Association of Acinetobacter baumannii with Soft Rot Disease of Carrot in India

Abstract

Soft rot disease of carrots is an important limiting factor of carrot production. In this study, carrot roots showing typical soft rot symptoms were identified in the fields, and diseased and healthy root samples were collected for pathogen identification. The pathogen was isolated using an enriched bell pepper method. The bell pepper developed a water-soaked lesion around the pricking region when it was pricked after stabbing the diseased root whereas, no symptoms were produced when bell pepper was pricked after stabbing a healthy carrot root. From samples of the infected roots, circular, whitish, smooth, mucoid, round, convex, and medium-sized colonies were formed on the nutrient agar medium and were morphologically identified as Acinetobacter spp. Pure culture for four isolates was obtained, and one of the isolates (AB1) was further subjected to 16S rDNA sequencing. The BLAST analysis of the 16S rDNA confirmed the identity of AB1 as Acinetobacter baumannii. Pathogenicity test using whole-root assay and slice assay proved AB1 as pathogenic on carrot by producing water-soaked lesion, maceration, and rotting symptoms, whereas water inoculated roots remain healthy. The rotting symptoms on the artificially diseased carrot roots were similar to those caused by Pectobacterium caratovorum and Klebsiella variicola on the carrot. Based on the colony morphology, biochemical tests, and 16S rDNA sequence identity followed by pathogenicity assays, it is evident that A. baumannii causes soft rot disease in carrots. This report is essential for developing specific diagnostics and management against this newly emerging bacterial pathogen of carrot.