Antifungal effect of borates against Fusarium sulphureum on potato tubers and its possible mechanisms of action
Highlights
► Borates strongly inhibited spore germination and mycelial spread of F. sulphureum. ► Morphological and ultrastructural alterations of hyphae treated with borates were observed. ► Borates have significant curative activity, but not preventive activity against F. sulphureum in vivo. ► Addition of borates significantly improved the effectiveness of TBZ to control dry rot.
Introduction
Dry rot, caused by Fusarium sulphureum, is one of the most important diseases of potato in Gansu province, the largest potato production area in China (He et al., 2004). Disease control is achieved by primarily postharvest application of fungicides, such as thiabendazole. However, the resistance of Fusarium spp. to thiabendazole (TBZ) is now widespread (Desjardins et al., 1993) and public concern regarding pesticide residues on fruit has been growing. Therefore, there is a need to develop alternative strategies for controlling dry rot of potato tuber that are safe, effective, economical, and compatible with commercial handling practices. A number of alternatives, including postharvest application of chitosan (Greyerbiehl and Hammerschmidt, 1998, Sun et al., 2008, Li et al., 2009a), sodium silicate (Li et al., 2009b), β-aminobutyric acid (Yin et al., 2010), K2HPO4 (Han et al., 2009) or citric acid (Zhang et al., 2009) have been investigated for the control of dry rot of potato tuber with some success.
Boron is an essential nutrient element and its deficiency reduces pollen germination and the growth of pollen tubes, which consequently results in the development of malformed fruits, thus lowering crop yield and reducing fruit quality (Lieten, 2002, Sharma et al., 2004). Hafez and Haggag (2007) found that boric acid applications could increase fruit firmness, total soluble solids, total sugar, total anthocyanin, while decreasing total acidity, weight loss percentage and fruit rot decay percentage of apples caused by Botrytis cinerea during cold storage at 5 °C for 60 d. Exogenous application of boron was also shown to reduce the occurrence of browning injuries in pears during controlled atmosphere storage (Xuan et al., 2005). Previous studies demonstrated that boron was effective to inhibit the growth Penicillium expansum (Qin et al., 2007, Huang et al., 2008), B. cinerea (Qin et al., 2010) and Monilinia laxa (Thomidis and Exadaktylou, 2010). The inhibition may be related not only to the halting of pathogen growth, but also to the increase of oxidative stress caused by boron through suppressing the expression of antioxidant enzymes in the pathogen (Qin et al., 2007, Huang et al., 2008, Shi et al., 2011). Although the preservative and antifungal properties of boron have been well known, borax is used as a food additive only in some countries with the E number E285, but is banned in the Unite States, Australia and Indonesia. So in order to reduce consumer health risk, studies on preservative and antifungal mechanisms of borates and integrated pest management (IPM) programs are still needed to reduce application dosage. There have been no reports concerning the possible role of boron in preventing the pathological decay of potato tubers.
This study was aimed at evaluating the activities of potassium tetraborate and borax against F. sulphureum in vitro, understanding the antifungal role of potassium tetraborate and borax in terms of morphological structures of the pathogen, and finally assessing the potential of potassium tetraborate and borax to control the dry rot of potato tubers.
Section snippets
Potato tubers
Potato tubers (Solanum tuberosum cv. Longshu no. 3) packed in net bags (15 kg/bag) were harvested from Dingxi, Gansu and transported to the laboratory within 24 h. Tubers were sorted by size while those without physical injuries or visible infection were stored at 5–8 °C. Before treatment, the tubers were surface-disinfected with 2% sodium hypochlorite for 3 min, then rinsed with tap water and air-dried.
Pathogen
F. sulphureum used in this study was provided by the Institute of Plant Protection, Gansu
Effects of borates treatment on spore germination and mycelial growth of F. sulphureum in vitro
Potassium tetraborate and borax at all concentrations showed antifungal activity against spore germination and mycelium growth of F. sulphureum (Table 1), with greater inhibitory effects at higher concentrations. The regression lines of the logarithm of the borate concentrations and the effectiveness indices transformed in probit, which were used to calculate the ED50, were highly significant (correlation coefficients between 0.92 and 0.98, p < 005; data not shown). At 20 g/L, both potassium
Discussion
In this study, we found that borates were effective in directly inhibiting spore germination and mycelial growth of F. sulphureum, although their effectiveness was less than that of TBZ. The study confirmed other findings about the antifungal activity of boron, as an essential micronutrient for plant, on spore germination, germ tube elongation and mycelial growth of many plant pathogens (Lloyd, 1996, Qin et al., 2010, Thomidis and Exadaktylou, 2010, Shi et al., 2011). The antimicrobial activity
Acknowledgments
This work was supported by Gansu Key Technology Research and Development Program (1011JKCA179), Gansu Science and Technology Major Project (1102NKDA025), Gansu Natural Science Foundation (1107RJZA232) and Gansu Provincial Key Laboratory of Arid Land Crop Science, Gansu Agricultural University (GSCS-2010-08).
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