Biological Control Efficacy of Indigenous Antagonistic Bacteria Isolated from the Rhizosphere of Cabbage Grown in Biofumigated Soil against <i>Pythium aphanidermatum</i> Damping-Off of Cucumber

Soil biofumigation is a widely used farming practice for soil-borne disease management in vegetable crops. Soil biofumigation is the practice of the incorporation of fresh plant materials from the members of the Brassicaceae family into the soil which release antimicrobial volatile organic compounds (VOCs), namely glucosinolates and their hydrolysis products. However, the VOCs may affect non-target beneficial soil biota including microbial biocontrol agents (MBAs) due to their broad-spectrum toxicity. In this study, native antagonistic rhizobacteria were isolated from cabbage plants grown in biofumigated fields and their potential in the management of <i>Pythium aphanidermatum</i> cucumber damping-off was evaluated with and also without biofumigation using cabbage leaf residue. A total of 20 morphologically distinct, culturable bacterial strains were recovered from the rhizosphere soil of cabbage cultivated in a biofumigated field, with the largest fraction of the isolates belonging to the genus <i>Bacillus</i>. The other bacterial genera isolated were <i>Pseudomonas</i>, <i>Serratia</i>, <i>Stenotrophomonas, Microbacterium</i> and <i>Priestia</i>. Of the 20 bacterial isolates, <i>Pseudomonas aeruginosa</i> B1-SQU, <i>Pseudomonas indica</i> B2-SQU, <i>Serratia marcescens</i> B3-SQU and <i>Pseudomonas brenneri</i> B4-SQU exhibited inhibitory activity against <i>P. aphanidermatum</i> in a co-culture assay. The deformation and shrinkage of <i>P. aphanidermatum</i> hyphae due to diffusible antimicrobial compounds from these bacterial strains were witnessed through scanning electron microscopy. A biofilm was formed by these bacterial strains in vitro and they exhibited compatibility with each other; however, they had no significant plant growth promotion effect on cucumber. These bacterial strains significantly reduced damping-off in cucumber under greenhouse conditions when applied to soil singly, but failed to show a significant effect when combined with biofumigation treatment, though the bacterial population in the soil was maintained at higher levels. Soil biofumigation alone was also not effective in suppressing the damping-off of cucumber. Gas chromatography–mass spectrometry analysis revealed that 1-butanol, n-butyl butyrate and butyl acetate were the predominant VOCs in the soil amended with cabbage leaf residue. The results suggest that soil biofumigation with cabbage leaf residue has no significant effect on <i>P. aphanidermatum</i> under high inoculum levels; furthermore, it affects the efficiency of the bacterial antagonists in controlling damping-off in cucumber.