Candidatus Liberibacter

Candidatus Liberibacter are carried in the heamolymph and salivary glands of psyllids. Since psyllids feed on sap, this provides bacteria the entry to phloem of the plant. [2] They induce significant metabolic and regulatory changes that damage the plants transport system and affects plants defense systems. These impairments have downstream negative effects on citrus microbiome of the infected plants. [4]

Since Candidatus Liberibacter cannot be cultivated outside of its vector or host, genetics, bacteria-vector and bacteria-plant interaction have not yet been thoroughly explored. Factors important for adaption and colonization or possible coevolution are not yet understood. [2][4] Liberibacter activates salicylic acid pathway in host, likely due to recognition of extracellular molecules such as lipopolyscacharides or flagella. Pathogen in turn likely mitigates the effects, because it encodes SA hydroxylase, that degrades salicylic acid. Liberibacters were shown to affect the spread of vector, by influencing the flight frequencies and sexual attraction of D. citri. On the other hand, infection with Liberibacter causes higher mortality of D. citri adults, but not nymphs. [4] Liberibacter is a part of the psyllid microbiota and co-existence with other bacteria likely has impact on the overall fitness of the insect, as well as outcome of the disease. [4]

Primary strategy for HLB disease management is a vector control. Antimicrobial treatment can suppress Candidatus Liberibacter spp., however usage of broad spectrum antibiotics in inadvisable due to adverse environmental effects. Alternative treatments, such as heat therapy, i.e. incubation of plant at temperatures above 40°C for several days, show varying effects. Another suggested alternatives include the use of compounds that alleviate disease symptoms and boost plants defense systems or reinforcing natural citrus microbiota in order to compete with Candidatus Liberibacter spp. [5] http://citrusindustry.net/2019/06/19/update-on-brassinosteroids-for-hlb-management/ https://portal.nifa.usda.gov/web/crisprojectpages/1005557-zinkicide-a-nanotherapeutic-for-hlb.html

Named species include:[6]

• Liberibacter africanus originated in Africa and is a causal agent of citrus greening disease, also known as huanglongbing, and vectored by the African citrus psyllid Trioza erytreae.[7]
• Liberibacter americanus is a novel species from Brazil described in 2005 and associated with huanglongbing and vectored by the Asian citrus psyllid Diaphorina citri.[8]
• Liberibacter asiaticus originated in Asia and is a causal agent of huanglongbing, vectored by the Asian citrus psyllid D. citri.[9]
• Liberibacter crescens[10] was isolated from papaya growing in Puerto Rico.
• Liberibacter europaeus is a novel species described in 2010, found in pear trees, where it seems to cause no symptoms and is vectored by the psyllid, Cacopsylla pyri.[11]
• Liberibacter psyllaurous is a novel species described in 2008 from solanaceous host plants potato and tomato and vectored by the potato tomato psyllid Bactericera cockerelli.[3] L. psyllaurous plays a dual role as a plant pathogen and as an inherited bacterial endosymbiont. This symbiont modifies tomato defenses in favor of itself and its psyllid vector.[12]
• Liberibacter solanacearum[13] is a causal agent of zebra chip disease in potatoes and vectored by the potato tomato psyllid Bactericera cockerelli.[14] There are five haplotypes described within this species, designated LsoA and LsoB (from solanaceous plants in North America), and LsoC, LsoD, and LsoE (from carrots and celery in Europe).[15][16] L. psyllaurous is a synonym for L. solanacearum haplotype A[17] (identical 16S rRNA genes).
• Liberibacter brunswickensis[18] associated with the psyllid Acizzia solanicola on eggplant in Australia.