First Report of Soft Rot Disease of Peach Caused by Pantoea ananatis in China.

In September 2014, the authors received peach fruit sealed in a shipping box, collected from a peach garden in Shanghai, China. Fruit were kept at room temperature for 7 days after harvest prior unsealing the shipping box. Upon opening, 25% of the fruit initially appeared water-soaked with brown pinpoint spots on the surface of the exocarp. Spots then enlarged into lesions with dark brown edges and coalesced after causing necrosis and collapse of the mesocarp tissues. Decaying fruit oozed liquefied mesocarp tissues when pressed. Lesions on the exocarp of five peaches were removed using sterilized tweezers, and 25 µl of liquid was sampled 1.5 cm into the mesocarp and was added to 500 µl LB medium. The inoculated LB medium was incubated at 28°C for 2 h, and then spread on LB agar. Eight representative bacterial colonies were obtained and selected for further characterization. Colonies were round, yellow, smooth, and translucent. Series of physiological tests were performed and indicated that all strains were gram negative, positive for indole production, and utilized glucose, sucrose, lactose, trehalose, pectinose and citrate, but not malonate, glycerol or salicin. Based on these tests, the strains were initially identified as Pantoea ananatis (Holt et al. 1994). In addition, the 16S rDNA was amplified by PCR using primers 27f and 1492r. Sequencing of the 16S gene revealed that it was 99% similar to that of P. ananatis strain LMG 20103 (CP001875.2). The additional three housekeeping genes including atpD, gyrB, and infB were amplified by PCR and sequenced using degenerate primers as described by Brady et al. (2008). All sequences of the eight strains were identical for each of the three genes. Phylogenetic trees based on the concatenated nucleotide sequences of these genes were built using MEGA6, and showed that strains were most similar to P. ananatis with 98% identity for atpD (KM853019), 100% for gyrB (KM675661), and 99% for infB (KM675662). To fulfill Koch’s postulates, 20 µl of washed-cell suspensions for two isolates (at 1 × 105 CFU/ml) and distilled water (control) were sprayed onto four peach fruit each. Fruit were then air dried and put into a 400 × 300 × 100-mm plastic moistened box. After incubating at 28°C for 5 days, similar water-soaked symptoms were observed on all the bacteria-treated fruits. The strains were reisolated from all the diseased tissues using the same method stated above and confirmed as P. ananatis by PCR and sequencing of atpD and gyrB. To our knowledge, this is the first report of P. ananatis causing soft rot disease on peach, which may influence the quality of harvested peach and result in economic loss during peach sales and the production of relevant peach products. © 2016 The American Phytopathological Society.