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: 10/21/2019
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. Trees that were selected from the Gauntlet screen from 2018 crosses were stick grafted with CLas-infected Valencia budwood for further selection of tolerant types; those exhibiting severe HLB symptoms were discarded, others that are mildly symptomatic remain. We completed DNA fingerprinting to verify the origins of the “Super-Root Mutants” that have been selected from in vitro propagations in a commercial nursery and found some of these to be of zygotic origin, not resulting from mutations. Field performance of some has been encouraging, so we will continue this work.
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, and we are monitoring fruit and seed development. A handful of fruit from interploid crosses are being harvested to begin embryo rescue to recover the triploid hybrids; diploid crosses are being held on the tree until seeds are close to maturity, to maximize the recovery of new hybrids.
3. Screen our ever-growing germplasm collection for more tolerant types and evaluate fruit quality of candidate selections. We have once again gone through the collection and evaluated tree health, to remove those trees that are failing from our list of tolerant types; the numbers of trees dropped from that list has been declining in the past evaluations, so our confidence in the performance of those remaining increases. There have been no collection-wide fruit quality evaluations conducted after early May, though we have noted a few selections with very late maturity.
4. Conduct studies to unravel host responses to CLas and select targets for genetic manipulations leading to consumer-friendly new scion and rootstock cultivars. We have completed anatomical studies of HLB-tolerant LB8-9 and Bearss lemon and demonstrated that phloem regeneration is an obvious physical mechanism of their apparent tolerance. We have looked at the metabolomic profiles of this same set of plants, and these data have been analyzed. We are drawing conclusions in comparison with similar metabolic studies using different chemical techniques. Once completed, we will submit a manuscript detailing the findings, including information on potential biomarkers for HLB-tolerance in scion cultivars.

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