‘Candidatus Liberibacter asiaticus’ colonization in citrus trees grafted onto rootstocks of citrus relatives

• Published in: European journal of horticultural science Vol. 90; no. 1
• Main Authors: Josiane Cecília Darolt, Laudecir Lemos Raiol-Junior, Everton Vieira Carvalho, Letícia Lima Andrade, Vanessa Silva Lima Oliveira, Pedro Augusto Gomes, Mônica Neli Alves, Alécio Souza Moreira, Haroldo Xavier Linhares Volpe, Juan Camilo Cifuentes-Arenas, Nelson Arno Wulff, Leandro Peña, Eduardo Augusto Girardi
• Format: Journal Article
• Language: English
• Published: CABI 15.08.2025
• Subjects: Citrus glauca; CLas-resistance; Clausena lansium; grafting; Huanglongbing; Oceanian citrus
• ISSN: 1611-4426; 1611-4434
• DOI: 10.1079/ejhs.2025.0016

Abstract Genetic resistance to ‘Candidatus Liberibacter asiaticus’ (CLas) is considered a promising long-term solution to control Huanglongbing (HLB), given that all commercial citrus varieties are susceptible to the disease. Only a few citrus relatives have been reported as putatively CLas resistant; however, these were only evaluated as scion varieties. Here, we assessed CLas-induced damages and titer in scion/rootstock combinations, including Citrus and certain putatively CLas-resistant relatives that were previously selected. In experiment I, ‘Valencia’ sweet orange (VSO) was grafted onto either BGC682 hybrid (a complex admixture of Australian citrus species) or ‘Rangpur’ lime (RL) as a control, and CLas was inoculated via bud grafting onto the scion. CLas was detected in most of VSO/RL and VSO/BGC682 scions, but it remained undetectable in BGC682 roots, while all RL tissues were CLas positive. In experiments II, III and IV, putatively CLas-resistant Clausena lansium (Clu) was used as the scion, rootstock or interstock. When inoculated by top-grafting, CLas was detected in most tissues from the control groups (only citrus stocks). No bacterium was detected in Citrus trifoliata (Ctr)/Clu and RL/Clu roots. Upon Asian citrus psyllid (ACP) inoculation, CLas infection levels were reduced, and ACP mortality rates were elevated in Clu-grafted treatments compared to the control (VSO/RL), likely attributed to nutrient deficiencies and slower development rates. Under field conditions with natural ACP-mediated CLas transmission, citrus controls were 100% infected with CLas. In contrast, Clu and Swinglea glutinosa (Swg) treatments had lower HLB incidence, but only Swg-grafted trees resulted in satisfactory plant growth compared to Clu-grafted trees. In summary, our findings suggest that Clu and BGC682, when used as citrus rootstocks, exhibit putative resistance to CLas. Although their practical application remains limited by graft incompatibility for Clu and high susceptibility to citrus tristeza virus of BGC682, further biotechnological and classic approaches can be used to incorporate their resistance into citrus breeding programmes. Significance of the study What is already known about this subject? A limited number of citrus relatives have been reported as putatively CLas resistant. Prior studies primarily focused on evaluating these relatives as scion varieties. What are the new findings? Two citrus relatives, Clausena lansium and the BGC682 Australian lime hybrid, evaluated as rootstocks for citrus trees, demonstrated putative resistance to Candidatus Liberibacter asiaticus (CLas), as evidenced by the absence of CLas detection in the roots of inoculated trees, despite limitations related to graft incompatibility and susceptibility to citrus tristeza virus. What is the expected impact on horticulture? These findings can aid in breeding new CLas-resistant rootstock cultivars in the future, using biotechnological approaches, providing long-term solutions to improve the management of Huanglongbing disease in citrus.