Huanglongbing (HLB): An unresolved citrus greening disease caused by bacteria vectored by insect, Psyllids with future perspective of research

Abstract

Citrus production in the world declined continuously due to ‘Huanglongbin’ (HLB) diseases, caused by phloem sap localized bacteria, Candidatus Liberibacter asiaticus (CLas) that was inoculated by Asian Citrus Psyllid (ACP), Diaphorina citri during stylet feeding. CLas was a phloem-limited, and gram-negative proteobacterium. Once inoculated by insect stylet probing, phloem environment favored aerobic respiration to ‘CLas’ resulting in rapid proliferation which caused blockage source-sink relationship, callose deposition, and cell collapse in the phloem tissue. The conventional control measure focused on reducing CLas titer by chemical spray, planting disease-free nursery stock, removing infected trees, and promoting root health etc. Unfortunately, these management practices evidenced as inadequate control measure to restrict HLB epidemics. The resistant plants produced a wide spectrum of defense compounds such as terpenoids, alkaloids, nonprotein amino acids, benzoxazinoids, cyanogenic glucosides, and phenolic compounds. But the mechanisms underpinning citrus tolerance to CLas has not been pinned yet. Cracking the ‘mystery’ was essential for sustainable development of CLas mediated citrus greening disease. This review lighted on future research perspectives which could be potentially explored in future.