The category of Candidatus was introduced to allow unambiguous reference to organisms that could not be cultivated in vitro. In plant pathology, a major impact of this novel taxonomic concept was to enable the classification of the diverse group of organisms, morphologically similar to the mycoplasmas, known by the trivial name of phytoplasmas. These plant pathogens were originally named according to the disease they caused. Later, extensive sequence analysis of the ribosomal RNA genes allowed the development of an evolution-based classification and the grouping of the phytoplasmas into phylogenetically distinct clades. Unfortunately, the phytopathological and the phylogenetic classification did not match and had striking contradictions. With the adoption of the category Candidatus, the description of the genus 'Ca. Phytoplasma' and several 'Ca. Phytoplasma' species, the scientific community is now attempting to provide a classification that takes into account both the phylogenetic and the biological/ecological characteristics of the organisms. Here we provide an outline of the characteristics and composition of the genus 'Ca. Phytoplasma'.
