Bacterial Stem Rot of Oncidium Orchid Caused by a Dickeya sp. (ex Pectobacterium chrysanthemi) in Mainland China.

In December 2007, stem rot symptoms on orchids (Oncidium Gower Ramsey) were observed at a flower nursery in the Zhejiang Province of China. Initial symptoms were water-soaked lesions starting at the base of the stem. As these lesions expanded and elongated, the stem and leaf tissues became soft and watery. When examined with a microscope, cut edges of symptomatic stem and leaf tissues consistently exhibited bacterial streaming. The bacteria were isolated by streaking on nutrient agar (3). All isolates were gram-negative, facultative, anaerobic rods with peritrichous flagella. Infiltration of tobacco leaves (Nicotiana tabacum cv. Samsun) with the bacterial suspension of 10⁸ CFU/ml resulted in typical hypersensitivity reactions within 24 h. Five representative isolates were further characterized by the Biolog Microbial Identification System, version 4.2 (Biolog Inc., Hayward, CA) and gas chromatography of fatty acid methyl esters (FAME) by the Microbial Identification System (MIDI Inc., Newark, DE) with aerobic bacterial library (TSBA50). The five isolates were identified as Erwinia chrysanthemi (Pectobacterium chrysanthemi) with a Biolog and FAME similarity index of 0.81 to 0.88 and 0.62 to 0.75, respectively. The transfer of P. chrysamthemi to a novel genus, Dickeya gen. nov., was recently proposed (2). The almost complete 16S rDNA sequence from Oncidium isolate SCH-01 (1,604 bp; EMBL Accession No. FM946179) was determined according to the method of Li et al. (1). A subsequent GenBank search showed that this isolate is 98% identical to that of type strain CFBP 1269T of Dickeya dadantii (EMBL Accession No. AF520707) and CFBP 1200T of Dickeya dianthicola (EMBL Accession No. AF520708). Nevertheless, species identification within genus Dickeya is still difficult since only a limited number of strains of each species have been characterized fully. Koch's postulates were completed with the inoculation of Oncidium seedlings with cell suspensions (10⁸ CFU/ml) by a pinprick at the base of the stem. All five representative isolates induced stem rot similar to that observed in natural infections. No symptoms were noted on the control plants inoculated with sterilized distilled water by the same method. The bacterium was reisolated from symptomatic stems of Oncidium plants. To our knowledge, this is the first report of stem rot on Oncidium orchid in Mainland China caused by the bacterium formerly referred to as P. chrysanthemi, now proposed as Dickeya sp. References: (1) B. Li et al. J. Phytopathol. 154:711, 2006. (2) R. Samson et al. Int. J. Syst. Evol. Microbiol. 55:1415, 2005. (3) N. W. Schaad et al. Laboratory Guide for Identification of Plant Pathogenic Bacteria. 3rd ed. The American Phytopathological Society. St. Paul, MN, 2001.