Genetic strategies to tackle citrus Huanglongbing

Citrus Huanglongbing (HLB), also called citrus greening disease, is the most destructive citrus disease worldwide. A phloem-restricted Gram-negative α-proteobacterium causes citrus HLB, and it can infect all the citrus cultivars and even some citrus relatives. Three forms of the bacteria (Candidatus Liberibacter asiaticus [CLas] Canditatus Liberibacter africanus [CLaf] and Candidatus Liberibacter americanus [CLam]) have been identified. Psyllids (Asian citrus psyllid, Diaphorina citri, and the African citrus psyllid, Trioza erytreae) transmit the bacteria. Infected trees present important physiological disorders, inducing hugely decreasing fruit quality and production, and frequent tree death. In the countries where the disease is present, it is at the origin of dramatic economical loss. The selection of scions and rootstocks tolerant and resistant to HLB is the main component for sustainable strategies aiming to tackle citrus HLB. All citrus species are susceptible to HLB, but strict resistance has been found in related genera and particularly in wild Australian citrus relatives (Eremocitrus and Microcitrus genus) sexually compatible with Citrus. Citrus breeding programs have been established worldwide to create new resistant varieties. In this review, we present the last research mobilizing conventional breeding and biotechnologies to develop more tolerant or resistant plants to maintain citrus production.