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Occurrence of Poinsettia Stem Flat Disease Caused by Phytoplasma in Korea.

In December 2009, commercially grown poinsettia (Euphorbia pulcherrima Willd cv. Ichibang) exhibited typical phytoplasma-like symptoms in 95% of an affected field in Yongin, Korea (Gyeonggi Province). Symptoms consisted of flat stems and fascicles and an abnormal number of apexes resulting in a cockscomb form of stem and flower bud proliferation. Leaf narrowing with curling of bracts was also associated with the disease. Symptomatic poinsettia plants were not marketable. Poinsettias, cv. Ichibang, without symptoms were obtained from a breeding collection in a glasshouse of the National Institute of Horticultural and Herbal Science in Suwon (Gyeonggi Province). The presence of phytoplasmas in symptomatic and healthy-looking flowers and stems of cv. Ichibang was demonstrated by PCR analysis with primer pair R16mF2/R16mR1 (1), which amplifies phytoplasma 16S rDNA regions. PCR products (~1,427 bp), were obtained from both symptomatic and healthy-looking plants, sequenced, and registered under the GenBank Accession Nos. of GU461275 and GU461277, respectively. Symptomatic poinsettia and healthy poinsettia sequences had 99.6 and 100% identity with U.S. PoiBI isolate FJ376625, indicating poinsettia stem flat disease is caused by PoiBI. A branch-inducing factor in poinsettia has been known for several decades, but only since 1997 was this graft transmissible factor identified as a PoiBI phytoplasma (1), belonging to the peach X-disease phytoplasma group (16S rRNA III) subgroup 16SrIII-H (2). Normally, phytoplasmas are associated with host quality loss, but abnormally, infection in poinsettia generates a desirable, free-branching growth habit. In this study we found that PoiBI could be detrimental to the quality of poinsettia depending on the cultivar and agronomic practice. Poinsettia stem flat disease presumably occurred because of increased levels of phytoplasma caused by successive stem cutting for commercial use. To our knowledge, this is the first report of PoiBI phytoplasma that affected marketability of poinsettia in Korea. References: (1) I. M. Lee et al. Nature Biotech. 15:178, 1997. (2) I. M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998.