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

Author
item Zhao, Yan
item SUN, QINGRONG - SHANDONG CHINA
item WEI, WEI - LIAONING CHINA
item Davis, Robert
item WU, WEI - LIAONING CHINA
item LIU, QINGZHONG - SHANDONG CHINA

Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2009
Publication Date: 10/1/2009
Citation: Zhao, Y., Sun, Q., Wei, W., Davis, R.E., Wu, W., Liu, Q. 2009. 'Candidatus Phytoplasma tamaricis', a novel taxon discovered in witches'-broom diseased salt cedar (Tamarix chinensis Lour.). International Journal of Systematic and Evolutionary Microbiology. 59:2496-2504.

Interpretive Summary: Salt cedar is an ornamental tree species native to Asia and East Europe, and was introduced into North America during 1800s. Being an Old World species, salt cedar remains a popular versatile tree in China as well as in other Asian countries, serving ornamental, medicinal, and wind-erosion control purposes. By contrast, in North America, especially in the western United States, salt cedar has recently been considered as an invasive species that negatively impacts the ecosystem due to its excessive water consumption, and natural predators are being sought to control its spread. In summer 2004, a disease characterized by uncontrolled branching (witches’-broom disease) was observed in salt cedar trees growing in their native habitat--China. In this study, we found that the salt cedar witches’-broom disease was associated with infection by a small bacterium called phytoplasma. We present DNA finger-printing results to demonstrate that this phytoplasma is a new species, hereby named ‘Candidatus Phytoplasma tamaricis’. We have found unique genetic markers for quick detection of this bacterium. This information will be of interest to scientists and extension personnel who are concerned with phytoplasma disease management as well as bio-control of salt cedar spread in North America.

Technical Abstract: 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 SCWB phytoplasma belonged to a subclade consisting of several mutually distinct ‘Candidatus Phytoplasma’ species including ‘Ca. Phytoplasma prunorum’, ‘Ca. Phytoplasma mali’, ‘Ca. Phytoplasma pyri’, and ‘Ca. Phytoplasma spartii’. Pairwise sequence identity scores calculated from an alignment of near-full-length 16S rRNA genes revealed that SCWB phytoplasma shared 96.6% or less sequence identity with each previously described or proposed ‘Ca. Phytoplasma’ species, justifying the recognition of SCWB phytoplasma as a novel taxon, ‘Candidatus Phytoplasma tamaricis’. The distinct virtual RFLP pattern derived from SCWB phytoplasma 16S rDNA, together with its lower-than-threshold similarity coefficient values with RFLP patterns of any of the 29 previously established groups, supports recognition of a new 16Sr group, designated 16SrXXX, salt cedar witches’-broom phytoplasma group.