Management of black rot in cabbage by rhizospheric Pseudomonas species and analysis of 2,4-diacetylphloroglucinol by qRT-PCR

Abstract

Black rot, an important and potentially destructive bacterial disease of crucifers, is caused by Xanthomonas campestrispv campestris (Xcc). The intent of this study was to evaluate rhizospheric bacterial antagonists to suppress black rot disease in cabbage. Fifty-four rhizosphere bacteria were screened against virulent strain of Xcc. Three isolates (TO7, SA3 and CA9) with inhibition diameter >14mm were selected and studied further in greenhouse. In greenhouse study, strains were evaluated for their effect in reducing number of lesions, vein discoloration, necrosis and chlorotic lesions. The TO7 strain was most effective in controlling the disease symptoms than SA3 and CA9. Further, TO7 strain was tested in the field and was very effective in controlling black rot in cabbage, when applied through root dip method. Based on nucleotide homology and phylogenetic analysis TO7 was identified as nearest homolog of Pseudomonas fluorescens. This strain produced 2,4-diacetylphloroglucinol (2,4-DAPG) which was analyzed in different physiological conditions through qRT-PCR analysis of phlD gene, followed by its subsequent effect on antagonistic activity. The TO7 culture of mid log and extended stationary growth phase with shaking had a maximum phlD expression. Growth temperature and pH had shown direct effect on phlD expression with maximum production at 16°C and at alkaline pH 8.5. Various carbon sources influenced phlD production by different degrees while iron had no significant effect on mRNA expression of phlD. The study demonstrated effective biological control by TO7, thereby indicating the possibility of application of rhizobacteria producing 2,4-DAPG for control of black rot.