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: 06/26/2019
Project: 18-011   Year: 2019
Category: Plant Improvement
Author: Fred Gmitter
Sponsor: Citrus Research and Development Foundation

This new project is a continuation of work that was ongoing under CRDF 15-010; the official start date for the project was 1 February 2019. We report below on the activities in the first two months of the project by objectives. 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 and are now under observation. Several crosses were made in spring 2019 for rootstock improvement at the diploid and tetraploid levels, using parents that have previously demonstrated good genetic combining ability, to generate new families for selection that combine tolerance of high pH calcareous soil and Phytophthora, with appreciable tolerance of HLB in grafted scions. Previous good parental combinations have included A+HBP x Sour Orange + Rangpur, among others at the tetraploid level. Recently we have used LB8-9 Sugar Belle as a seed parent with various pollen parents and have selected several strong survivors in the Gauntlet. We have repeated some of these crosses to increase the likelihood of finding superior performers. We are using DNA fingerprinting techniques to verify the origins of the “Super-Root Mutants” that have been selected from in vitro propagations in a commercial nursery. 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. A large number of crosses were made in the spring 2019, to produce families for selection of individuals that may meet this objective and be valuable for commercialization following advanced testing, as well as to continue the process of parent development to improve future outcomes from breeding. The former will meet the needs of the industry in the near-term while the latter will lay the basis for a future of quantitatively enhanced tolerance of HLB. Over 25 crosses were made for grapefruit development, using at least one parent that has been shown to be highly tolerant of HLB and citrus canker; 10 crosses were made to create families of sweet orange like hybrids, including 2 using Parson Brown as pollen parent at the recommendation of some citrus growers; and 40 crosses for mandarin hybrid development and parent building. Some of the crosses for mandarin improvement may yield also sweet orange-like hybrids. 3. Screen our ever-growing germplasm collection for more tolerant types and evaluate fruit quality of candidate selections. We evaluated the fruit quality attributes of several very tolerant types previously identified, but we have not carried out whole collection assessments of tree health vis a vis HLB in these two months.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.     

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