Phytoplasmen

Phytoplasmen, or phytoplasmas, are plant-pathogenic bacteria that lack a cell wall and inhabit the phloem of plants and certain insect vectors. They belong to the class Mollicutes and are designated as “Candidatus Phytoplasma,” with numerous 16S rRNA-based subgroups. They are obligate parasites and cannot be cultured on standard artificial media.

Habitat and transmission: Phytoplasmas reside in sieve elements of host plants and are transmitted by phloem-feeding insects such as leafhoppers and planthoppers in a persistent, circulative manner. Infected plants display symptoms including witches’ broom, yellowing, stunting, leaf deformities, and reduced vigor. The host range covers many crops and ornamentals.

Genomics and biology: Phytoplasmas have small, reduced genomes (~0.5–1.0 Mb) and lack a cell wall and many metabolic pathways, relying on their hosts. They are not cultivable on standard media, which complicates research.

Diagnosis and management: Detection uses molecular methods such as PCR and sequencing of the 16S rRNA gene, often with group-specific primers; FISH is also used. Management focuses on vector control, removal of infected plants, use of certified seed or propagation material, and, where available, resistant cultivars.

Taxonomy and research: Phytoplasmas are organized under provisional taxonomy as Candidatus Phytoplasma species; classification relies on 16S rRNA gene groups rather than formal species. Ongoing genomics and vector–host studies aim to improve detection, taxonomy, and control.