phloemlimited

Phloem-limited is a term used in plant pathology to describe organisms whose distribution within a plant is restricted to the phloem, the vascular tissue that transports sugars. In practical terms, these organisms colonize the sieve elements and their associated companion cells and typically do not invade the xylem or other tissues. The phloem restriction influences disease development, tissue tropism, and detection, and it often corresponds to transmission by phloem-feeding insects.

The term is most commonly applied to phytopathogenic Mollicutes such as phytoplasmas and spiroplasmas, which inhabit phloem tissue and spread systemically through the phloem. Phytoplasmas are wall-less bacteria classified by 16S rRNA sequences and are responsible for diseases including aster yellows and grapevine yellows in various hosts. In addition, some plant viruses are described as phloem-limited or phloem-restricted, with luteoviruses and certain closteroviruses demonstrating replication in phloem-associated cells and dependence on phloem-feeding vectors for spread.

Diagnosis of phloem-limited infections relies on sampling phloem-rich tissues such as petioles, stems, or bark, followed by molecular methods like PCR or qPCR targeting the pathogen’s ribosomal RNA genes, or by in situ hybridization or serology. Because these pathogens are often not cultivable outside the plant, detection from non-phloem tissues can be challenging and may require tissue-specific assays.

Management emphasizes vector control, sanitation, and the use of certified disease-free planting material. Because the pathogens persist in perennial hosts and may have wide host ranges, prevention is the primary strategy, though research continues into phloem colonization, transmission dynamics, and host-pathogen interactions.