Postharvest sour rot control in lemon fruit by natamycin and an Allium extract

Citrus sour rot caused by Geotrichum citri-aurantii is one of the most important postharvest diseases in citrus fruit, causing huge economic losses. Traditionally, it has been controlled by the postharvest application of guazatine and propiconazole fungicides, but restrictions in their use make it urgent to find an alternative for sour rot management. Natamycin, a common food preservative, and the organosulfuric compounds extracted from Allium species are safe food additives that control different foodborne pathogens. In the present study, the curative activities of commercial formulations of natamycin (Fruitgard Nat 20) and an Allium extract (PTSO: propyl thiosulfinate oxide; Proallium FRD®), were evaluated for the control of G. citri-aurantii in artificially inoculated lemon fruit. Trials in laboratory and in commercial conditions were carried out to explore the feasibility of including both compounds as part of a safe postharvest sour rot disease control strategy. Under controlled laboratory conditions, sour rot was significatively reduced by 500 mg L^-1 of natamycin, 580 mL L^-1 of PTSO and 290 mL L^-1 of PTSO + 4% of a food coat, applied by immersion. Nevertheless, the maximum dose of PTSO (580 mL L^-1) caused phytotoxicity on the fruit rind. In commercial drenching conditions, 290 mL L^-1 of PTSO + 4% of a food coat reduced sour rot incidence similar to conventional treatment. In a packing line treatment, spray application of 500 mg L^-1 of natamycin with a previous dip in sodium bicarbonate, resulted in nearly 70% reduction of disease incidence compared to conventional salt application. A second commercial experiment revealed that fruit drenching with 290 mL L^-1 of PTSO + 4% food coat followed by an in-line cascade application of 500 mg L^-1 of natamycin is completely effective for sour rot control after 20 days at 5 °C. Further exposure at room temperature for 7 d showed a 61% reduction in sour rot incidence compared to the control. Results revealed that natamycin and PTSO are promising tools for sour rot control used alone or combined as part of an integrated postharvest strategy.