Antifungal effect of borates against Fusarium sulphureum on potato tubers and its possible mechanisms of action

Abstract

Potassium tetraborate and borax (Na₂B₄O₇) were evaluated for their in vitro activity against Fusarium sulphureum and for their curative and preventive efficacy against postharvest dry rot infections on potato tubers. Potassium tetraborate and borax strongly inhibited spore germination (effective inhibitory dose [ED50] = 2.1 and 2.4 g/L, respectively) and mycelial spread (ED50 = 2.8 and 3.2 g/L, respectively) of F. sulphureum in the culture medium and the inhibitory effect was highly correlated with the concentrations used in this study. Morphological changes such as sparse, blasted, wizened and cupped hyphae treated with potassium tetraborate, and asymmetrical, thinner, distorted and broken hyphae treated with borax were observed by scanning electron microscopy (SEM) observation. Transmission electron microscopy (TEM) observation further documented ultrastructural alterations of hyphae treated with both borates. These changes included abnormally contracted, distorted and irregularly thickened hyphal cell walls, unshaped hyphae and some inclusion bodies in the hyphal cells. Dry rot development in tubers treated with two borates after fungal inoculation (curative activity) was significantly (p < 0.05) reduced. However, potato tuber treated before inoculation did not show any lesion diameter reduction of dry rot (preventive activity). The addition of borates significantly improved the effectiveness of thiabendazole (TBZ) to control dry rot. These findings suggest that borates are promising as a natural fungicide to partially substitute for the utilization of synthetic fungicides in potato tuber.

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.