Effect of the presence of an endemic pathogen in the citrus phloem on Candidatus Liberibacter asiaticus establishment and vector competency

Huanglongbing (HLB), a devastating disease of citrus, is thought to be caused by any of three species of phloem-limited alpha-proteobacteria, Candidatus Liberibacter spp., with Candidatus Liberibacter asiaticus (CLas) as the most widespread in the world. This disease continues to spread through much of the citrus growing regions in the world, including into warmer and drier citrus growing regions that have the endemic, phloem-limited pathogen Spiroplasma citri (Sc), causal agent of Citrus stubborn disease, present. In greenhouse and laboratory studies, co-infection by the two pathogens and the impact on the establishment of either or both pathogens was investigated in seedling Washington navel on Carrizo rootstock plants. The effect of the presence of both pathogens on vector competency of the Asian citrus psyllid (Diaphorina citri Kuwayama; Hemiptera: Liviidae), one vector of CLas, was also investigated. Both pathogens were found co-infecting citrus seedlings, and time to first detection of a pathogen was longer in dually-infected plants as compared to plants infected with only one pathogen. When only one pathogen was detected in seedlings that were inoculated with both pathogens, the times to first detection of the single pathogen were variable. The Asian citrus psyllid did not vector Sc and could still acquire and transmit CLas from a plant infected with both Sc and CLas. Possible mechanisms of co-infection in seedlings and planned future studies are discussed.