Relative influence of rootstock and scion on Asian Citrus psyllid infestation and Candidatus Liberibacter asiaticus colonization

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

Submitted to: HortScience
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. Relative influence of rootstock and scion on Asian Citrus psyllid infestation and Candidatus Liberibacter asiaticus colonization. HortScience. 58(4):395–403. 2023. https://doi.org/10.21273/HORTSCI17039-22.
DOI: https://doi.org/10.21273/HORTSCI17039-22

Interpretive Summary: The citrus disease huanglongbing (HLB) is devastating the Florida citrus industry and damaging many other regions of citrus production around the world. The disease is commonly spread between trees by the insect vector Asian citrus psyllid (ACP). Most common citrus fruiting cultivars are highly susceptible to the disease and greatly damaged by infection, but some cultivars used as rootstocks have been shown to highly tolerant to infection and improve overall health in the grafted trees. This study examined the attractiveness of grafted trees with different cultivars in the position of scion (the above-ground part of the tree) and rootstock (the below-ground part of the tree). When trees contained the susceptible sweet orange cultivar in the scion position, the top of the tree was highly attractive to the vector ACP and became infected by the disease quicky, regardless of the rootstock. When trees were formed with the HLB-tolerant rootstock in the scion position, they were less attractive to the vector ACP and became infected with the disease more slowly and with less symptoms. The evidence indicates that the scion has a more important influence on HLB disease severity in the grafted tree than the rootstock, and that the rootstock influence to improve tree health following HLB-infection is not due to the induction of greater resistance to ACP in the scion or reduced infection by the disease in the scion. These findings help to define the targets for developing new scions and rootstocks with better tolerance to HLB.

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 disease 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 CLas titers were low and did not increase throughout the duration of the study. In contrast, leaf CLas titers increased and were considerable 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.