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

Abstract

Although the citrus scion cultivar primarily determines the characteristics of the fruit, the rootstock cultivar of the graft combination has a major role in determining the horticultural performance of the tree. The disease huanglongbing (HLB) is particularly devastating to citrus, and the rootstock has been demonstrated to modulate tree tolerance. However, no existing rootstock is entirely suitable in the HLB-endemic environment, and citrus rootstocks are particularly challenging to breed because of a long life cycle and several biological characteristics that interfere with breeding and commercial use. This study with Valencia sweet orange scion documents the multi-season performance of 50 new hybrid rootstocks and commercial standards in one trial that forms the first wave of a new breeding strategy, with the aim of identifying superior rootstocks for commercial use now, and mapping important traits to be used in selection for the next generation of outstanding rootstocks. A large assortment of traits were quantified for all trees in the study, including traits associated with tree size, health, cropping, and fruit quality. Among the quantitative traits compared between rootstock clones, all except one were observed to have significant rootstock influence. 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. Pedigree information was integrated with quantitative trait measurements to dissect the genetic components of rootstock-mediated tree performance. Results suggest there is a significant genetic component underlying rootstock-mediated tolerance to HLB and other critical traits, and that integration of pedigree-based genetic information with quantitative phenotypic data from trials should enable marker-based breeding approaches for the rapid selection of next-generation rootstocks with superior combinations of traits that are needed for commercial success. The current generation of new rootstocks included in this trial is a step toward this goal. Based on results from this trial, the new hybrids US-1649, US-1688, US-1709, and US-2338 were considered the four most promising new rootstocks. Release of these rootstocks for commercial use is being considered, pending the evaluation of continuing performance in this trial and the results from other trials.

Response of Citrus Germplasm Seedlings to Candidatus Liberibacter Asiaticus Infection under Controlled Greenhouse Conditions

“…Although many Poncirus hybrids produce juice with a bitter flavor and aftertaste, a recent investigation identified four hybrids resulting from crosses between Citrus and P. trifoliata with a favorable taste (Deterre et al 2023). Microcitrus species have also been used in rootstock and scion breeding programs, partly because of their tolerance against HLB (Bowman et al 2023;Ramadugu et al 2016). However, Microcitrus hybrids generally have not performed well as rootstocks (Bowman et al 2023), and their fruits are generally unsuitable for juicing or fresh consumption (Bowman et al 2019;Shaw et al 2000), likely requiring several generations of backcrossing to obtain suitable fruit quality in progeny for use as a commercial scion.…”

“…Foliar symptoms. Blotchy mottle and nutrient deficiency-like symptoms were scored in each canopy quadrant using a 5-point standard scale as previously described (Bowman et al 2023). Blotchy mottle was scored as follows: 1 5 no blotchy mottle; 2 5 less than 25% of leaves with blotchy mottle; 3 5 26% to 50% of leaves with blotchy mottle; 4 5 51% to 75% of leaves with blotchy mottle; and 5 5 more than 75% of leaves with blotchy mottle.…”

“…During a field study in Brazil, the HLB incidence of the 'Valencia' scion was lower on Flying Dragon (P. trifoliata) than on Rangpur lime [C. ×limon (L.) Osbeck] rootstock (Rodrigues et al 2020). Therefore, significant efforts are underway to develop new citrus scions and rootstocks with high levels of tolerance to HLB (Bowman and Joubert 2020;Bowman et al , 2023McCollum et al 2016).…”

Four New SuperSour Rootstocks for Improved Production of Sweet Orange in a Huanglongbing Environment

“…Swingle and standard sour orange are two of the most common rootstocks used in Florida. A detailed report of rootstock performance during this trial was published (Bowman et al 2023). Overall tree survival through 2021 was 94%, and tree testing by reverse-transcription polymerase chain reaction revealed that 100% of the trees were infected by CLas and 90% were infected by citrus tristeza virus.…”

“…The four hybrid rootstock selections originated from crosses made at the USDA A.H. Whitmore Foundation Farm by Kim Bowman between 2000 and 2006, and each has distinctly different parentage (Table 1). These hybrids are the product of a new USDA citrus rootstock breeding strategy, termed SuperSour, that has been previously described (Bowman and Joubert 2020;Bowman et al , 2023. Generally, the SuperSour strategy uses a broad range of parental material that includes either standard sour orange itself (C. aurantium) or some combination of the parental species that gave rise to standard sour orange, the mandarin (C. reticulata), and pummelo (C. maxima).…”