Antagonistic potential of Bacillus amyloliquefaciens and B. velezensis as biocontrol agents against Lasiodiplodia theobromae

Abstract

ABSTRACT Various fungi are known to cause important plant diseases leading to significant losses in agricultural products. To prevent crop diseases, biocontrol of these phytopathogenic fungi is necessary, most importantly by replacing the application of harmful pesticides from environmentally friendly management practices. The aim of this study was to isolate bacterial strains from soil and longan fruit with the capacity to inhibit Lasiodiplodia theobromae, a cosmopolitan fungal pathogen. Of 114, only four bacterial strains (3.51%) displayed significant inhibitory activity against L. theobromae by dual culture method. Strains CMU2 and RS5 were the most prominent isolates in controlling the tested pathogen and based on 16S rRNA gene sequence, they were identified as Bacillus velezensis and B. amyloliquefaciens, respectively. Cell-free supernatants (CFSs) of both antagonistic bacteria were prepared and found to be active against all tested L. theobromae strains. The optimum pH and temperature for antifungal activity of both CFSs were 7 and 30°C. In addition, conducted experiments on UV exposure and proteinase treatments did not affect the CFS’s antifungal activity. Characterisation of the chemical constituents using gas chromatography-mass spectrometry revealed the presence of various metabolites exhibiting antifungal activity. Furthermore, the in vivo bioassays also exhibited a significant impact on the banana rot lesion size, although it should be noted that the disease inhibition depends on the application procedure. In this study, the reduction of the disease development in bananas was most efficient when the antagonistic bacteria were applied before and/or at the same time with the fungal inoculation, emphasising the preventive measure. Our results demonstrated the effectiveness of B. amyloliquefaciens (CMU2) and B. velezensis (RS5) as the biological control agents. These findings would be useful in developing and implementing the management of L. theobromae pathogen.