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First Report of Xanthomonas axonopodis pv. poinsettiicola Causing Bacterial Leaf Spot of Euphorbia pulcherrima in Slovenia

In September 2009, water-soaked spots, 2 mm in diameter, surrounded by a pale yellow halo were observed on leaves of pot-grown poinsettia plants (Euphorbia pulcherrima L.) cv. Christmas Feeling in a commercial greenhouse in Slovenia. Several spots per leaf developed on 10% of 84 plants used for propagation and slowly progressed to necrotic brown spots. While all plants were watered by overhead irrigation until mid-September, and afterward by flooding, no symptoms were observed on parent plants of four other separately grown cultivars. Propagated cuttings were all grown together, and in addition to cv. Christmas Feeling, an estimated 90% of 315, 35% of 29, 10% of 240, and 5% of 840 plants of cvs. Crazy Marble Star, Crazy Christmas, Lemon Snow, and Cortez Red, respectively, developed leaf spots. Yellow, smooth, and butyrous colonies with entire margins were isolated from symptomatic leaves of poinsettia parent plants of cv. Christmas Feeling on yeast peptone glucose agar (YPGA). They were identified as a Xanthomonas sp. based on biochemical tests (oxidase negative, positive for hydrolysis of H₂S, starch and tributiryn and acid production from sucrose) and the isolates caused a hypersensitive reaction in leaves of tomato cv. Moneymaker. Partial sequences of gyrase subunit B-like (gyrB) gene (2) from an isolate (Accession No. HQ215596, 676 bp) showed highest similarity to Xanthomonas axonopodis pv. poinsettiicola strain LMG 5401 (Accession No. GU144264.1, 99% identity, 98% coverage) and 98% identity with gyrB sequence of X. axonopodis pv. poinsettiicola pathotype strain LMG 849 (Accession No. GU144273.1, 99% coverage, 3 gaps). Repetitive BOX-PCR (3) revealed high similarity of our isolate to pathotype strain LMG 849 with one additional band of approximate size of 1,500 bp present in our isolate. The pathogenicity of two isolates from parent plants of cv. Christmas Feeling was confirmed on four young poinsettia plants each. Plants were inoculated with a bacterial suspension of approximate concentration of 10⁶ CFU/ml by spraying on the under side and upper side of the leaves, some of which were pricked with a sterile needle (1). Plants were then incubated under high relative air humidity (minimum 85%), 12 h of daylight, and 25°C day and 20°C night temperature regimens. After 10 days, all inoculated plants developed faint leaf spots, consistent with mild symptoms observed in the greenhouse. Colonies isolated from the developed spots had identical morphology and BOX-PCR profile to original isolates. Mock inoculated, negative control plants did not develop characteristic symptoms and no colonies similar to X. axonopodis pv. poinsettiicola were isolated from them. Bacteria isolated from leaf spots of other poinsettia cultivars had the same biochemical characteristics and BOX-PCR profiles as the first isolate. Since no leaf blight was observed on poinsettias in the greenhouse in the previous season and no host plants were kept between the seasons, imported parent plants are the most likely source of infection. To our knowledge, this is the first report of X. axonopodis pv. poinsettiicola on poinsettia in Slovenia, providing further data on the occurrence and potential economic damage of leaf blight of poinsettia in Europe.

References: (1) R. A. Lelliott and D. E. Stead. Host tests. In: Methods in Plant Pathology. Vol 2. Blackwell, Oxford, 1987. (2) N. Parkinson et al. Int. J. Syst. Evol. Microbiol. 59, 264, 2009. (3) J. Versalovic et al. Methods Mol. Cell Biol. 5:25, 1994.