First report of Pectobacterium carotovorum and Pectobacterium brasiliense causing bacterial soft rot of bok choy in Taiwan

Bok choy ( var. ) is one of the most popular leafy green vegetables in Asia (Wang et al. 2019; Zhang et al. 2014). In May 2022, disease resembling bacterial soft rot was observed in a commercial greenhouse located in Xiluo, Yunlin County, Taiwan. Affected plants exhibited maceration, primarily close to the base of the plants (Fig. S1). Almost all bok choy plants (about 1,800 plants in total) on site were symptomatic. Macerated tissues were collected from six plants. The samples were homogenized in 10 mM MgCl and bacteria were isolated on nutrient agar (NA) by streak plating. After 1 day of culturing at 28°C, creamy-white, round colonies were consistently grown on all the plates, and six strains (Br1 to Br6) were obtained; each isolated from a different plant. The strains were able to ferment glucose and induced maceration on potato tuber slices (Schaad et al. 2001) but could not produce indigoidine on NGM medium (NA added with glycerol and MnCl ; Lee and Yu 2006). The DNA samples of these strains were tested with -specific primers Y1 and Y2 (Darrasse et al. 1994) and all samples produced the expected amplicon. To identify the isolated pathogens, 1,592-bp sequences concatenated from fragments of the (452 bp), (492 bp), and (648 bp) genes (GenBank accession nos. OP360013-OP360021) were obtained for each strain as previously described (Portier et al. 2019). Three genotypes were detected, the sequences of strains Br1, Br2, Br4, and Br5 were identical, while strains Br3 and Br6 each belong to a different genotype. The sequence identity between Br3 and Br6 was 98.2%. The concatenated sequences ( - - ), along with those of type strains from known species, were subjected to maximum likelihood analysis. The reconstructed trees showed that strains Br1, Br2, Br4, and Br5 grouped with . CFBP2046 (Fig. S2); the sequence identity between the isolated strains and the type strain was 98.7%. Strains Br3 and Br6 clustered with . CFBP6617 (Fig S2); the sequence identity between CFBP6617 and Br3 and Br6 were 97.5% and 98.4%, respectively. The six strains were inoculated onto 55-day-old bok choy plants using previously described prick inoculation methods (Wei et al. 2019). Autoclaved toothpicks, each carrying 9.3 x 10 - 5.6 x 10 cfu of bacteria, were used to inoculate the base of plant leaves. All six strains were tested, and each strain had three replicates. Plants in the control group were stabbed with bacteria-free toothpicks. The plants were enclosed in clear plastic bags