The term phytoplasmas refers to pathogenic microorganisms of plants, similar to bacteria, from which they differ in the absence of cell walls and usually smaller dimensions.
The phytoplasma is a plant parasite belonging to the Mollicutes class, whose survival is possible only within the host plant and the vector insect. Phytoplasmas are bacteria without a cell wall and therefore pleomorphic.
Phytoplasmas, therefore, can develop exclusively on a host’s live tissue: they occupy the phloem of plants and are systemic.
They do not have a real nucleus even if they contain both DNA and RNA. They are cultivable in vitro on artificial growth substrates, but not in purity and not on selective substrates.
Unlike viruses, a phytoplasma has its own metabolism, but very reduced, such that many vital molecules, indispensable for their existence, must be acquired by host cells (obligatory biotrophic vitality).
The classification of phytoplasmas is quite complex. They have been distinguished thanks to molecular techniques, from mycoplasmas, pathogens for humans and animals; and the name of phytoplasmas is assigned to exclusively phytopathogenic organisms.
From a systematic point of view, the study of the 16S ribosomal gene led to their classification into ribosomal groups and different species ‘Candidatus Phytoplasma’.