Effectiveness of an indigenous Bacillus subtilis B6 strain in the control of postharvest apple fruit rot

Antifungal activity of 108 Bacillus spp. isolates, originating from the rhizosphere of different cultivated plants, against several causal agents of apple fruit rot (Monilinia fructicola, Colletotrichum acutatum and Botrytis cinerea) were studied in vitro. Based on the results of dual culture and in vitro antifungal assays, a B6 strain was selected for further investigation. Molecular characterization of nucleotide sequences derived from PCR-amplified 16S rDNA fragments confirmed that the B6 strain belongs to the species Bacillus subtilis. In addition, PCR analysis showed that the strain genome included genes involved in the production of antimicrobial substances (iturin, bacillomycin and surfactin). Regarding the antimicrobial activity of B6 filtrates, the results indicated that competition for nutrients and antibiosis were the most probable modes of action. Based on the results obtained in in vivo experiments on apple fruits with demonstrated efficacy of 70-93%, three different liquid formulations were developed and their effectiveness was tested against M. fructicola. As the most efficient product, formulation 3 (SE formulation prepared with esterified rape seed and nonionic emulsifier) was chosen for further testing, and the results showed an efficacy ranging from 60 to 80%. In conclusion, this work demonstrated that the SE bioproduct based on B. subtilis strain B6 offers a potentially suitable approach for M. fructicola management on apple fruits.