In vitro Inhibitory Activity of Bacillus amyloliquefaciens Isolates against Botrytis cinerea Mycelial Growth and Determination of the Minimal Inhibitory Concentration

Gray mold disease, caused by Botrytis cinerea is a major postharvest disease impacting fruits such as strawberries. Bacillus species are promising agent for the biological control of postharvest diseases. Biological control agents at suitable population, should be able to effectively interact with pathogens to produce satisfactory disease control. Knowledge on the relationships between biocontrol agent and pathogen inoculum concentration can determine the population levels of the biocontrol agent required to achieve adequate disease control. This study aimed to determine the inhibitory effect of nine Bacillus amyloliquefaciens isolates from rhizospheric soil and roots of strawberries plants against Botrytis cinerea mycelial growth in vitro.   Nine bacterial isolates (I1, I2, I3, I18, B3, B24, B12, RA9, and RA12) were selected from rhizospheric soil and roots of healthy strawberry plants. The bacterial isolates at different concentrations (3x101 to 3x107 cfu/mL) were tested for their inhibitory activity against mycelial growth of Botrytis cinerea using the plate confrontation assay in a potato dextrose agar. The results showed that all the isolates inhibited Botrytis cinerea mycelial growth in a concentration-dependent manner, with isolates B3 and B24 exhibiting the most effective activity showing 50.68% inhibition at 3x103 cfu/mL (B3), and 31.91% inhibition at 3x101 cfu/mL (B24). The minimum inhibitory concentration (MIC) for both B3 and B24 was 3x105 cfu/mL.  The MIC for isolates I1, I2 and I18 was 3x106 cfu/mL, while the other isolates had MIC ≥.3x107 cfu/mL. These findings suggest that the two isolates B3 and B24 could serve as biocontrol agents for Gray mold.