Plant Diseases Associated with Insect-Transmitted Phloem-Limited Bacteria

Abstract

Phloem-limited, insect-transmitted bacteria represent one of the most destructive groups of plant pathogens affecting global agriculture. These pathogens including Candidatus Phytoplasma and Candidatus Liberibacter species reside exclusively in sieve elements, where nutrient-rich phloem sap supports their survival, systemic movement, and rapid spread through host plants. Their transmission is primarily mediated by phloem-feeding insects such as psyllids, leafhoppers, and planthoppers, which facilitate dissemination across a wide range of economically important crops. Phytoplasmas are associated with disorders such as witches’ broom, phyllody, flavescence dorée, and lethal yellowing, while Candidatus Liberibacter species cause citrus Huanglongbing (HLB), zebra chip disease in potato, and other destructive conditions. These pathogens manipulate plant and insect physiology through secreted effector molecules that interfere with developmental regulators, defense pathways, and hormonal balance. Despite significant advances in genomics and molecular diagnostics, management remains challenging due to long latency periods, asymptomatic infections, lack of culturable isolates, and the complexity of vector–host–pathogen interactions. Understanding phloem biology, pathogen effector functions, and plant immune responses including PAMP-triggered and effector-triggered immunity is essential for developing durable resistance. This review synthesizes current knowledge on the biology, transmission, virulence mechanisms, and host responses associated with phloem-limited bacteria, highlighting the global threat they pose and the need for integrated management strategies.