Candidatus Liberibacter asiaticus (C.L.asiaticus) is the cause of Citrus greening disease, also known as Huanglongbing (HLB), a destructive and lethal citrus disease. C.L.asiaticus (named after its discovery in Asia) is a gram negative phloem-restricted bacteria that belongs to the genus Candidatus Liberibacter. The two other Candidatus species present in this genus (Candidatus Liberibacter africanus and Candidatus Liberibacter americanus) also cause HLB.
Symptomatically, HLB in the early stages is noticeable through the development of yellow shoots. Infection with HLB affects over 600 genes including those important for plant defence and sugar metabolism to name a few. The phloem are disrupted and sucrose accumulates within the leaves, leading to nutrient deficiencies. The infected trees are stunted and only bear a few small, unpleasant tasting and deformed fruits that are poorly coloured (known as greening). Upon infection, these symptoms can appear within a few months or after several years. This is dependent upon a range of factors including the age and health of the tree, season and environmental conditions.
Management of HLB demands quarantine and control of the psyllid vectors with insecticides, removal of symptomatic trees and geographical isolation. Essentially the most effective form of control for HLB would be to develop resistant cultivars, but resistance genes have not yet been identified. Between 2007 and 2014, citrus greening disease was reported to have caused Florida’s industry a cumulative loss of $2.9 billion in grower revenues. With this in mind, fast and accurate detection with real time PCR can be extremely beneficial in the diagnosis and disease management of HLB.