First Report of Enterobacter mori and Serratia marcescens Causing Bacterial Wilt of Ginger in Ethiopia

Ginger (Zingiber officinale Rosc.) is an important spice crop in Ethiopia, valued for culinary and medicinal uses. However, production is constrained by several pathogens, including bacterial wilt, with disease incidence reaching up to 95%. Symptomatic plants showed yellowing, wilting, and rhizome rot, symptoms typical of bacterial wilt caused by R. solanacearum (Yu et  al., 2003). A field survey conducted during the 2022 cropping season (August - November) assessed the incidence of bacterial wilt in eight key production districts of the former Southern Nations, Nationalities, and Peoples’ Region, Ethiopia (5°00′ - 8°00′ N, 35°00′ - 39°00′ E). Ginger rhizomes were collected for pathogen isolation. Samples were surface sterilized in 1% NaOCl for 5 min, rinsed with sterile distilled water, and aseptically sectioned. Tissues from the margin between healthy and diseased areas were macerated and streaked onto nutrient agar, incubated at 28 - 30°C for 24 - 48 h. Colonies were purified and stored at -80 °C. Enterobacter isolates formed creamy-white colonies, whereas Serratia isolates produced reddish colonies. The 16S rRNA gene was amplified using primers 27F/1492R and sequenced (Li et al., 2010). BLASTn analysis identified 15 isolates as Enterobacter and 10 as Serratia. For species-level identification, the hsp60 and rpoB genes were analysed (Hoffmann & Roggenkamp, 2003; Miyoshi-Akiyama et al., 2013; Mollet et al., 1997). Based on hsp60 sequences, six isolates were identified as E. mori (PZ362820, PZ405498 - PZ405502; 287 bp) showed 98.3% similarity to PQ567000 and clustered in the same clade, while four S. marcescens isolates (PZ371310 - PZ371313; 300 bp) showed 100% similarity to KT992365 and grouped in the same clade. Similarly, rpoB analysis confirmed E. mori (PZ371300; 592 bp) with 99.3% similarity to OL771192 and OL771193 (PP = 0.53), while sequences PZ405491 – PZ405496 showed 97% similarity to OR555745 (PP = 1). S. marcescens isolates (PZ371302 - PZ371308; 758 bp) showed 99.7% similarity to KT992367 (PP = 0.99). Pathogenicity tests were conducted on tissue culture-derived variety “Boziab” ginger seedlings grown in sterilized sandy soil under greenhouse conditions. At the four-leaf stage, plants were inoculated at the stem base with 6 ml bacterial suspension (1 × 10⁸ CFU/ml) using a sterile syringe, while control plants received an equal amount of sterile distilled water, with three replicates per treatment. Symptoms were assessed weekly. Plants inoculated with E. mori and S. marcescens developed wilting symptoms consistent with those observed in the field, whereas control plants remained asymptomatic. Sequence analysis of the 16S rRNA, hsp60 and rpoB genes also confirmed the re-isolated bacteria as E. mori and S. marcescens. This contrasts with earlier reports attributing ginger bacterial wilt to R. solanacearum without molecular confirmation (Hunduma et al., 2016). This is the first report of E. mori and S. marcescens causing bacterial wilt of ginger in Ethiopia and Africa. This study expands the known diversity of pathogens associated with ginger wilt and underscores the importance of accurate pathogen identification. The practice of in situ storage of ginger planting material by farmers in the study area may contribute to increase the risk of bacterial disease outbreaks by prolonging exposure to soil-borne pathogens, highlighting the need for integrated disease management, including disease-free planting material and strict field sanitation.