Phyllospheric bacteria isolated from broccoli (Brassica oleracea L. var. italica) as biocontrol agents against Ralstonia solanacearum and as plant growth promoting bacteria

Abstract

Importance of the work: Phyllospheric bacteria play a crucial role in shaping plant microenvironments
and offer sustainable alternatives to synthetic fertilizers and pesticides. The increasing environmental
concerns associated with agrochemicals highlight the need to explore plant growth-promoting
bacteria (PGPB) for enhancing crop productivity and disease resistance. One major phytopathogen,
Ralstonia solanacearum, causes bacterial wilt and greatly impacts a wide range of crops.
Objectives: 1) To characterize bacterial isolates from the broccoli phyllosphere; 2) to evaluate their
antagonistic activity against R. solanacearum; 3) to assess their plant growth-promoting traits; and
4) to identify the most promising isolate using 16S rRNA gene analysis.
Materials and Methods: Bacterial isolates were obtained from broccoli phyllosphere samples and
screened for antagonistic activity against R. solanacearum using the disc diffusion method. Plant
growth-promoting traits were evaluated based on nitrogen fixation, phosphate solubilization and
indole-3-acetic acid (IAA) production assays. Molecular identification of the selected isolate was
performed using 16S rRNA sequencing.
Results: In total, 10 bacterial isolates were obtained, among which isolate FB5 had the highest
antagonistic activity, with an inhibition zone diameter of 16.37 mm. FB5 tested negative for
nitrogen fixation but was positive for phosphate solubilization and IAA production. Molecular
identification revealed 91.43% similarity to Bacillus velezensis, indicating that the isolate belonged
to the genus Bacillus.
Main finding: Bacillus sp. FB5 had strong antagonistic activity against R. solanacearum and
displayed key plant growth-promoting traits, highlighting its potential as a biocontrol agent and
biofertilizer for Solanaceae crops such as potato, tomato and chili.