EPPO Global Database

Phytoplasmas constitute a large and diverse group of pathogens which cannot be cultured in vitro and which are associated with several hundreds of plant diseases. In EPPO RS 2005/060, the current classification of phytoplasmas was summarized in a table. More recently, Firrao et al. (2005) provided a short taxonomic guide to the genus ‘Candidatus Phytoplasma’. This short guide gives useful explanations on how the scientific community is attempting to provide a classification which takes into account both the phylogenetic and the biological/ecological characteristics of phytoplasmas.

It is recalled that the category ‘Candidatus’ was introduced to allow unambiguous reference to organisms that could not be cultivated in vitro. In principle, a Candidata species had to be defined on the basis of habitat and nucleic acid sequence data. So for phytoplasmas, it was understood that they would be described on the basis of their host plants and sequence data of the 16S rRNA gene (which is the main studied one). According to the original proposal, classification should be based on a unique sequence, but this was not practical for phytoplasmas which show a great diversity. This would have lead to an impractical number of species. It was agreed that a strain that shared more than 97.5% of its 16S rRNA gene sequence with an already described species should not be described as a new Candidata species unless it was demonstrated that the organism belonged to an ecologically distinct population. The analysis of more than 200 different sequences of phytoplasma 16S rRNA gene has delineated less than 20 clusters of strains at 97.5 % similarity. But, as within these clusters, some phytoplasmas presented different biological, phytopathological and other molecular properties a few more species were proposed. The following rules were agreed when describing new phytoplasma species which shared more than 97.5% of their 16S rRNA gene. These species can be considered as different only when: they are transmitted by different vectors; they have different natural host plants (or at least show a different behaviour on the same host); and there is evidence of significant molecular diversity (using hybridization tests with cloned DNA probes, serological or PCR-based tests).

As a result, the new genus ‘Candidatus Phytoplasma’ now comprises 21 species (see below) which have been validly described and 6 additional species which have an informal description (‘Ca. Phytoplasma cocosnigeriae’, ‘Ca. Phytoplasma cocostanzaniae’, ‘Ca. Phytoplasma luffae’, ‘Ca. Phytoplasma palmae’, ‘Ca. Phytoplasma pruni’, ‘Ca. Phytoplasma solani’, ‘Ca. Phytoplasma vitis’). This short taxonomic guide summarized the details needed for the identification of each species (host plants and distinctive nucleotide sequences). For practical reasons, the EPPO Secretariat has extracted the following details about species names and host plants, data about the nucleotide sequences can be found in the original paper.