Beyond Single-Pathogen Models: Understanding Mixed Infections Involving Phytoplasmas and Other Plant Pathogens

Abstract

Phytoplasmas are wall-less, phloem-restricted bacteria responsible for numerous significant plant diseases worldwide. An increasing body of evidence indicates that phytoplasmas can coexist with other pathogens in mixed infections, including various 16Sr group phytoplasmas, 'Candidatus Liberibacter' species, viruses, spiroplasmas, fungi, and other difficult-to-culture phloem-limited bacteria. These interactions challenge established views regarding the causes, detection, and management of plant diseases. This review consolidates existing knowledge on the diversity and epidemiology of phytoplasma-related mixed infections, with a particular emphasis on documented co-infections across various host plants and regions, especially in tropical and subtropical areas. Mixed infections affect disease severity, symptom expression, vector behavior, and pathogen dissemination, highlighting the limitations of pathogen-specific diagnostic and control strategies. The necessity for tools to detect multiple pathogens, enhanced understanding of pathogen-pathogen and host-pathogen interactions, and comprehensive surveillance systems is emphasized. Ultimately, breeding for resistance must consider the complexities of natural co-infections to ensure effective protection of crops. Addressing the challenges presented by phytoplasma-related mixed infections is crucial for developing resilient and sustainable plant health strategies in the face of increasing ecological and agricultural pressures.

Keywords: co-infection; plant disease prevention; plant pathobiome paradigm; unculturable plant pathogen; ‘one pathogen, one disease’ concept.

Figure 1

A phylogenetic tree constructed using the neighbor-joining method based on 16S rRNA gene sequences derived from various phytoplasma strains, including C. maxima phytoplasma CmPII-hn and CmPXXXII-hn strains, which belong to the 16SrII and 16SrXXXII groups, respectively, as well as the ‘Ca. Liberibacter asiaticus’ CmLas-hn strain, identified in C. maxima plants on Hainan Island, China. In this tree, the scale bar indicates inferred character-state changes, revealing the genetic diversity and evolutionary history of the analyzed strains. Branch lengths represent the number of inferred character-state changes, and the scale bar indicates genetic distance. Bootstrap values based on 1000 replicates are shown next to each branch, providing statistical support for the tree topology. The identification of strains from different clades co-infecting the same region suggests potential mixed infections, as highlighted by the accompanying images of symptomatic C. maxima plants. This analysis enhances our understanding of phylogenetic relationships between plant pathogens, which can impact plant health and management strategies [33].