Biological control of Botrytis cinerea, Penicillium expansum, and Mucor piriformis on 'Gala' and 'McIntosh' apples using Pseudomonas fluorescens strains

Postharvest disease is a serious issue faced by the pome fruit industry worldwide. Three major postharvest fungal pathogens, Penicillium expansum, Botrytis cinerea, and Mucor piriformis, commonly infect apples and cause rot during storage in British Columbia, Canada. Fungicides have been applied extensively to reduce postharvest losses, but pathogen resistance is emerging and public pressure to reduce fungicide application has led to an increase in research for safer alternatives such as biocontrol agents (BCA). Three strains of Pseudomonas fluorescens 4-6, 1-112, and 2-28, isolated from the rhizosphere of pulse crops in western Canada, were studied as potential BCA under commercial cold and controlled atmosphere (CA) storage with two apple cultivars, 'Gala' and 'McIntosh'. Percent infection of apples inoculated with each of the three pathogens and biocontrol strains was determined after 15 weeks in commercial cold storage or 17-22 weeks in CA storage and compared with the fungicide Scholar® (fludioxonil) and the BCA Bio-Save® (P. syringae). All three isolates significantly inhibited the growth of B. cinerea, P. expansum, and M. piriformis in vitro. Efficacy of P. fluorescens strains varied with target pathogen, apple cultivar, and storage conditions. Percent infection by P. expansum and M. piriformis was lower in 'Gala' and 'McIntosh' apples treated with P. fluorescens strain 4-6 and stored in CA than for those in cold storage. Strains 1-112 and 2-28 decreased the percent infection caused by B. cinerea on 'Gala' apples in CA storage when compared with those in cold storage. 'McIntosh' apples, which had higher titratable acidity than 'Gala', exhibited greater percent infection than 'Gala'. Strain 4-6 showed the most consistent efficacy against all three pathogens and on both apple cultivars. The disease control was comparable to Bio-Save® but less effective than Scholar®. These results suggest that P. fluorescens has the potential to control common postharvest fungal pathogens during cold and CA storage.