Citation: Firrao, G., Davis, R.E., Gundersen, D.E., Lee, I. and others. 2004. Candidatus Phytoplasma, a taxon for the wall-less, non-helical prokaryotes that colonize plant phloem and insects. International Journal of Systematic and Evolutionary Microbiology. 54:1243-1255. Interpretive Summary: The trivial name 'phytoplasma' has been adopted to refer to unusual bacteria that lack a cell wall, cause diseases only in plants, and are carried from plan to plant by insects. Thus far, several hundred plant diseases caused by phytoplasmas have been reported worldwide. Due to their inability to grow in cell-free media, phytoplasmas were poorly characterized until the advent of molecular biology. Molecular evidence revealed that phytoplasmas constitute a large, coherent phylogenetic cluster, representing a new Genus. Using molecular methods more than 20 distinct phytoplasma groups were identified. The International Phytoplasma/Spiroplasma Working Team-Phytoplasma taxonomic group has reached a consensus that each group represented at least one species. According to a recently adopted taxonomic rule, the properties of uncultured organisms should be recorded by a Candidatus designation. The present paper provides a formal description of the genus 'Ca. Phytoplasma' and its species composition, and provides guidelines for naming a new taxon. This information will aid scientists, diagnosticians, extension agents, and growers to identify phytoplasmas and/or diagnose the phytoplasmal diseases, and will help scientist in APHIS to design and implement measures to prevent the potential introduction of foreign phytoplasma strains.
Technical Abstract: The trivial name 'phytoplasma' has been adopted to collectively name wall-less, non-helical prokaryotes that colonize plant phloem and insects, which were formerly known as mycoplasma-like organisms. Although phytoplasmas have not yet been cultivated in vitro, phylogenetic analyses based on various conserved genes have shown that they represent a distinct, monophyletic clade within the class Mollicutes. It is proposed here to accommodate phytoplasmas within the novel genus 'Candidatus (Ca.) Phytoplasma'. Given the diversity within 'Ca. Phytoplasma', several subtaxa are needed to accomodate organisms that share <97.5% similarity among their 16S rRNA gene sequences. This report describes the properties of 'Ca. Phytoplasma', a taxon that includes the species 'Ca. Phytoplasma aurantifolia (the prokaryote associated with witches'-broom disease of small-fruited acid lime), 'Ca. Phytoplasma australiense' (associated with Australian grapevine yellows), 'Ca. Phytoplasma fraxini' (associated with ash yellows), 'Ca. Phytoplasma japonicum (associated with Japanese hydrangea phyllody), 'Ca. Phytoplasma brasiliense' (associated with hibiscus witches'-broom in Brazil), 'Ca. Phytoplasma castaneae' (associated with chestnut witches-broom in Korea), 'Ca. Phytoplasma asteris' (associated with aster yellows), 'Ca. Phytoplasma mali (associated with apple proliferation), 'Ca. Phytoplasma phoenicium' (associated with almond lethal disease), 'Ca. Phytoplasma trifolii (associated with clover proliferation), 'Ca. Phytoplasma cynodontis' (associated with Bermuda grass white leaf), 'Ca. Phytoplasma ziziphi' (associated with jujube witches-broom), 'Ca. Phytoplasma oryzae' (associated with rice yellow dwarf) and six species-level taxa for which the Candidatus species designation has not been formally proposed (for the phytoplasmas associated with X-disease of peach, grapevine flavescence doree, Central American coconut lethal yellows, Tanzanian lethal decline of coconut, Nigerian lethal decline of coconut and loofah witches'-broom, respectively). Additional species are needed to accommodate organisms that, despite their 16S rRNA sequence being >97.5% similar to those of other 'Ca. Phytoplasma' species, are characterized by distinctive biological, phytopathological and genetic properties. These include 'Ca. Phytoplasma pyri' (associated with pear decline), 'Ca. Phytoplasma prunorum' (associated with European stone fruit yellows), 'Ca, Phytoplasma spartii' (associated with spartium witches'-broom), 'Ca. Phytoplasma rhamni' (associated with buckthorn witches'-broom), 'Ca. Phytoplasma allocasuarinae' (associated with allocasuarina yellows), 'Ca. Phytoplasma ulmi' (associated with elm yellows) and an additional taxon for the stolbur phytoplasma. Conversely, some organisms, despite their 16S rRNA gene sequence being <97.5% similar to that of any other 'Ca. Phytoplasma' species, are not presently described as Candidatus species, due to their poor overall characterization.