Relative influence of rootstock and scion on Asian citrus psyllid colonization and Candidatus Liberibacter asiaticus distribution

Authors: TARDIVO, CAROLINE - University Of Florida; QURESHI, JAWWAD - University Of Florida; Bowman, Kim; ALBRECHT, UTE - University Of Florida

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2023
Publication Date: 2/28/2023
Citation: Tardivo, C., Qureshi, J., Bowman, K.D., Albrecht, U. 2023. Relative influence of rootstock and scion on Asian citrus psyllid colonization and Candidatus Liberibacter asiaticus distribution. Scientia Horticulturae. 58(4):395-403. https://doi.org/10.21273/HORTSCI17039-22.
DOI: https://doi.org/10.21273/HORTSCI17039-22

Interpretive Summary: Citrus crops in much of the world are severely damaged by the bacterial disease huanglongbing, and the disease is spread from tree to tree and sometimes over long distances by an insect vector called Asian citrus psyllid. The use of specific tolerant and resistant citrus scion and rootstock cultivars is one method by which disease spread may be slowed and disease severity may be reduced. This study examined the influence of eight different citrus clones, used as scions and rootstocks, on Asian citrus psyllid colonization and egg laying, along with the development of bacterial infection and disease symptoms. The study found that: 1) The Asian citrus psyllid prefers unifoliate-type scions over scions with hybrid trifoliate-type parentage, 2) the HLB bacteria is detected more consistently and at higher levels in the leaves than in the fibrous roots of citrus trees, and 3) The citrus scion has a greater relative influence on HLB disease progression than the rootstock. These findings provide knowledge that will improve disease monitoring and management, as well as provide insights for the development of additional future solutions to the problem.

Technical Abstract: Citrus rootstocks are critical to a viable citrus industry. In addition to their inherent horticultural characteristics and influence on the grafted scion, rootstocks may impact colonization by the Asian citrus psyllid (ACP), the vector of Candidatus Liberibacter asiaticus (CLas). CLas is a phloem-limited bacterium associated with huanglongbing (HLB) disease which is devastating citrus industries worldwide. While most scions are susceptible to HLB, some rootstocks are HLB tolerant. The objective of this study was to elucidate the relative influence of rootstock and scion on insect vector preference and CLas colonization under natural HLB-endemic conditions. Seven commercial rootstock cultivars with different genetic backgrounds were self-grafted or grafted with Valencia sweet orange (Citrus sinensis) and planted in an open field where ACP and CLas were abundant. The ACP infestation was determined weekly during the flushing period, and leaves and roots were analyzed every three months to determine CLas titers. Trees grafted with Valencia in the scion position were more attractive to the psyllids and more susceptible to HLB than self grafted (non-Valencia) trees with the rootstock cultivars in the scion position. This was corroborated by higher CLas titers and a larger abundance of foliar HLB symptoms. Oviposition was also affected by the scion-rootstock combination. In some trees, CLas was detected first in the roots three months after planting but root titers remained low throughout the study. In contrast, leaf CLas titers increased over time and were considerably higher than in the roots after 6 months of field growth. This suggests that roots may not be suitable for accurately detecting new infections in young citrus trees. Leaf titers were influenced by the scion, but not by the rootstock. The results from this study demonstrate the greater relative influence of the scion than the rootstock in the HLB disease process.