First Report of radish tubers rot caused by Enterobacter asburiae in China

Radish (Raphanus sativus L.) is a widely consumed vegetable in China. However, radish is susceptible to diseases, which limits its yield and development in Harbin, China. In September 2021, rotten white radish tubers were observed in the field. The incidence of this disease reached 70% in October 2021, which led to huge economic losses (i.e., 30%–40%). Water-soaked lesions appeared on the radish tubers and appeared brown–yellow, which looked similar to ginger tuber rot caused by Enterobacter asburiae (Zhang et al. 2020). The interior was rotten with no considerable smell. Over time, the lesions gradually spread into all tubers of radish. Small square pieces of radish (0.5 cm × 0.5 cm) were excised from the junction of diseased and healthy tuber, disinfected with 75% alcohol, and washed three times with distilled water then ground to prepare tissue suspensions for plating. Under 28 ℃ for 16h, single colonies were isolated from the beef extract culture medium. Single colonies appeared oval, white, and smooth, with bright and slightly raised surfaces, and with moist neat edges. Gram-negative bacterial strain CCGL 988 was obtained, with an average size of 1–2 µm × 0.5–1 µm, and 3–4 flagella. Physiological and biological test results showed that strain CCGL 988 produced acid utilizing sucrose, glucose, maltobiose, D-Sorbitol, and mannitol; negative for Voges-Proskauer, methyl red, malanate, ornithine decarboxylase, arginine decarboxylase, and lysine decarboxylase. According to the results, strain CCGL 988 was identified as Enterobacter asburiae (Hoffmann et al. 2005). The 16S rDNA region of the strain was amplified using PCR with 27F/1492R primers (López et al. 2019), and partial gyrB, atpD, rpoB genes were amplified according to Zhang et al. (2020), infB gene was amplified with primers (F:TCAATGCGTGCTCGTGGTGCTC; R: TCGATACAGTGCCACTTCACG). The 16S rDNA, gyrB, atpD, rpoB and infB sequences were deposited in GenBank under accession numbers: ON999069, OP006448, OP006449, OP006450, and OP542231, respectively. These five sequences shared 99.80%, 100%, 100%, 100% and 100% of identity with E. asburiae (GenBank Accession: NO. CP011863). Maximum-likelihood phylogenetic tree clustered CCGL 988 with E. asburiae (MEGA7, bootstrap n = 1,000). Strain CCGL 988 was able to produce pectate lyases, polygalacturonases, cellulases, proteases, and extracellular polysaccharide using the methods described by Hugouviex-Cotte-Pattat et al. (2014), and Condemine et al. (1999). Koch’s postulates were conducted by inoculating 20 µl of the bacterial suspension (108 CFU/ml) on the needle wound on the surface of six healthy radish tubers; six radish tubers incubated with sterile water were negative controls. Radish tubers were incubated at 28 ℃ with 80% humidity. The inoculated radish was slightly rotten after 7 days. Water-soaked lesions with light yellow were initially observed; after 12 days, the lesions expanded gradually and appeared deep yellow. No symptoms were found in the control radish. This experiment was carried out three times, each time with three replications. The bacterium was reisolated from infected radish tuber and was confirmed to be E. asburiae by the same molecular and morphological characterization as described above. This study is the first report of E. asburiae causing radish tuber rot in China. It serves as a basis for future studies to develop management strategies for the disease to prevent radish yield loss.