Pseudomonas lactucae sp. nov., a pathogen causing bacterial rot of lettuce in Japan

Two phytopathogenic bacteria, MAFF 301380^T and MAFF 301381, isolated from rot lesions of lettuce (Lactuca sativa L. var. capitata L.) in Japan, were characterized using a polyphasic approach. The cells were Gram-reaction-negative, aerobic, non-spore-forming, rod-shaped and motile with one to three polar flagella. Analysis of the 16S rRNA gene sequences showed that the strains belong to the genus Pseudomonas and are closely related to Pseudomonas cedrina subsp. cedrina CFML 96-198^T (99.72 %), Pseudomonas cedrina subsp. fulgida P515/12^T (99.65 %), Pseudomonas gessardii DSM 17152^T (99.51 %), Pseudomonas synxantha DSM 18928^T (99.44 %), Pseudomonas libanensis CIP 105460^T (99.44 %) and Pseudomonas lactis DSM 29167^T (99.44 %). The genomic DNA G+C content was 60.4 mol% and the major fatty acids consisted of summed feature 3 (C_16 : 1  ω7c/C_16 : 1  ω6c), C_16 : 0 and summed feature 8 (C_18 : 1  ω7c/C_18 : 1  ω6c). Phylogenetic analysis using the rpoD gene sequences and phylogenomic analyses based on the whole genome sequences demonstrated that the strains are members of the Pseudomonas fluorescens subgroup but formed a monophyletic and robust clade separated from their closest relatives. The average nucleotide identity and digital DNA-DNA hybridization values between the strains and their closely related species were 88.65 % or less and 36.3 % or less, respectively. The strains could be distinguished from their closest relatives by phenotypic characteristics, pathogenicity towards lettuce and whole-cell MALDI-TOF MS profiles. The evidence presented in this study supports the classification of the strains as representing a novel Pseudomonas species, for which we propose the name Pseudomonas lactucae sp. nov., with the type strain MAFF 301380^T (=ICMP 23838^T).