Bacterial Soft Rot of Oncidium Orchids Caused by a Dickeya sp. (Pectobacterium chrysanthemi) in Florida.

Oncidium orchids have been subjected to extensive cultivation in the pot-plant and cut flower industries because of their attractive and numerous flowers. In August 2008, approximately 50 Oncidium 'Gower Ramsey' orchids were discovered at a commercial orchid nursery in South Florida with brown, macerated leaves typical of soft rot disease reported in other orchids. Ten plants were selected, and sections were removed from the edge of symptomatic tissue and bacteria were isolated according to the method described by Schaad et al. (3). All isolates were gram negative, anaerobic, degraded pectate, grew at 37°C, produced blue-to-brown pigment on nutrient agar-glycerol-manganese chloride (NGM) medium (1), were sensitive to erythromycin, oxidase negative, and positive for phosphatase and indole production. Further analyses were performed on four of the isolates. MIDI analysis (Sherlock version TSBA 4.10; Microbial Identification, Newark, DE) identified the isolates as Erwinia chrysanthemi (SIM 0.880 to 0.929). Polymerase chain reactions were performed with the 16S primers 27f and 1495r (4) and 1,423 bp of the 16S rDNA gene showed 98 to 99% sequence identity to Pectobacterium chrysanthemi (GenBank Accession No. FM946179). Sequences were deposited in GenBank (Nos. HQ287572-HQ287575). Pathogenicity tests were performed by injecting 10 Oncidium 'Gower Ramsey' orchids with 100 μl of a bacterial suspension at 1 × 10⁸ CFU/ml. Ten plants were inoculated with 100 μl of sterile water as controls. Plants were placed in a greenhouse at 26.0°C to 30.0°C and 50 to 83% relative humidity. Soft rot symptoms were observed on all inoculated plants within 24 h while control plants appeared normal. A Dickeya sp. was reisolated and identified according to the method described above. Oncidium orchids are known to be highly susceptible to P. carotovora (= E. carotovora) and soft rot caused by P. carotovora is known to occur frequently on Oncidium orchids (2). Although, an Erwinia sp. has been reported to cause soft rot symptoms on Oncidium aureum, to our knowledge, this is the first report of a Dickeya sp. (= P. chrysanthemi) causing soft rot symptoms on Oncidium orchids grown in large-scale commercial production in the United States. References: (1) Y. A. Lee and C. P. Yu. J. Microbiol. Methods 64:200, 2006. (2) C. H. Liau et al. Transgenic Res. 12:329, 2003. (3) N. W. Schaad et al. Erwinia soft rot group. Page 56 in: Laboratory Guide for Identification of Plant Pathogenic Bacteria. 3rd ed. N. W. Schaad et al., eds. The American Phytopathological Society, St. Paul, MN, 2001. (4) W. G. Weisburg et al. J. Bacteriol. 173:697, 1991.