Part A – The UF/CREC Core Citrus Improvement Program (Complementary to Part B – The UF/CREC Citrus Improvement Program’s Field Trial Evaluations)

Part A - The UF/CREC Core Citrus Improvement Program (Complementary to Part B - The UF/CREC Citrus Improvement Program's Field Trial Evaluations)

Report Date: 02/08/2020
Project: 18-011   Year: 2019
Category: Plant Improvement
Author: Fred Gmitter
Sponsor: Citrus Research and Development Foundation

1. Develop new rootstocks that impart HLB-tolerance to scion cultivars. Twelve new rootstock crosses were made in spring 2019 and seeds were harvested and planted into the calcareous soil/ Phytophthora screen, the first stage of the “gauntlet” screening protocol. Seeds were harvested from many of the UFRs for distribution to nurseries. Seeds from various unreleased rootstock candidates, showing good performance (HLB tolerance, good yields and fruit quality, some tree size controlling with more efficient canopies) in several field trials throughout the industry were harvested to be available for future trials with interested industry partners in Florida, as well as for new trials in other citrus production areas. New hybrids from 2018 crosses for rootstock improvement were field planted. 2. Develop new, HLB-tolerant scion cultivars from sweet orange germplasm, as well as other important fruit types such as grapefruit, mandarins, and acid fruit. Spring crosses for this objective were numerous. We harvested fruit from more than 2 dozen interploid crosses, including several designed to produce sweet orange-like fruit, using HLB tolerant parents selected by our program previously. In addition, selected HLB and canker tolerant pummelo breeding parents were crossed with diploid and tetraploid grapefruit, to produce new grapefruit hybrids with enhanced tolerance of HLB. Embryo rescue has been completed and plant production is underway. Over 100 new somaclones have been regenerated in vitro from EV1 and EV2, for further selection. Four new grapefruit hybrid selections were made in this autumn 2019 that produce fruit very similar to grapefruit in appearance, color and flavor, but with improved fruit quality attributes and substantially more tolerant trees, and these have been entered into the DPI Parent Tree Program for cleanup. Finally, the OLL-20 sweet orange was approved for release to the Florida citrus industry. 3. Screen our ever-growing germplasm collection for more tolerant types and evaluate fruit quality of candidate selections. We have explored some new approaches to quantifying tree responses to HLB, in addition to the previously used subjective approaches. Specifically, we have begun measuring photosynthetic parameters and leaf canopy indexes, to produce repeatable and reliable quantitative data in support of further genetic analyses of tolerant types. This work is ongoing, and it will improve the precision with which we can define HLB tolerance genes. We have evaluated fruit quality of the more tolerant types of sweet orange-like hybrids, as well as mandarins and grapefruit hybrids, and identified some worthy of further evaluation as potential new cultivars. 4. Conduct studies to unravel host responses to CLas and select targets for genetic manipulations leading to consumer-friendly new scion and rootstock cultivars. Using the quantitative data described in 3. above, we are preparing to conduct additional GWAS to validate previously identified or to identify new genomic regions associated with HLB tolerance and/or sensitivity. Several new genetic constructs have been developed using newly identified citrus specific promoters (phloem and root tissue), and new putative disease resistance genes, or downstream genes. Transgenic plants have been produced with some of these constructs, and additional transformation experiments have been begun.    

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