Control of major citrus postharvest diseases by sulfur-containing food additives

Sodium metabisulfite (SMBS), potassium metabisulfite (PMBS), aluminum sulfate (AlS) and aluminum potassium sulfate (AlPS), common sulfur-containing salts used as food additives, were evaluated for their antifungal activity against Penicillium digitatum, Penicillium italicum and Geotrichum citri-aurantii, the most economically important pathogens causing postharvest diseases of citrus fruits. In vitro radial mycelial growth was measured on potato dextrose agar (PDA) Petri dishes amended with five different concentrations of the salts (10, 20, 30, 50, 100 mM) after 7 d of incubation at 25 °C. SMBS and PMBS at all concentrations, and AIS and AIPS above 20 mM, completely inhibited the growth of these fungi. The curative antifungal activity of the four salts to control citrus green (GM) and blue (BM) molds and sour rot (SR) was evaluated on 'Valencia' oranges artificially inoculated in rind wounds with P. digitatum, P. italicum and G. citri-aurantii, respectively. In vivo primary screenings showed no significant antifungal activity of AlS and AlPS to control the three diseases at any dose tested, but SMBS and PMBS reduced the incidence and severity of GM, BM and SR at various concentrations. Effective salts and concentrations were selected for in vivo dip treatments in small-scale trials. Dips at room temperature (20 °C) in SMBS and PMBS at 20 and 50 mM for 60 or 120 s significantly reduced the incidence and severity of GM and BM, with PMBS at 50 mM for 120 s the most effective treatment. Conversely, dips in SMBS and PMBS at 50 mM for 60 or 120 s did not reduce SR incidence and severity. SMBS and PMBS treatments are potentially new tools to be included in reduced-risk non-polluting strategies to control Penicillium diseases, but not SR, on citrus fruits.