Efficacy of Pseudomonas fluorescens for control of Mucor rot of apple during commercial storage and potential modes of action.

The ability of Pseudomonas fluorescens isolates 1-112, 2-28, and 4-6, to control Mucor piriformis (Mucor rot) on Gala, McIntosh, Ambrosia, and Spartan apple cultivars in commercial cold storage and their possible mechanisms of action were investigated. Isolates 1-112 and 2-28 provided significant levels of disease control on McIntosh and Spartan apples, while isolate 4-6 provided control of Mucor rot on Gala and Spartan apples, compared with control fruits after 15 weeks of storage at 0 °C. Mycelial growth of M. piriformis was markedly inhibited by cell-free supernatant and volatile organic compounds produced by P. fluorescens isolates, in vitro. In filter-sterilized apple juice, living cells of all 3 P. fluorescens isolates or their metabolites significantly inhibited spore germination by 99.8% and 61.6%, on average, respectively. Electron microscopy indicated that all 3 isolates of P. fluorescens colonized the hyphae of M. piriformis, but only isolate 1-112 was observed to colonize M. piriformis spores in vitro. In the wounds of apple, all 3 isolates formed a biofilm on the fungal hyphae and on the fruit tissue. Potential mechanisms of antagonism utilized by P. fluorescens against M. piriformis may include competition for nutrients and space, production of inhibitory metabolites and volatiles, and biofilm formation, leading to inhibition of spore germination and mycelial growth.