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First Report of Leaf Blight Caused by Pantoea agglomerans on Rice in Korea

In September 2009, leaf blights were observed on rice (Oryza sativa L., variety Dongjin 1 and Hopyeong) in paddy fields located in Gwangyang and Naju, Jeonnam Province, Korea. Lesions appeared first as water-soaked stripes or light brown-to-slightly reddish spots on the upper blades of the leaves, ultimately causing leaf blight and stalk rot. Ten strains of bacteria were isolated from the blighted leaf samples and four isolates (EML-ORY1, -ORY2, -ORY3, and -ORY4) suspected to be Pantoea spp. were selected on the basis of colony types and sampling sites. The isolates readily grew at 27 to 32°C but growth was significantly lower at 35°C. Using the API 20E system, EML-ORY1, 2, and 3 showed the same reaction patterns and gave 15 positive reactions whereas EML-ORY4 gave 11 positive reactions, but results were negative for arginine dihydrolase, citrate utilization, sorbitol fermentation, and rhamnose fermentation. All strains were considerably different from Pantoea agglomerans ATCC27155, which produced nine positive reactions. The strains were identified based on the 16S rRNA gene sequence analyses. A neighbor-joining tree was generated for the four isolates using PHYLIP with the following known bacterial strains: P. agglomerans DSM3493; P. vagans LMG24199; P. eucalypti LMG24197; P. ananatis ATCC19321; and Kluyvera georgiana ATCC51603. The four isolates from rice formed a monophyletic cluster and were most closely related to P. agglomerans DSM3493 (GenBank AJ2334231) with an average 16S rRNA sequence similarity of 99.0%. GenBank Accession numbers for the four isolates are: EML-ORY1, HM854282; -ORY2, HM854283; -ORY3, HM854284; and -ORY4, HM854285. On the basis of molecular phylogenetic analyses and API 20E test, we determined that the causal pathogen might be a subspecies of P. agglomerans. Pathogenicity tests were performed on 2-week-old rice seedlings (variety Hopyeong) in duplicate with bacterial suspensions containing 1.5 × 10⁹ CFU/ml with 0.001% Tween 20. Of the isolates, EML-ORY3 demonstrated the strongest pathogenicity to rice seedlings when evaluated by five scoring systems on the basis of symptom development and severity levels. Disease symptoms appeared 3 days after artificial inoculation. Symptoms on the inoculated leaves were similar to those of natural infection and included water-soaked stems with a light brown color, blighted leaves, and stalk rot, with no symptoms found on water-treated controls. P. agglomerans, formerly called Enterobacter agglomerans (or Erwinia herbicola), is a group of gram-negative bacteria that belong to the family Enterobacteriaceae (3). Pantoea spp. are known to cause different diseases on a broad range of host plants including gypsophila, cotton, pineapple, maize, barley, onion, melons, and eucalyptus and also have been implicated as opportunistic pathogens in humans (1,2). P. agglomerans has been widely found in nature on leaves, fruits, and the seeds of many crops and is a known endophyte (1,2). To our knowledge, this is the first report of rice leaf blight caused by a putative subspecies of P. agglomerans in Korea. The importance of this pathogen to rice production in Korea is unknown.

References: (1) Y. Feng et al. J. Appl. Microbiol. 100:938, 2006. (2) S. Manulis and I. Barash. Mol. Plant Pathol. 4:307, 2003. (3) M. P. Starr. The genus Erwinia. Page 1260 in: The Prokaryotes: A Handbook on Habitats, Isolation and Identification of Bacteria. Springer-Verlag, NewYork, 1981.