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item Lee, Ing Ming
item Bottner, Kristi

Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2006
Publication Date: 7/1/2006
Citation: Lee, I., Bottner, K.D., Secor, G., Rivera-Varas, V. 2006. "Candidatus Phytoplasma americanum", a phytoplasma associated with a potato purple top wilt disease complex. International Journal of Systematic and Evolutionary Microbiology. 56:1593-1597.

Interpretive Summary: Phytoplasmas are very small bacteria that lack a cell wall and that cause several hundred economically important diseases in plants worldwide. Potato purple top wilt (PPT) and similar diseases have caused tremendous damage to potato tuber production in South America, Mexico, and the US. In 2002 and 2003, a major epidemic of PPT occurred in Washington and Oregon, causing great economic damage to the potato industry. A phytoplasma belonging to the clover proliferation phytoplasma group was the causal agent. In 2004 and 2005, a new potato disease with symptoms similar to those of PPT occurred in Texas and Nebraska, causing patchy brown discoloration of chips produced from commercial processing potatoes. We have identified two phytoplasmas that were associated with this new PPT disease in Texas and Nebraska. By analysis of a gene involved in protein synthesis, we have found that one belonged to the aster yellows phytoplasma group and the other was a new phytoplasma that is distinct from all previously reported phytoplasmas. We propose that the new PPT phytoplasma, termed American potato purple top wilt (APPTW) phytoplasma, represents a new species, ‘Candidatus Phytoplasma americanum’. The information will aid implementation of quarantine regulations and help extension workers and plant diagnosticians to determine how to combat the disease.

Technical Abstract: Potato purple top wilt (PPT) is a devastating disease occurring in various regions in North America and Mexico. At least three distinct phytoplasma strains belonging to three different phytoplasma groups (16SrI, 16SrII, and 16SrVI) are known to be associated with this disease. A new disease with symptoms similar to PPT was recently observed in Texas and Nebraska. Two distinct phytoplasma strain clusters were identified. One belongs to 16SrI phytoplasma group, subgroup A, and the other is a new phytoplasma that is most closely related to, and shares 96.6% 16S rDNA sequence similarity with, a member of group 16SrXII. Phylogenetic analysis of 16S rDNA sequences of the new PPT-associated phytoplasma strains, previously described ‘Candidatus Phytoplasma spp’, and other distinct unnamed phytoplasmas indicated that the new phytoplasma, termed American potato purple top wilt (APPTW) phytoplasma, represented a distinct lineage and shared a common ancestor with stolbur phytoplasma, ‘Ca. P. australiense’, ‘Ca. P. japonicum’, ‘Ca. P. fragariae’, bindweed yellows phytoplasma, ‘Ca. P. caricae’, and ‘Ca. P. graminis’. On the basis of unique 16S rDNA and biological properties, we propose that the new PPT phytoplasma represents a novel taxon, ‘Candidatus Phytoplasma americanum’.