Bacillus subtilis strain BS-F3:: Colonization of pear organs and its action as a biocontrol agent

The experiments were conducted following the "Guideline for the Efficacy Evaluation of Bactericides - E. amylovora" of the European and Mediterranean Plant Protecton Organization. Bacillus subtilis BS-F3 strain gave the best relative protection (62%) against blossom infections, when applied prior to and after inoculation with the pathogen. The highest shoot protection level (95%) was obtained when the bacterial antagonist was applied as a suspension from lyophilized cells. A spontaneous rifampicin resistant mutant (Rif(r)) of BS-F3 strain was used to study bacterial population dynamics on flowers and leaves in a pear orchard under field conditions throughout the growing season. On flowers, viable bacteria were found till petal fall, reaching a population size of about 10(3) cfu/sample after 14 days. On leaves, the antagonist was still present at a high population level 10 days after initial application; thereafter, the population size decreased gradually, and at 150 days no live bacteria were detected on the recovery medium. BS-F3 strain was found to produce peptide antibiotic(s) with a wide-spectrum of action against E. amylovora strains from different geographic origin and different hosts. The timing of synthesis and release of antimicrobial compounds coincided with spore formation. None of the amino acids tested (asparagine, glutamic acid, aspartic acid, serine and alanine) were able to inactivate antimicrobial compounds produced by BS-F3.