Potassium silicate reduces postharvest nectarine decay caused by Rhizopus stolonifer

Rhizopus soft rot (RSR) is a destructive postharvest disease in nectarine. Rhizopus stolonifer produces abundant airborne spores which are easily spread by air currents. As the inoculum of R. stolonifer is frequently present in packinghouses of peaches and nectarines, protective measures are necessary to control RSR. In Brazil only dicloran is registered to control RSR after harvest. Silicon-based products such as potassium silicate are generally recognized as safe compounds with antifungal activity on several plant pathogens. In this study, the protective effect of potassium silicate was compared to potassium chloride and potassium hydroxide for RSR control on nectarines after harvest. The effect of potassium silicate, potassium chloride, potassium hydroxide, monosilicic acid, and polyethylene glycol on spore germination was also assessed in vitro (optical microscopy) and over fruit skin (scanning electron microscopy). Potassium silicate reduced the incidence of nectarine decay (20-85%), and the highest RSR reduction was achieved at 300 mM (pH=11.3). Potassium chloride was inefficient for RSR control and potassium hydroxide favored R. stolonifer infection due to its abrasive damage on fruit surface. Spore germination was inhibited only by potassium silicate (65 and 80 mM) and potassium hydroxide (270 mM). The correlation between spore germination and pH was significantly negative (r=-0.98). The effectiveness of potassium silicate in reducing RSR may be associated, at least in part, with the reduction of spore germination due to the alkalinisation of fruit surface. Potassium silicate is a promising and useful alternative compound for reducing postharvest nectarine decay.