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

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.

► 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.