Development and Commercialization of Improved New Disease Resistant Scions and Rootstocks – the Key For a Sustainable and Profitable Florida Citrus Industry

Development and Commercialization of Improved New Disease Resistant Scions and Rootstocks - the Key For a Sustainable and Profitable Florida Citrus Industry

Report Date: 05/24/2019
Project: 15-010   Year: 2019
Category: Plant Improvement
Author: Fred Gmitter
Sponsor: Citrus Research and Development Foundation

1. Development of rootstocks that can impart HLB tolerance/resistance to grafted scions. Seedlings grown from over one dozen unreleased rootstocks, selected on the basis of their abilities to control tree size, size support good fruit loads, and to have minimal HLB symptom expression, are being budded with sweet orange scions for field planting at the St. Helena site next season. As part of the `gauntlet’ screening, we stick-grafted approximately 75 new candidate rootstock hybrids produced from HLB-tolerant parents in 2017 with HLB+ Valencia sweet orange for HLB screening; these are currently under evaluation, as are approximately 100 gauntlet candidate rootstock hybrids (including 47 hybrids crossing HLB-tolerant `LB8-9′ Sugar Belle® with complementary rootstock germplasm such as salt tolerant pummelo/mandarin hybrids and trifoliate orange 50-7). `Super Root Mutants’ of 10 selections of UFR and other rootstocks, including 3 mutants of UFR-1, 3 of UFR-3, one of UFR-4, one of UFR-17 and one of SO+50-7, are being used to produce whole trees for further evaluations of HLB tolerance.  One of these from UFR-1 has been found to be a zygotic triploid, and not a mutant from the original line.2. Breeding of HLB tolerant/resistant processing sweet oranges and orange-like hybrids. New hybrids produced have been potted up to grow until field planting next season. Fruit and juice samples of existing UF releases and new oranges under consideration for release were presented to industry at Fruit Display days in November and December. Blends of sweet orange like-selections also were presented and ranked higher than most sweet orange juice samples. New selections were identified from field plantings of orange-like hybrids and likewise were well received.  The newer selections have been entered into the DPI PTP for cleanup and certification.3. Screening of the UF-CREC germplasm collection to identify and validate HLB tolerant or resistant selections. Another season of observations was begun in fall-winter 2018 and the new data, combined with previous seasons’ data, are being reanalyzed to more accurately identify and characterize tolerant individuals. This information is also being used to target specific genome regions that may harbor genes for tolerance, to be used in other projects.4. Advanced field trials, release and commercialization of promising HLB tolerant/resistant scion and rootstock cultivars. We continued focused effort on field trial data management, analysis and interpretation. Files from more than 80 sites have been opened, conditions of the trials have been noted, and based on this information plans for 2018-19 field data collection were developed, prioritized, and implemented. Efforts to review and summarize data have continued, and information was organized for inclusion in our website, which was launched in late 2018. This website can be accessed through this link: We visited 18 different field trial sites in fall 2018 and evaluated trees fir HLB responses and overall tree health, and we harvested fruit from several for juice quality analysis.  Yield data were also collected from some of these trials and those data are being entered into our database. Finally, a very substantial effort was undertaken this year to rescue promising individual trees of diverse scion and rootstock germplasm from our 50-acre research block at the GCREC in Balm. These blocks have not been irrigated since early fall of 2017, and we were forced to leave the site. All trees in this block were subjectively assessed for potential HLB tolerance, as well as general overall health and appearance, using a 0-4 scale (0=dead; 4=completely heathy appearing tree). We harvested budwood from ~2300 individuals with scores = 3 and propagations for field planting in another location. Between June and end of September 2018, we had to revisit the block and collect additional budwood from trees that were not successfully propagated. We are growing off these trees for planting in 2019 at a new location; it appears that we were successful in recovering ~1800 of the selected individuals. 

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