First Report of Bacterial Leaf Spot on Sweet Potatoes Caused by Acinetobacter seifertii in China.

Abstract

Sweet potato (Ipomoea batatas (L.) Lam), a globally essential yet underutilized staple crop, is primarily cultivated in Asia, Africa, and Latin America. In 2021, China alone accounted for 53% of global production, yielding 47 million tons and supplying essential nutrients such as proteins, minerals, and provitamin A carotenoids (Li et al. 2024). In June 2024, a new bacterial leaf spot disease was identified in a sweet potato nursery in Hengzhou City, Guangxi Province, China, with an incidence exceeding 50%. Early symptoms appeared as brownish-black necrotic spots, which progressed to widespread yellowing of stems, veins, and leaves, ultimately leading to tissue blackening and plant death. To isolate the causal agent, 12 tissue samples, including leaf blades and stalks from six symptomatic plants, were surface-sterilized (75% ethanol for 45 s, 2% NaClO for 1 min) and rinsed three times with sterilized water. A 10-fold serial dilution of homogenized tissue was prepared, and 100 μL of suspension was plated onto nutrient agar (NA) medium and incubated at 30°C for 48 h in the dark. A predominant colony type was found on purified NA, and a representative isolate, designated J-5, was selected for further study. The colonies were small, round, smooth, and white. Biochemical tests identified the isolate as gram-negative, methyl red positive, catalase positive, Voges-Proskauer negative, nonmotile, and highly salt-tolerant. The 16S rRNA gene was amplified and sequenced using primers 27F and 1492R (Lane 1991). BLAST analysis of the sequence against the NCBI database showed a 99.79% identity with Acinetobacter seifertii (GenBank accession no. OK178959.1). Additionally, partial sequences of four housekeeping genes (dnaK, fyuA, gyrB, and rpoD) were amplified and sequenced (Young et al. 2008), with sequences deposited in GenBank (16S rRNA: PQ638888; dnaK: PQ757048; fyuA: PQ757047; gyrB: PQ757043; rpoD: PQ757045). BLAST analysis confirmed a 99.15%-99.93% identity and 98%-100% coverage with A. seifertii. Pathogenicity was assessed by spraying a bacterial suspension (10⁸ CFU/mL) onto slightly scratched stems of 10 healthy sweet potato seedlings. A control group of 10 healthy sweet potato seedlings was treated with sterile nutrient broth. Seedlings were maintained in a greenhouse at 25°C with 70% relative humidity for 7 days. At seven days post-inoculation, inoculated plants developed symptoms identical to those observed in the nursery, while control plants remained asymptomatic. The pathogen was successfully reisolated from necrotic tissues and confirmed as A. seifertii through amplification in the polymerase chain reaction assay and sequencing of the 16S rRNA gene. A. seifertii, a species within the genus of the bacterium known for its ecological versatility in soil and water, plays a significant role in biodegradation (Budkum et al. 2022; Yang et al. 2016). To the best of our knowledge, this is the first report of A. seifertii that causes bacterial leaf spot on sweet potato (Ipomoea batatas (L.) Lam) in China. This finding is expected to contribute to the research on the management of this newly emerging disease.