'Candidatus phytoplasma japonicum', a new phytoplasma taxon associated with Japanese Hydrangea phyllody.

A phytoplasma was discovered in diseased specimens of field-grown hortensia (Hydrangea spp.) exhibiting typical phyllody symptoms. PCR amplification of DNA using phytoplasma specific primers detected phytoplasma DNA in all of the diseased plants examined. No phytoplasma DNA was found in healthy hortensia seedlings. RFLP patterns of amplified 16S rDNA differed from the patterns previously described for other phytoplasmas including six isolates of foreign hortensia phytoplasmas. Based on the RFLP, the Japanese Hydrangea phyllody (JHP) phytoplasma was classified as a representative of a new subgroup in the phytoplasma 16S rRNA group I (aster yellows, onion yellows, all of the previously reported hortensia phytoplasmas, and related phytoplasmas). A phylogenetic analysis of 16S rRNA gene sequences from this and other group I phytoplasmas identified the JHP phytoplasma as a member of a distinct sub-group (sub-group Id) in the phytoplasma clade of the class Mollicutes. The phylogenetic tree constructed from 16S rRNA gene sequences was consistent with the hypothesis that the JHP phytoplasma and its closest known relatives, the Australian grapevine yellows (AUSGY), Phormium yellow leaf (PYL), Stolbur of Capsicum annuum (STOL) and Vergilbungskrankheit of grapevine (VK) share a common ancestor. The unique properties of the DNA from the JHP phytoplasma clearly establish that it represents a new taxon, 'Candidatus Phytoplasma japonicum'.