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Biological control of common postharvest diseases of apples with Pseudomonas fluorescens and potential modes of action Wallace, Rhiannon Louise


Postharvest diseases are a serious issue faced by the pome fruit industry worldwide. Three major postharvest fungal pathogens, Penicillium expansum, Botrytis cinerea, and Mucor piriformis, commonly infect and rot apples in storage in British Columbia, Canada. Fungicides have been applied extensively to reduce postharvest loss, but pathogen resistance is emerging and public pressure to reduce fungicide use has led to increased research for safer alternatives such as biological control agents (BCAs). Three isolates of Pseudomonas fluorescens, 1-112, 2-28, and 4-6, isolated from the rhizosphere of pulse crops in Western Canada, were studied as potential BCAs under commercial cold and controlled atmosphere (CA) storage with apple varieties ‘Gala’, ‘McIntosh’, ‘Ambrosia’, and ‘Spartan’. Disease incidence and lesion diameters of apples inoculated with each of the three pathogens and biological control strains were determined after 15 weeks in commercial cold storage or varying periods of time in CA storage and compared with the fungicide Scholar® (fludioxonil) and the BCA BioSave® (Pseudomonas syringae). On apples, among the isolates of P. fluorescens tested, isolates 1-112 and 4-6 provided the best control of postharvest disease. Of all the biological and chemical treatments tested, Scholar® consistently performed the best, but when P. fluorescens isolate 4-6 was combined with sodium bicarbonate (SBC), the biological control activity of the antagonist was comparable to the fungicide. Overall, the efficacy of the P. fluorescens isolates varied with pathogen, apple variety, and storage environment. In dual culture and volatile tests, all three isolates or their metabolites significantly inhibited the mycelial growth and spore germination of P. expansum, B. cinerea, and M. piriformis in vitro. Scanning electron microscopy (SEM) indicated that all three P. fluorescens isolates adhered to the fungal hyphae of P. expansum, B. cinerea, and M. piriformis in vitro and in vivo, and colonized the wounds of apples. The ability of P. fluorescens to compete for nutrients and space, form a biofilm and produce inhibitory metabolites that target spore germination and mycelial growth may be the basis for its biological control capabilities. Collectively, these results suggest that P. fluorescens has potential to control common postharvest fungal pathogens during commercial storage.

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