Alternative methods to control postharvest decay caused by Penicillium expansum in plums (Prunus domestica L.)

In the latest years, investigation on postharvest treatments has been increasingly addressed to preserve human health and environment safeguard. Several preservative compounds, physical treatments and biological control agents to restrain postharvest pathogens on horticultural products have been widely studied. Among them potassium sorbate (KS) has been generally recognized as safe for use in foods and personal care products. It acts as microbial growth inhibitor and fungistatic agent in foods, including vegetable and fruit products. The efficacy of KS, used alone or combined with heat treatments or biocontrol agents, has been demonstrated in Citrus and stone fruits. Here we report the results of 3 experiments aimed at controlling Penicillium expansum Link decay with the use of KS on a yellow ('Shiro') and a red ('Sanguigna di Bosa II', from the Sardinian germplasm) plum cultivar. An integrated approach, combining ultrasounds (US) as a physical mean and KS solutions at different concentrations, has been employed. In the first experiment, 360 fruits were wounded twice and divided into 6 sets (6 x 60), three of which were inoculated with an isolate of P. expansum (20 microl of a 10(5) cfu x mL(-1)). Then, 180 fruit (half inoculated) were treated by pipetting into each wound 20 microl of a KS solution containing 0, 1.5 or 3% (w/v), respectively. In exp. 2, all fruit (number) was wounded and inoculated, and after 24 h treated by immersion (1 min) into solutions containing 0, 1.5 or 3% (w/v) of SK, with or without the use of US. In exp. 3, wounded fruits were treated by immersion or sonication like in exp. 2, while inoculation took place after 24 h. Then, plums were kept at 25 degrees C and 75-80% RH and the infection degree was monitored after 3 and 6 days. In both cultivars, the 1.5% KS solution significantly reduced the natural infection, while the 3% KS solution resulted effective only on the red one. Moreover, the 1.5% solution was effective in controlling decay of artificially inoculated fruit, achieving a 56% reduction compared to control. Similar results were attained in exp. 2 and 3, where the combination of salt and sonication improved the efficacy, likely by increasing the salt diffusion into the wounds.