Biological control of apple gray mold caused by Botrytis mali with Pseudomonas fluorescens strains

Two hundred and ten Pseudomonas spp. strains were obtained from leaf surfaces and apple fruit using serial dilution methods on semi-selective King B medium. These strains were screened in vitro against Botrytis mali, a casual agent of gray mold of apple. B. mali was originally described in the state of Washington and has been recently revived in British Columbia, Canada. When compared to Botrytis cinerea it differs by producing smaller sclerotia and has a different DNA sequence on the β-tubulin gene. Ten promising Pseudomonas fluorescens strains from the 210 Pseudomonads were selected and evaluated as potential biocontrol agents for control of gray mold on apple in vitro and in vivo. Dual culture, cell free metabolite and volatile tests showed that all 10 strains of P. fluorescens inhibited growth of B. mali. Inhibition varied from 49% to 68% in the dual culture tests; 75–99% in the cell free metabolite tests; and 52–97% in the volatile tests. Apple fruit wounds were inoculated with 20 μL of a bacterial suspension (1011 CFU L−1) of P. fluorescens followed 48 h later by B. mali (108 conidia L−1). The apples were then incubated at 20 °C for 20 d or at 5 °C for 25 d. All 10 fluorescent Pseudomonads appeared to be good antagonists of gray mold on apple at 20 and 5 °C. P. fluorescens strain Pf1 appeared to be the best biocontrol agent preventing B. mali from expanding to no more than 0.6 cm2 compared to 14.5 cm2 for the control at 5 °C.