Isolation and characterization of black rot- causing bacteria of cabbage, and isolation of its bacteriophage as a biocontrol agent.

Abstract

Black rot caused by Xanthomonas campestris pv. campestris (Xcc) is one of the most destructive bacterial diseases of economically important Brassicaceae vegetables including cabbage. Available methods to control black rot in cabbage are only preventive and ineffective in controlling the disease once the plant is infected. Phage-mediated biocontrol is considered as a more promising, safe, and environmentally friendly method to control bacterial plant diseases. Therefore, this study was focused on isolating the causative bacteria of black rot of cabbage and isolating their bacteriophages that can be used as a biocontrol agent. Bacterial pathogens were isolated from cabbage leaf samples showing typical black rot symptoms using standard microbiological techniques. Two isolates, namely Xcc-1 and Xcc-2, with yellow-coloured, mucoid colonies were purified and preliminarily identified as Xanthomonas spp. using morphological and biochemical characteristics. The UV-visible spectrophotometric analysis of the methanolic extract of the yellow intracellular pigment produced by the bacterial isolates showed a maximum peak near 440 nm with shoulder peaks, which was characteristic of the Xanthomonadin pigment of Xanthomonas spp. The pathogenicity of the two bacterial isolates was confirmed by inoculating healthy cabbage leaves. Characteristic disease symptoms were observed after 10 days of incubation. Further, bacteriophages were isolated from diseased cabbage leaves using agar overlay method. The phage isolate Xcc-Ph1, formed visible plaques with both bacterial isolates Xcc-1 and Xcc-2. Although further studies are needed to confirm the biocontrolling ability of Xcc-Ph1, these findings provide a great initiative to develop a bacteriophage-based method to manage Black rot disease in cabbage in the future.