First Report of Bacterial Soft Rot of Potato Caused by Pectobacterium aroidearum strains in China.

Potato (Solanum tuberosum L.) is a globally important staple crop, and China has contributed more than 20% of the world's production. In Guangdong Province, potato has become one of the most important winter crops, increased the farmer income. However, the potato were seriously affected by soft rot disease caused by Pectobacterium spp. in the past decade. In February 2024, typical symptoms of potato soft rot were observed on the plants of cultivar "Daxiyang" in Enping City, Guangdong Province, where the disease incidences ranged 3%-5% in the investigated fields. The stems adjacent to the tubes showed typical inky black symptoms, and the plants appeared yellow. The interior of the tubes appeared water-soaked and had developed to overall decay, with an evident smell. Eleven symptomatic plants were collected and used to isolate the causal agents. Pieces of tube tissue (about 5×5 mm) were removed between the rot-symptomatic and non-symptomatic margins, and surface sterilized using 75% ethanol for 30 s and 2% NaClO for 1 min. The pieces were rinsed in sterile water for three times, and then plated on Luria-Bertani (LB) medium agar for 36 h at 30°C. The produced colonies were selected for hypersensitive test on tobacco, and the colonies with a positive reaction were subcultured three times. Two representative strains, EP51-2 and EP51-3, were processed for gene sequencing, including house-keeping genes, danX, leuS, and recA (Portier et al. 2019). The gene sequences (GenBank accessions no. PP870934 to PP870939) of these two representative strains were matched with Pectobacterium aroidearum strain NCPPB 929 (CP166097.1) with identities ranging from 97.15 to 100% and the coverage of 100%. The sequences of three genes were concatenated and used for phylogenetic analyses, along with representative strains from 19 other Pectobacterium species. Phylogenetic analyses supported that EP51-2 and EP51-3 were grouped into P. aroidearum with the representative strains. Koch's postulates were applied to determine the pathogenicity of P. aroidearum strain EP51-2. According to the external inoculation method on potato stems (Czajkowski et al. 2010), each of six pots of healthy potato plants was inoculated at the basal stem with 100 µl of bacterial suspension (10⁸ CFU/ml), while six pots inoculated with LB served as controls. The plants were then kept at 30°C and maintained at 95% humidity. After 2 days, all six plants inoculated with the bacteria exhibited typical symptoms on the stems, which progressed to the collapse of the whole plant; the controls remained symptom-free during the observation. Single colonies reisolated from the symptomatic stem of the inoculated plants were confirmed using PCR and sequencing with recA primers as described above, and the causal agent was identified as P. aroidearum strains, fulfilling Koch's postulates. P. aroidearum has been reported on Chinese cabbage (Xie et al. 2018) and leaf mustard (Chu et al. 2024) in China, as well as on crops in the family Araceae, such as konjac (Wei et al. 2021), taro (Zhou et al. 2022), and Pinellia ternata (Du et al. 2024). To our knowledge, this is the first report of P. aroidearum causing potato soft rot in China. This pathogen has been prevalent in the taro fields in Guangdong Province and has caused severe losses. This report highlights an expansion of the host range for this pathogen. Attention should be focused on this newly emerging pathogen affecting potatoes, and immediate measures should be implemented to control its spread.