phytoplasmas

Phytoplasmas are a group of plant-pathogenic bacteria-like organisms belonging to the class Mollicutes. They are wall-less, pleomorphic, obligate intracellular parasites that inhabit the phloem sieve elements of infected plants and are transmitted by sap-sucking insects, mainly leafhoppers, planthoppers, and psyllids. They cannot be cultured on artificial media, which complicates isolation and study.

Taxonomy is based on 16S ribosomal RNA gene sequences. Because phytoplasmas are uncultured, many taxa are referred to with “Candidatus Phytoplasma” names, representing clades rather than formally described species.

Genomes are small and highly reduced (roughly 0.5–1.0 Mb) and lack many biosynthetic pathways, including components for cell wall synthesis, making the organisms highly dependent on their plant and insect hosts. They encode a set of secreted effector proteins (SAPs) that manipulate host development and metabolism and contribute to disease symptoms such as witches’ broom, phyllody, virescence, leaf yellowing, and shoot proliferation.

Phytoplasmas affect a broad range of plants, including major crops and ornamentals, and can cause substantial yield losses in fruit trees, vegetables, and ornamental species. Symptoms are often nonspecific and can be mistaken for nutrient deficiencies or viral diseases.

Detection relies on molecular methods targeting the 16S rRNA gene, including nested PCR and real-time PCR; field-applicable methods such as LAMP are also used.

Management focuses on preventing spread rather than curing infected plants: use of certified disease-free planting material, removal (roguing) of infected plants, vector control to reduce transmission, and, in some cases, resistance breeding. Quarantine and sanitation measures are important to limit introduction into new areas.