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

Authors: BODAGHI, SHAHRZAD - UNIVERSITY OF FLORIDA; PUGINA, G - 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: 1/1/2022
Citation: Bodaghi, S., Pugina, G., Meyering, B., Bowman, K.D., Albrecht, U. 2022. Different sweet orange-rootstock combinations infected by Candidatus Liberibacter asiaticus under greenhouse conditions: Effects on the roots. HortScience. 57(1), 56-64. https://doi.org/10.21273/HORTSCI16206-21.
DOI: https://doi.org/10.21273/HORTSCI16206-21

Interpretive Summary: 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 second part of the study, physical and physiological characteristics of the root systems for the ten rootstocks were compared with and without disease infection. Huanglongbing disease infection had a striking influence on many characteristics of the root system, regardless of the rootstock. Among the traits examined, CLas titers of the root system were most significantly influenced by the rootstock, with some rootstocks greatly reducing the amount of bacteria in the root tissues. More tolerant rootstocks also exhibited higher rates of metabolism than less tolerant rootstocks, suggesting this may also be a useful trait to help in preselecting rootstocks with the greatest potential for good field performance.

Technical Abstract: Grafting a scion onto a rootstock results in physical and physiological changes in plant growth and development, which can affect tree vigor, productivity, and tolerance to stress and disease. Huanglongbing (HLB) is one of the most destructive citrus diseases and has become endemic in Florida since its introduction in 2005. It is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), which cause severe metabolic disruptions in affected plants. Although most scion cultivars are highly susceptible, some rootstock cultivars are tolerant and allow the grafted tree to cope better with the disease. The objectives of this study were to identify rootstock traits that can be used to assess cultivars under controlled greenhouse conditions in advance of longer-term field trials. We used 10 commercially important rootstocks with different genetic backgrounds and known field performance in graft combination with ‘Valencia’ sweet orange scion. Trees were graft-inoculated with CLas and compared against mock-inoculated trees. Tree health and CLas populations were assessed regularly, and root growth was monitored using a minirhizotron imaging system. Plants were excavated and destructively sampled 21 months after inoculation to assess biomass distributions and other CLas-induced effects. We found significant differences between healthy and infected trees for most variables measured, regardless of the rootstock. In contrast to leaf CLas titers, root titers were significantly influenced by the rootstock, and highest levels were measured for ‘Ridge’ sweet orange and sour orange. Root growth and root biomasses were reduced upon infection but differences among rootstocks did not always agree with reported field performances. Despite severe biomass reductions plants maintained their relative distribution of biomass among different components of the root system, and no dead roots were observed. Root respiration was reduced by CLas infection and was overall higher in tolerant cultivars suggesting its potential as a physiological marker. This study improves our knowledge about the strengths and weaknesses of assessing rootstock traits of grafted trees in a controlled greenhouse setting. Results from the study suggest that in addition to HLB-tolerance, other rootstock traits will ultimately have major contributions to field survival and productivity of the grafted trees in an HLB endemic production environment.