Recent progress in the understanding of Citrus Huanglongbing: from the perspective of pathogen and citrus host

Citrus Huanglongbing (HLB) is a devastating disease spread by citrus psyllid, causing severe losses to the global citrus industry. The transmission of HLB is mainly influenced by both the pathogen and the citrus psyllid. The unculturable nature of the HLB bacteria (Candidatus Liberibacter asiaticus, CLas) and the susceptibility of all commercial citrus varieties made it extremely difficult to study the mechanisms of resistance and susceptibility. In recent years, new progress has been made in understanding the virulence factors of CLas as well as the defense strategies of citrus host against the attack of CLas. This paper reviews the recent advances in the pathogenic mechanisms of CLas, the screening of agents targeting the CLas, including antimicrobial peptides, metabolites and chemicals, the citrus host defense response to CLas, and strategies to enhance citrus defense. Future challenges that need to be addressed are also discussed.