Control of Major Plum Postharvest Diseases by Apple Thinning By-Product Extracts

Fresh Japanese plums are highly perishable and susceptible to postharvest diseases caused by fungal pathogens. Current management strategies rely on synthetic fungicides, raising concerns about resistance and sustainability. Immature-thinned apples (ITA), an underutilized agricultural by-product, are rich in polyphenols with potential antifungal properties. This study evaluated the potential of ITA ultrasound-assisted extracts of the cultivars 'Gala', 'Golden Delicious', and 'Opal' for controlling plum postharvest diseases. Extracts were chemically characterized for total polyphenol content and total antioxidant activity, with chlorogenic acid and phloridzin identified as major compounds, especially in 'Gala' extracts. In in vitro tests, all three extracts inhibited the mycelial growth of Botrytis cinerea and Monilinia fructicola by more than 90%, but none of them reduced the growth of Rhizopus stolonifer, Penicillium expansum, and Geotrichum candidum by more than 80%. In in vivo trials with plums artificially inoculated, treated 2 or 24 h later, and stored either at 20°C for 6 days or at 1°C and 90% RH for 4 weeks, all three extracts reduced the incidence of gray mold and brown rot by up to 90 and 85%, respectively. These reductions on cold-stored plums were up to 87 and 71% after a shelf-life period of 4 days at 20°C, with no significant differences between apple cultivars. This high curative activity highlights the potential of ITA extracts as sustainable, natural bio-based alternatives to synthetic chemical fungicides for stone fruit postharvest decay control, while aligning with circular bioeconomy principles through the valorization of agricultural waste.