Identification and characterization of pathogenicity of Lelliottia nimipressuralis causing soft rot of Codonopsis pilosula (dangshen) roots in China

The roots of Codonopsis pilosula, a crucial ingredient in Chinese medicine, are prone to soft rot disease soon after transplanting seedlings in the second year. This almost always causes severe losses in yield at harvest, with severe economic consequences for farmers of this perennial vine. However, the pathogens involved remain understudied, although some gram-negative bacteria of the Enterobacteriaceae family are well-known pathogens of soft rot in many other plants. Here, the isolate B5-2-1-30 was suspected of being a key pathogen of C. pilosula in China according to counts of bacteria from diseased, healthy and adjacent tissues of roots. Results of morphological and phylogenetic analyses, including microscopic observations, in addition to a multilocus sequence analysis (MLSA) of the concatenated partial 16S rDNA, rpoB, atpD and gyrB genes, revealed that this isolate belongs to Lelliottia nimipressuralis. In vitro and in vivo pathogenicity tests showed that L. nimipressuralis B5-2-1-30 could cause root soft rot in seedlings of C. pilosula, with symptoms similar to those observed in the field. Further experiments suggested the typical symptoms of softened rotting roots and wilting shoots arose from phloem necrosis and vessel plugging, as inferred by observed anatomical characteristics of roots, providing clues for elucidating the infection mechanism of phytopathogens in L. nimipressuralis.