'Candidatus Phytoplasma tamaricis', a novel taxon discovered in witches'-broom-diseased salt cedar (Tamarix chinensis Lour.)

1 Molecular Plant Pathology Laboratory, USDA-Agricultural Research Service, Beltsville, MD 20705, USA 2 Shandong Institute of Pomology, Taian 271000, PR China Correspondence Yan Zhao yan.zhao{at}ars.usda.gov Salt cedar trees with pronounced witches'-broom symptoms were observed in their natural habitat in China. 16S rRNA gene sequences unique to phytoplasmas were detected in every DNA sample extracted from stem and leaf tissues of the symptomatic trees, revealing a direct association between phytoplasma infection and the salt cedar witches'-broom (SCWB) disease. Phylogenetic analysis of the SCWB phytoplasma 16S rRNA gene sequence indicated that the SCWB phytoplasma belonged to a subclade consisting of several mutually distinct ‘ Candidatus Phytoplasma’ taxa including ‘ Ca. Phytoplasma prunorum’, ‘ Ca. Phytoplasma mali’, ‘ Ca. Phytoplasma pyri’ and ‘ Ca. Phytoplasma spartii’. Pairwise sequence similarity scores calculated from an alignment of near full-length 16S rRNA genes revealed that SCWB phytoplasma shared 96.6 % or less sequence similarity with each previously described or proposed ‘ Ca. Phytoplasma’ taxon, justifying the recognition of SCWB phytoplasma as a novel taxon, ‘ Candidatus Phytoplasma tamaricis’. The distinct virtual RFLP pattern derived from the SCWB phytoplasma 16S rRNA gene sequence, together with its lower-than-threshold similarity coefficient values with RFLP patterns of any of the 29 previously established groups, supported the recognition of a new 16Sr group, designated 16SrXXX, salt cedar witches'-broom phytoplasma group. Abbreviations: SCWB, salt cedar witches'-broom disease The GenBank/EMBL/DDBJ accession number for the 16s rRNA gene sequence of ‘ Candidatus Phytoplasma tamaricis’ is FJ432664 . The results of PCR amplification from samples of symptomatic salt cedar trees are available as supplementary material with the online version of this paper.