SuperSour: A New Strategy for Breeding Superior Citrus Rootstocks

Abstract

Citrus crops have a long history of cultivation as grafted trees on selected rootstock cultivars, but all current rootstocks have significant limitations and traditional methods of rootstock breeding take at least 2–3 decades to develop and field test new rootstocks. Citrus production in the United States, and other parts of the world, is impaired by a wide range of biotic and abiotic problems, with especially severe damage caused by the disease huanglongbing (HLB) associated with Candidatus Liberibacter asiaticus. All major commercial citrus scion cultivars are damaged by HLB, but tree tolerance is significantly improved by some rootstocks. To overcome these challenges, the USDA citrus breeding program has implemented a multi-pronged strategy for rootstock breeding that expands the diversity of germplasm utilized in rootstock breeding, significantly increases the number of new hybrids evaluated concurrently, and greatly reduces the time from cross to potential cultivar release. We describe the key components and methodologies of this new strategy, termed “SuperSour,” along with reference to the historical favorite rootstock sour orange (Citrus aurantium), and previous methods employed in citrus rootstock breeding. Rootstock propagation by cuttings and tissue culture is one key to the new strategy, and by avoiding the need for nucellar seeds, eliminates the 6- to 15-year delay in testing while waiting for new hybrids to fruit. In addition, avoiding selection of parents and progeny based on nucellar polyembryony vastly expands the potential genepool for use in rootstock improvement. Fifteen new field trials with more than 350 new hybrid rootstocks have been established under the SuperSour strategy in the last 8 years. Detailed multi-year performance data from the trials will be used to identify superior rootstocks for commercial release, and to map important traits and develop molecular markers for the next generation of rootstock development. Results from two of these multi-year replicated field trials with sweet orange scion are presented to illustrate performance of 97 new hybrid rootstocks relative to four commercial rootstocks. Through the first 7 years in the field with endemic HLB, many of the new SuperSour hybrid rootstocks exhibit greatly superior fruit yield, yield efficiency, canopy health, and fruit quality, as compared with the standard rootstocks included in the trials.

Survey and detection for citrus tristeza virus in Florida groves with an unconventional tool: The Asian citrus psyllid

“…The perennial production of citrus trees allows for repeated encounters with pathogens year after year, especially in the HLB endemic environment in Florida. The observed tolerance of sour orange to HLB has also brought back interest in replanting trees with this rootstock and its hybrids ( Albrecht et al., 2012 ; Stover et al., 2016 ; Castle et al., 2020 ; Bowman et al., 2021 ). Together this shows the current transforming status of CTV in Florida and stresses the importance of mitigating potentially larger CTV epidemics.…”

“…CTV is phloem-limited, can infect all commercial citrus, and is vectored by aphids (Hemiptera: Aphididae ) ( Norman and Grant, 1956 ; Simanton and Knorr, 1969 ; Bar-Joseph et al., 1979 ; Roistacher and Bar-Joseph, 1987 ; Bar-Joseph et al., 1989 ). CTV can be devastating in sweet orange ( Citrus sinensis ) trees grafted onto sour orange ( Citrus aurantium ), a historically popular rootstock for its exceptional traits ( Bowman et al., 2021 ). However, following the initial detection of CTV in Florida in 1952 ( Grant, 1952 ), the distribution and diversity of the virus continue to pose significant challenges to citrus in the state ( Hilf and Garnsey, 2002 ).…”

“…However, following the initial detection of CTV in Florida in 1952 ( Grant, 1952 ), the distribution and diversity of the virus continue to pose significant challenges to citrus in the state ( Hilf and Garnsey, 2002 ). The susceptibility of sour orange rootstock to CTV-decline during the latter half of the twentieth century drastically reduced its use in Florida ( Bowman et al., 2021 ).…”

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

“…Work has been conducted over many years at several institutions to develop and evaluate superior new citrus rootstock cultivars, including at the United States Department of Agriculture (USDA) ( Wutscher and Bowman, 1999 ; Bowman and Rouse, 2006 ; Bowman et al., 2016b , Bowman et al, 2021 ), University of Florida ( Castle, 2010 ; Grosser et al., 2016 ; Grosser et al., 2020 ; Kunwar et al., 2021 ), University of California ( Bitters, 1986 ; Roose et al., 1989 ), Valencian Institute of Agricultural Research (IVIA) in Spain ( Forner-Giner et al., 2003 ; Forner-Giner et al., 2014 ; Martínez-Cuenca et al., 2016 ), CRA-Research Center for Citriculture and Mediterranean Crops (CRA-ACM; CREA) in Italy ( Russo and Reforgiato Recupero, 1984 ; Reforgiato Recupero et al., 2009 ), and in Brazil under the Brazilian Agricultural Research Corporation (EMBRAPA), The Sylvio Moreira Citrus Culture Center, Fundecitrus-IDR-Paraná, and University of São Paulo ( Cantuarias-Avilés et al., 2010 ; Costa et al., 2020a ; Costa et al., 2020b ; Soratto et al., 2020 ; Domingues et al., 2021 ). These efforts have involved a combination of testing rootstock clones already in existence (including those imported from other regions) and creating new clones by sexual hybridization between two parental species.…”

“…Although citrus breeding has been a focus of international research for over 100 years, the long lifecycle and widespread apomixis in most citrus species has limited development of foundational genetic information associated with rootstocks and effective tools for genetic evaluation of a broad array of citrus parents and progeny ( Bowman et al, 2021 ). Recent work has made important strides to bring molecular genetic tools to bear in the genetic improvement of citrus ( Gois et al., 2016 ; Imai et al., 2016 ; Minamikawa et al., 2017 ; Lima et al., 2018 ; Imai et al., 2019 ; Raveh et al., 2020 ; Shimizu et al., 2020 ), but efforts to date have generally focused on scion germplasm and traits most relevant to genetic improvement of citrus scions rather than rootstocks.…”