Genetic modulation of valencia sweet orange field performance by 50 rootstocks under huanglongbing-endemic conditions

Authors: Bowman, Kim; MCCOLLUM, GREG - Retired ARS Employee; SEYMORE, DANELLE - University Of California, Riverside

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2023
Publication Date: 2/8/2023
Citation: Bowman, K.D., Mccollum, G.T., Seymore, D. 2023. Genetic modulation of valencia sweet orange field performance by 50 rootstocks under huanglongbing-endemic conditions. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2023.1061663.
DOI: https://doi.org/10.3389/fpls.2023.1061663

Interpretive Summary: World citrus production is severely affected by the disease huanglongbing (HLB), associated with the bacterium Candidatus Liberibacter asiaticus (CLas), and a new SuperSour breeding strategy has been implemented within the USDA rootstock breeding program to improve and accelerate efforts to develop and test more tolerant rootstocks. A large assortment of performance traits were quantified for all trees in the study, including those associated with tree size, tree health, disease symptoms, fruit crop size, and fruit quality. Nearly all of the traits were observed to have significant differences by rootstock clone and by parental combinations. Pedigree information was integrated with quantitative trait measurements to determine the genetic components of rootstock-mediated tree performance, and additive genetic values were calculated ed for each individual to select the most promising HLB-tolerant hybrids and facilitate future breeding decisions. Results indicate there is a significant genetic component underlying rootstock-mediated tolerance to HLB and that the SuperSour strategy should enable marker-based breeding approaches for the rapid and effective selection of next-generation rootstocks with superior combinations of traits needed for commercial success. The current generation of new rootstocks included in this trial contains several hybrids that appear superior to existing rootstocks for commercial use with sweet orange scion in the HLB-endemic environment.

Technical Abstract: Production of citrus crops in much of the world is severely affected or threatened by the disease huanglongbing (HLB), associated with the bacteria Candidatus Liberibacter asiaticus (CLas). Most commercial scion cultivars are highly susceptible, methods to effectively control or manage the disease are impractical, and reliable methods to select or screen for HLB-tolerant cultivars in advance of field testing have not been identified. To address this problem, a new SuperSour breeding strategy has been implemented within the USDA rootstock breeding program, and incorporates several changes to more traditional citrus rootstock breeding efforts with the goal of more rapidly developing superior new rootstocks with the necessary traits. The SuperSour strategy currently includes about 350 new hybrid rootstocks, from a broad range of parental combinations, in numerous replicated field trials, with the aim of identifying superior rootstocks for the HLB-endemic environment now, and mapping important traits to be used in selection for the next generation of superior rootstocks. One of these trials with 50 different rootstocks is the focus for this study of rootstock effects on Valencia sweet orange tree performance. After planting into the field, trees were evaluated through seven years in an HLB-endemic environment for traits associated with field performance. In addition to 46 new hybrid rootstocks, the trial included four standard rootstocks for general comparison with other trials and reference to industry usage. A large assortment of phenotypic traits were quantified for all trees in the study, including traits associated with tree size, tree health, HLB symptoms, fruit crop size, and fruit quality. Of the 29 quantitative traits compared between rootstock clones, 28 were observed to have significant differences by rootstock clone. Multiple progeny from eight different parental combinations were included in the trial study, and significant differences between parental combinations of the rootstocks were observed for 27 of the 32 traits compared. For this subset of hybrids, pedigree information was integrated with quantitative trait measurements to dissect the genetic components of rootstock-mediated tree performance and HLB tolerance using two methods, with similar results. In addition, additive genetic values were predicted for each individual to select the most promising HLB-tolerant hybrids and facilitate future breeding decisions. Results suggest there is a significant genetic component underlying rootstock-mediated tolerance to HLB and that integration of pedigree-based genetic information with extensive quantitative phenotypic data collected from a common scion is an effective strategy for selecting new rootstock hybrids. The expansion of these approaches in citrus, including across similar populations, and the inclusion of genome-wide genetic information will inform the choice of the best parental combinations for particular target characteristics, and enable marker-based breeding approaches for the rapid and effective selection of next-generation rootstocks with superior combinations of traits needed for commercial success with citrus crops. The current generation of new rootstocks included in this trial is a step toward this goal, and contains several hybrids that appear superior to existing rootstocks for commercial use with sweet orange scion in the HLB-endemic environment.