Insight into resistance to ' Liberibacter asiaticus,' associated with Huanglongbing, in Oceanian citrus genotypes.

Huanglongbing (HLB), the most destructive citrus disease, is associated with unculturable, phloem-limited Liberibacter species, mainly L. asiaticus (Las). Las is transmitted naturally by the insect . In a previous study, we determined that the Oceanian citrus relatives , , , and and three hybrids among them and were full-resistant to Las. After 2 years of evaluations, leaves of those seven genotypes remained Las-free even with their susceptible rootstock being infected. However, Las was detected in their stem bark above the scion-rootstock graft union. Aiming to gain an understanding of the full-resistance phenotype, new experiments were carried out with the challenge-inoculated Oceanian citrus genotypes through which we evaluated: (1) Las acquisition by fed onto them; (2) Las infection in sweet orange plants grafted with bark or budwood from them; (3) Las infection in sweet orange plants top-grafted onto them; (4) Las infection in new shoots from rooted plants of them; and (5) Las infection in new shoots of them after drastic back-pruning. Overall, results showed that insects that fed on plants from the Oceanian citrus genotypes, their canopies, new flushes, and leaves from rooted cuttings evaluated remained quantitative real-time polymerase chain reaction (qPCR)-negative. Moreover, their budwood pieces were unable to infect sweet orange through grafting. Furthermore, sweet orange control leaves resulted infected when insects fed onto them and graft-receptor susceptible plants. Genomic and morphological analysis of the Oceanian genotypes corroborated that and are pure species while our accession is an × hybrid and probably a × hybrid. × hybrid was found coming from a cross between and mandarin or tangor. × hybrid is a complex admixture of , , and while the last hybrid is an × admixture. Confirmation of consistent full resistance in these genotypes with proper validation of their genomic parentages is essential to map properly genomic regions for breeding programs aimed to generate new -like cultivars yielding immunity to HLB.