Use performance of 300 hybrids in established trials to map Huanglongbing tolerance/resistance genes and release superior new rootstocks

Research by: USDA-ARS in Ft. Pierce, FL; University of California, Riverside in Riverside, CA, University of Florida in Wimauma, FL and Immokalee, FL

Article written by: Danelle Seymour
Article edited by: Kim Bowman, Peggy G. Lemaux

What is the research?

The USDA rootstock breeding program has previously identified several rootstocks with tolerance to HLB that are significantly more productive than other industry standard rootstocks in HLB-endemic environments. However, the current best HLB-tolerant rootstock still suffers a yield loss when diseased and further gains in tolerance or resistance to HLB are needed to sustain the industry. Field citrus trees in Florida are now uniformly infected with Candidatus ?Liberibacter asiaticus (CLas), providing an opportunity to screen thousands of trees for the ability to yield high quality fruit and juice in the presence of HLB. Beginning in 2014, a series of replicated field trials across three HLB-endemic growing regions in FL were initiated by the USDA rootstock breeding program (Figure 1). These trials now include over 300 new rootstock hybrids that have been evaluated in replicated field trials for up to eight years. Traits associated with field performance, fruit and juice quality, and disease progression in the common sweet orange scion have been measured yearly across more than 6,000 trees. Ultimately, this material will deliver new HLB-tolerant rootstocks to growers. In addition, the scale of these trials is being leveraged to dissect genetic control of tolerance or resistance to HLB in order to boost the efficiency of citrus rootstock breeding in the future.

What are major successes to date?

Several rootstock selections have been made that were identified with superior performance relative to industry standards. These selections have been submitted to clean budwood programs to facilitate their testing across different environments to determine if the productivity and overall performance observed in these trials hold up in multiple locations. Additionally, we have been able to demonstrate that there is genetic control of rootstock-mediated HLB tolerance, with substantial heritability observed in traits related to yield, tree size, and disease progression. This is good news for selective breeding for tolerance and/or resistance to HLB, indicating that breeders can continue to improve levels of rootstock-mediated tolerance to HLB.

Who is working on this project?

Kim Bowman from the USDA-ARS in Ft. Pierce, Florida, Danelle Seymour and Philippe Rolshausen from the University of California, Riverside, and Ute Albrecht and Zhanao Deng from the University of Florida.

What are the challenges and opportunities?

There is strong HLB disease pressure in Florida, which enables large-scale field testing for HLB-tolerance or resistance that is impossible in other regions. As a result, several promising new rootstock hybrids with superior performance under HLB-endemic conditions were identified in the trials established in 2014. We will continue to evaluate trials planted at later dates and expect additional rootstock hybrids with enhanced HLB tolerance to be identified. Because these hybrids have been evaluated in Florida only, although often in one or two locations, it is a challenge to predict their performance in other regions, each with unique biotic and abiotic stress conditions. In the future, we hope to initiate focused testing of rootstock selections for region-specific stresses so that recommendations can be tailored to the needs of citrus growers in each region.

Funding source: USDA-National Institute of Food and Agriculture (NIFA) - Citrus Disease Research and Extension Program Award # 2021-70029-36052