Different sweet orange-rootstock combinations infected by Candidatus Liberibacter asiaticus under greenhouse conditions: Effects on the scion

Authors: BODAGHI, SHARZHAD - UNIVERSITY OF FLORIDA; MEYERING, BO - UNIVERSITY OF FLORIDA; Bowman, Kim; ALBRECHT, UTE - UNIVERSITY OF FLORIDA

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2021
Publication Date: 12/21/2021
Citation: Bodaghi, S., Meyering, B., Bowman, K.D., Albrecht, U. 2021. Different sweet orange-rootstock combinations infected by Candidatus Liberibacter asiaticus under greenhouse conditions: Effects on the scion. HortScience. 57(1), 144-153. https://doi.org/10.21273/HORTSCI16205-21.
DOI: https://doi.org/10.21273/HORTSCI16205-21

Interpretive Summary: Citrus crops are primarily grown as grafted trees on rootstock varieties, and diseases such as huanglongbing severely damage the production of citrus. Citrus production in areas affected by huanglongbing can be significantly improved by using disease-tolerant rootstocks. We report on the results from a detailed greenhouse evaluation of sweet orange trees on ten rootstocks that were artificially infected by huanglongbing and used to assess tree tolerance to the disease. In the first part of the study, performance of the sweet orange scion of the tree was compared by physical characteristics and physiology. Huanglongbing disease infection had a striking influence on many characteristics of the tree, regardless of rootstock. Among the traits examined, leaf size of the sweet orange scion appeared the most closely associated with rootstock tolerance to the disease, and may be used to help predict rootstock tolerance.

Technical Abstract: The devastating citrus disease huanglongbing (HLB) associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas) has caused a more than 70% reduction in citrus production since its discovery in Florida in 2005. Most citrus scion cultivars are sensitive to HLB, whereas some cultivars used as rootstocks are tolerant. Using such tolerant rootstocks can help trees to cope better with the disease’s impact. Evaluating rootstock effects on a grafted scion in the field takes many years, but shorter-term evaluation is imperative to aid in rootstock selection for an HLB-endemic production environment. In this study, we investigated grafted healthy and CLas-infected citrus trees under controlled greenhouse conditions. The objectives were to identify traits suitable for assessing grafted tree tolerance in advance of longer-term field studies and aiding in the selection of superior rootstock cultivars. We assessed ten commercially important rootstocks grafted with ‘Valencia’ sweet orange scion and with known field performance. At six, nine, 15, and 21 months after graft-inoculation (mai), leaf CLas titers were determined, and canopy health was evaluated. Plants were destructively sampled at 21 mai to assess plant biomasses and other physiological and horticultural variables. There was little influence of the rootstock cultivar on CLas titers. Surprisingly, few HLB foliar disease symptoms and no differences in soluble and non-soluble carbohydrate concentrations were measured in infected compared to healthy plants, despite high CLas titers and significant reductions in plant biomasses. Most trees on rootstocks with trifoliate orange parentage were less damaged by HLB than other rootstocks, although results did not always agree with reported field performance. Among the different variables measured, leaf size appeared to be most predictive for grafted tree assessment of HLB sensitivity. The results of this study provide a better understanding of the strengths and weaknesses of assessing rootstock influence on grafted tree performance in a controlled greenhouse environment. Although such studies provide valuable information for cultivar tolerance to HLB, other rootstock traits will ultimately contribute to field survival and productivity in an HLB endemic production environmen