First Report of Pectobacterium polaris Causing Soft Rot of Potato in Poland

Bacteria belonging to the genus Pectobacterium are causal agents of soft rot disease all over the world, resulting in severe economic losses (Toth et al. 2011). In Poland, P. atrosepticum, P. parmentieri, P. carotovorum subsp. carotovorum, P. carotovorum subsp. odoriferum, and P. carotovorum subsp. brasiliense have been isolated from symptomatic plants to date (Motyka et al. 2017). In 2017, P. carotovorum strains were reclassified, and a new species, Pectobacterium polaris, was described (Dees et al. 2017). Therefore, we tested over 250 isolates that had been collected from plants with soft rot symptoms since 1995 in Poland to determine if P. polaris was present. A multilocus sequence analysis (MLSA) based on five housekeeping genes (gyrA, recA, recN, rpoA, and rpoS) (Waleron et al. 2018) was utilized to taxonomically characterize isolates. Based on our results, we found that the five strains isolated from potato (IFB5220, IFB5222, IFB5225, IFB5226, and IFB5252), and one from bittersweet (IFB5223) previously identified as P. carotovorum subsp. carotovorum (Waleron et al. 2002), should be renamed as P. polaris. All isolates were gram negative, facultative anaerobes exhibiting pectinolytic activity and were negative for oxidase, urease, indole production, gelatin liquefaction, and acid production from d-arabitol, dulcitol, sorbitol, raffinose, and melibiose. All strains were unable to utilize malonate and citrate. They were catalase positive, produced acid from lactose, rhamnose, saccharose, xylose, and trehalose, and were tolerant to 5% NaCl. All strains exhibited enzyme activity of cellulase, protease, α-amylase, α- and β-glucosidase, as well as ornithine and lysine decarboxylase. All strains except for IFB5252 and IFB5225 grew at 37°C. Strain IFB5226 produced reducing substances from sucrose and acid from maltose, utilized α-methyl-d-glucoside. All strains isolated from potato and the strain isolated from bittersweet caused soft rot symptoms on potato tuber tissues. The mean diameter of rotting potato tuber tissue (12 mm) was measured after incubation in 28°C and 95% relative humidity for 72 h. Surface-sterilized potato tubers were inoculated by inserting a pipette tip containing 25 μl of bacterial suspension (2 × 10⁷ CFU/ml) 10 mm into each tuber. Three tubers were inoculated for each strain, and the inoculation was done in triplicate. As a control, sterile water was used. Sequence analysis of the partial 16S rRNA gene (1,435 bp) (MH166801 to MH166803, K