Putative new species of the genus Dickeya as major soft rot pathogens in Phalaenopsis orchid production

Bacterial soft rots are a serious limitation to the production of orchids and other horticultural plants. Here, the characterization of causative bacteria isolated from Phalaenopsis orchids showing symptoms, from a commercial production site, is reported. The most commonly isolated bacteria were identified as Dickeya spp. Partial sequencing of 16S rDNA, fliC and dnaX showed diversity among the isolates and divided the isolates into two groups, with greatest similarity to previously reported undefined Dickeya lineages from orchids (UDL‐3 and UDL‐4). Two isolates (B16, S1) were sequenced using next‐generation sequencing, which has provided draft genomes of these two isolates for further studies (Ali? et al., 2015 ). Newly developed fliC‐based lineage‐specific quantitative real‐time PCR assays were used to distinguish among the lineages and to assess their relative abundances in diseased tissues. Virulence and aggressiveness comparison tests in vivo on Phalaenopsis orchids, potato plants and witloof chicory leaves indicated high virulence and extreme maceration potential of these novel Dickeya isolates, compared to a reference panel of other Dickeya spp. Pantoea cypripedii (formerly Pectobacterium cypripedii), which has previously been reported as a soft rot pathogen of orchids, was not detected, and isolates obtained from culture collections did not cause symptoms on artificially infected Phalaenopsis orchids.