Phytoplasma

Phytoplasmas are a group of plant-pathogenic bacteria classified within the class Mollicutes. They are wall-less, pleomorphic organisms that inhabit the phloem sieve elements of plants and are obligate intracellular parasites. Because they cannot be cultured on artificial media, phytoplasmas are typically detected through molecular methods and direct plant or insect sampling rather than routine bacterial culture.

Transmission and life cycle: Phytoplasmas are spread by sap-sucking insects, primarily leafhoppers, planthoppers, and psyllids. Infected insects acquire phytoplasmas while feeding and can transmit them to healthy plants in a persistent, propagative manner. Within plants, phytoplasmas colonize the phloem and induce a range of symptoms; in insects, they circulate and multiply, enabling ongoing transmission to new host plants.

Genomics and biology: Phytoplasmas have among the smallest bacterial genomes, roughly 0.5 to 1.0 megabases, with extensive gene reduction and limited biosynthetic capability. They rely on their plant and insect hosts for many nutrients and metabolic precursors. They lack a cell wall and therefore exhibit pleomorphic shapes and osmotic sensitivity, characteristics shared with other Mollicutes. Because they cannot be cultured in artificial media, their taxonomy is based on molecular data, particularly 16S rRNA gene sequences, under the designation “Candidatus Phytoplasma” with diverse 16S rRNA groups.

Symptoms and impact: Infected plants may show witches’ broom, virescence or phyllody, leaf yellowing, stunting, and abnormal flowering or shoot proliferation. Phytoplasmal diseases affect a wide range of crops and ornamentals, causing yield losses and trade implications in affected regions.

Diagnosis and control: Detection relies on PCR-based methods and sequencing of the 16S rRNA gene, along with in situ hybridization in some cases. Management focuses on vector control, removal of infected material, and use of certified disease-free planting stock, as there are no widely available cures.