China, and in 2020, the country promulgated GB 28234 − 2020 (Hygienic requirements for acid electrolytic water generator).
Acidic electrolyzed water is a promising fungicide due to its efficiency, safety, and minimal pollution (Zhao et al., 2021; Chen et al., 2019). Numerous studies have shown that its effective chlorine component damages the fungal cell membrane structure, causing intracellular substance leakage and deoxyribonucleic acid denaturation (Zhang et al., 2023; Liao et al., 2017). Accumulation of reactive oxygen species (ROS) further destroys intracellular substances such as nucleic acids, proteins, and lipids, promoting cell death (Tang et al., 2021; Shi et al., 2020).
Acidic electrolyzed water is a potential bacteriostatic agent. After acidic electrolyzed water treatment, the surface of the mycelium appeared collapsed, twisted and contracted, which suggested a significant reduction in the contents of the cells as well as leakage of intracellular substances of the mycelium. Abnormal increase of cell membrane permeability is the early manifestation of cell injury. The cell membrane plays an important role in maintaining the contents, such as sugars, proteins, inorganic salts, which are crucial for cell activities and survival. Damage to the cell membrane can lead to leakage of cellular contents. This study demonstrated that the electrical conductivity of acidic electrolyzed water treated group was higher than that of the control group, which indicated that acidic electrolyzed water could increase the permeability of the cell membrane of pathogenic fungi, thus increasing the extracellular electrical conductivity of pathogenic fungi. The increase in the characteristic nucleic acid wavelength of 260 nm normally suggest the leakage of nucleic acid and protein, which cause irreversible damage to cell membrane and cell. A260 in the treatment group was significantly higher than that in the control group, which indicated that increased leakage of nucleic acid and other substances in the cells of pathogenic fungi, suggesting that the genetic process in the pathogenic fungal cells was greatly affected by acidic electrolyzed water. pH controls DNA transcription, protein synthesis and enzyme activity. This study demonstrated that acidic electrolyzed water treatment increased the extracellular pH of pathogenic fungi, which caused decreased intracellular pH and acidification of cells by proton accumulation, resulting in cytoplasmic acidification, denaturation, and energy loss. The results indicated that acidic electrolyzed water could disrupt the cell membrane integrity of Penicillium citrinum and Aspergillus sydowii, resulting in intracellular components leakage and perturbation of the intracellular environment, leading to accelerated cell death of the pathogens.