Postharvest application of natamycin to control gray mold in table grapes

Abstract

Gray mold caused by Botrytis cinerea is the most economically important postharvest disease of table grapes in California. In this study, the efficacy of the new biofungicide natamycin in controlling gray mold and its effects on fruit quality of table grapes were evaluated on two varieties. Four rates of natamycin treatment, ‘full’ (a.i. 920 mg L−1), ‘half’ (a.i. 460 mg L−1), ‘quarter’ (a. i. 230 mg L−1), and ‘eighth’ (a.i. 115 mg L−1) of the label rate, administered through dipping or spraying, were tested on grape berries that were with or without inoculation with B. cinerea prior to the treatment at 20 °C for 4 d or at 1 °C for 4 w. Natamycin significantly reduced percentage and severity of gray mold on the grape berries inoculated with B. cinerea, regardless of application methods, grape varieties, and incubation temperatures. Natamycin at all four rates also significantly reduced the percentage of grape berries showing fruit rots resulting from naturally occurring infections after the storage at 1 °C for 4 w, in comparison with the non-treated control, and remained effective when the fruit were evaluated after four-week cold storage followed by additional two days at 20 °C. Natamycin, however, had little effect on the spread of gray mold (nesting) within the grape cluster via fruit contact by mycelium of B. cinerea. Natamycin did not have any negative effects on the fruit quality parameters such as the appearance of grape rachis, firmness of berries, soluble solids, and titratable acidity. Our findings indicated that natamycin can be an effective postharvest treatment for managing gray mold on table grapes during storage.