Poinsettia (Euphorbia pulcherrima) is the biggest flowering potted-plant culture in Norway with approximately six million plants produced yearly. A considerable percentage is produced from imported cuttings. In September 2010, we received diseased poinsettias with necrotic leaf spots from a commercial poinsettia grower in Hordaland County. Leaf spots on the upper sides of leaves were brownish, necrotic, irregular in shape, and surrounded by yellow halos. Small, grayish brown, water-soaked spots were observed on the abaxial sides of the leaves from the same plants. Some of the latter carried crusty, dried residues of bacterial exudates. Leaves were surface sterilized and small pieces were excised from the transition area between healthy and diseased tissue. Leaf pieces were soaked in 0.2 ml of sterile phosphate buffered saline (SPBS) for 30 min. The resulting solution was diluted and streaked on several common media suited for the recovery of plant pathogenic bacteria, among them YDC (yeast dextrose chalk agar). The plates were incubated at 25°C in the dark. After 48 to 72 h, pale yellow, smooth, convex, round, and shiny colonies appeared on YDC. On the basis of plant symptoms and colony morphology, the isolated bacteria were expected to be Xanthomonas axonopodis pv poinsettiicola, which is a known pathogen of poinsettia. One isolate was analyzed by fatty acid methyl ester (FAME) analysis according to Sasser (2) and partial gyrase B sequencing as described by Ah-You et al. (1). A strain of X. axonopodis pv. poinsettiicola (NCPPB 581) from the National Collection of Plant Pathogenic Bacteria (UK) was included as a control in both analyses. The isolates were identical to NCPPB581 with respect to the FAME analysis (species level) and the gyrase B sequence. Furthermore, the gyrB sequence was identical to the sequence of strain LMG 849 in GenBank (Accession No. EU015342.1; identities = 774 of 774). Leaf inoculation of disease-free poinsettia was carried out by spraying a solution (approximately 108 CFU ml–1) on the leaves, covering the plants with wetted plastic bags, and placing the plants in a greenhouse maintained at 21°C for 4 weeks. Leaf spot symptoms consistent with the previously observed ones appeared after 2 weeks of incubation. No symptoms were observed on the negative control plant, which was sprayed with SPBS only. The bacterium was successfully reisolated from the induced symptoms and identified by FAME analysis and gyrase B sequencing. In the period following the first detection, Norwegian poinsettia growers were advised to inspect their produce. Suspected samples were sent to us from 28 producers from around the country. The pathogen was detected at 15 production places. Growers were recommended to disinfect their premises and be vigilant with respect to starting up the new season with healthy propagation material. To our knowledge, this is the first report of X. axonopodis pv. poinsettiicola causing bacterial leaf spot on poinsettia in Norway, providing further data on the occurrence of the disease in Europe.
References: (1) N. Ah-You et al. Int. J. Syst. Evol. Microbiol. 59:306, 2009. (2) M. J. Sasser. MIDI Tech. Note No. 102. MIDI, 115 Barksdale Prof. Center, Newark, DE, 1990.