Effect of a biological agent and GRAS compounds on the control of apple postharvest diseases in long-term storages

Abstract: The two most important postharvest diseases of apples in Canada in long term storages are blue mould caused by Penicillium expansum and gray mould caused by Botrytis cinerea. The objective of this study was to test the efficacy of an antagonistic rhizobacterium alone and in combination with three food additives that are generally regarded as safe (GRAS) compounds for the control of postharvest diseases in cold air storages using local apple varieties and storage conditions. Three food additives, 0.5% sodium biocarbonate (SBC), 1.0% salicylic acid (SA) and 1.0% calcium chloride (CaCl2), were tested in combination with a bacterial antagonist, Pseudomonas fluorescens strain 4-6, against blue mould and gray mould on fruit from two apple cultivars, ‘Gala’ and ‘McIntosh’ in Ontario (ON). The wounded apples were co-treated with 1 ×104 conidia/ml of pathogen of either TBZ-resistant P. expansum or B. cinerea and/or in combination with 1 ×108 cfu/ml of Pseudomonas fluorescens (Ps. fluorescens) antagonist 4-6, and GRAS compounds. Positive controls, the bacterial biocontrol Ps. syringae and the chemical fungicide fludioxonil (Scholar) were applied at recommended rates. Control treatment had no antagonists or fungicides. Treated fruit were incubated in cold storage at 4 °C for up to 2 months. Blue mould or gray mould disease incidence was recorded at monthly intervals. Results showed that the antagonist, Ps. fluorescens 4-6 alone or in combination with each of the three GRAS compounds gave good control for up to 29 days following the treatments, and after which disease incidence increased significantly. The antagonist Ps. fluorescens 4-6 gave similar disease reduction as the commercial biocontrol agent Ps. syringae in ‘McIntosh’ apple. In conclusion, of the three combination treatments tested, the antagonist Ps. fluorescens 4-6 and CaCl2 and antagonist Ps. fluorescens 4-6 and SBC were found most promising against postharvest blue mould and gray mould diseases on both apple cultivars for up to 29 days in storage. Further studies with different apple cultivars are underway.