First Report of Wilt Disease of Ginger Caused by Achromobacter xylosoxidans in China

Abstract

Ginger (Zingiber officinale) is an important commercial crop that has been widely cultivated in China for more than 2500 years. One variety, Tongling white ginger, has been grown in the Yi'an District of Tongling city, Anhui province (30°45 N, 117°43 E), China. In August 2022, symptoms of yellowing and wilting were observed on ginger plants, with a disease incidence rate exceeding 20% in the field. The stem base of the diseased plants became soft, brown and water-soaked. Additionally, the rhizomes displayed symptoms of browning and water-soaked rot, resembling those caused by Ralstonia solanacearum and Enterobacter cloacae (Yu et al. 2003; Nishijima et al. 2004; Liu et al. 2021). Consequently, ginger bacterial wilt disease may potentially emerge from a combination of infections by diverse pathogenic bacteria. To identify novel pathogens causing the wilt disease, stem tissues of the diseased plants from different locations were sterilized with 1% sodium hypochlorite (NaOCl) for 10 min, followed by at least three time rinses with sterile water. The sterilized samples were then ground with 0.9% saline solution and left at room temperature for 30 min. A 20 μL aliquot of the suspension was serially diluted and cultured on Luria-Bertani (LB) medium at 28°C. A total of 217 isolates was picked and purified for taxonomic identification by 16S rRNA gene analyses with the primer 27F/1492R (Weisburg et al. 1991). Among these isolates, 23 single colony isolates were identified as A. xylosoxidans through NCBI BLASTn analyses. Furthermore, three single isolates from different locations were randomly selected for further experiments. The growing colonies appeared opaque white and round. Microscopic evaluation revealed that cells were rod-shaped with an average length of 1.95 µm and average width of 0.46 µm. The three isolates shared identical 16S rRNA sequences, displayed 99.72% identity with the sequence from A. xylosoxidans strain SeqID2 (GenBank accession NO. MH266081.1). The glutamate synthase (gltB), GTP-binding membrane protein (lepA), NADH:ubiquinone oxidoreductase subunit L (nuoL), RNA polymerase beta-subunit (rpoB), and the enolase (eno) genes of the three isolates were amplified by PCR using primer pairs gltB-F/gltB-R, lepA-F/lepA-R, nuoL-F/nuoL-R, rpoB-F/rpoB-R and eno-F/eno-R, respectively (Spilker et al. 2012; Vandamme et al. 2016). The gene sequences of 16S rRNA (OQ711945, OQ740153 and OR230037), gltB (OR242732, OQ737692 and OR262112), lepA (OR233727, OQ737693 and OR262113), nuoL (OR233726, OQ737694 and OR262114), ropB (OR233725, OQ737695 and OR262115) and eno (OR242733, OQ737696 and OR262116) for the isolates ZOR02, ZOR05 and ZOR12 were deposited in GenBank database. The gltB, lepA, nuoL, rpoB and eno sequences of the isolates ZOR02, ZOR05 and ZOR12 showed 98.66-99.16%, 98.9-100%, 96.28%-97.34%, 98.47-99.44% and 99.27-99.82% similarity to A. xylosoxidans strain AX27, respectively. Phylogenetic trees were constructed based on the 16S rRNA and gltB-lepA-nuoL-rpoB-eno multilocus using the Neighbor-Joining (NJ) method with 1000 bootstrap replicates in MEGA11.0 software (Álvarez et al. 2018). For pathogenicity tests, bacterial suspensions were initially prepared in sterile water at a final concentration of 108 CFU mL-1. Subsequently, 10 μL of bacterial suspensions was injected into the stem base of two-month-old ginger plants, while sterile water was used as a control (Wang et al. 2022). These plants were then incubated at 28°C and 70% relative humidity. There were three replicates for each treatment, and each replicate contained five plants. After six days of inoculation, the ginger plants injected with bacterial suspensions alone exhibited severe wilting symptoms similar to those observed in the field. However, water-soaked symptoms were not observed on rhizome tissues from the pathogen-infected plants. Bacterial pathogens were re-isolated from the diseased plants and identified using the morphological and molecular methods to meet Koch's hypothetical tests. To our knowledge, this is the first report of A. xylosoxidans causing wilt disease of ginger in China. In 2022, the average yield loss due to wilt disease in the Yi'an District of Tongling exceeded 20%, posing a major threat to local ginger cultivation. Effective disease management strategies are needed to develop for the control and prevention of the disease.