Citrus greening (huanglongbing, HLB) has reduced Florida’s grapefruit production by 75% due to the high sensitivity of commercial grapefruit cultivars to the disease. New combinations of cultivars and hybrid rootstocks may provide better performance than current commercial selections, particularly in the Indian River District (IRD), a geographical area that is recognized for its high-quality fresh grapefruit production since the 1950s. The objective of this study was to evaluate and compare the early performance of several new grapefruit and grapefruit-like cultivars grown on three commercial rootstocks, by measuring tree growth and HLB tolerance. A large-scale field trial was established in Fort Pierce, FL. and included 17 grapefruit/grapefruit-like cultivars and sour orange, US-942 and X-639 rootstocks. Tree canopy volume, trunk diameter, CLas titer, HLB severity index, and leaf nutrient concentrations were evaluated during the first two years of tree growth. Significant differences among rootstock-scion combinations were found for most assessed traits. Overall, tree size significantly varied upon scion and rootstock used over the course of the evaluation. CLas infection varied with cycle threshold (Ct) values of 35–40, and HLB visual index differed significantly among scion-rootstock combinations. Trees grew vigorously over the beginning of the experiment and HLB is already present. Production and fruit quality need to be evaluated to determine the best scion-rootstock combinations that can confer consistent economic advantages amidst the ongoing HLB crisis in the IRD.

Field trials are critically important to the citrus industry in that they provide scientific data from replicated plantings to determine which rootstock-scion combinations to plant. Field trials allow for growers to make evidence-based decisions regarding cultivar selection and to determine for themselves which rootstocks work best for selected scions in their individual growing environments. The field trial reported herein was a semi-commercial scale scion trial involving several selections of ‘Valencia’ and ‘Midsweet’ on the ‘Carrizo’ citrange rootstock. The purpose of this trial was to evaluate ‘Valencia’ and ‘Midsweet’ scion selections from various sources to identify accessions that could complement the existing scion cultivar range and provide a wider harvest window. The scions consisted of advanced seedless somaclonal ‘Valencia’ selections along with irradiated budlines of ‘Midsweet’ from the Citrus Research and Education Center’s Plant Improvement Team. In addition to the somaclones and irradiated material, there was an assortment of other selections including accessions from Australia, Brazil, and California. Of special interest were young (juvenile) line sources. A secondary objective was to assess the response of the selections to huanglongbing (HLB). The location of this trial was in Lake County, FL and the trees were planted in March 2000. The design was randomized with complete blocks and four replications. The plot size was 14 trees with 15 ft. × 25 ft. spacing, which totaled 116 trees per acre.

Field trials are critically important to the citrus industry because they provide scientific data from replicated plantings that can be used to determine which rootstock-scion combinations to plant. The field trial reported here was a semi-commercial-scale scion trial involving several selections of ‘Valencia’ and one ‘Vernia’ selection on the ‘Carrizo’ citrange rootstock. The purpose of this trial was to evaluate ‘Valencia’ and ‘Vernia’ scion selections from various sources to identify accessions that could complement the existing scion cultivar range and provide a wider harvest window. The scions consisted of advanced seedless somaclonal ‘Valencia’ selections, irradiated material, and an assortment of other selections, including accessions from Australia, Brazil, and California. Of special interest were young (juvenile line) sources. A secondary objective was to assess the relative response of the selections to huanglongbing (HLB), which was discovered in Florida during this trial. The location of this trial was Lake County, FL, USA, and the trees were planted in Mar 2000. A randomized complete block design was used with four replications of multiple tree plots. Data collected included total soluble solids, titratable acidity, sugar-to-acid ratio, pounds-solids per box, number of boxes per tree, pounds-solids per acre, cumulative pounds-solids per acre, and HLB rating. There were significant differences among the scions for several variables, including boxes of fruit per tree, pounds-solids per acre, HLB rating, and others. The findings of this research suggest that there are differences in performance among scion selections for a variety of horticultural traits important to citrus growers and processors.

Huanglongbing (HLB), which is believed to be caused by the phloem-restricted bacterium Candidatus Liberibacter asiaticus (CLas), has decimated Florida’s citrus production. Grapefruit production has declined 75%, mandarin 78%, and sweet orange 52% due to the high sensitivity of commercial scions and rootstocks to the disease. New combinations of scions and hybrid rootstocks may provide better performance than current commercial selections for Florida’s fresh citrus production, particularly in the Indian River District. The objective of this study was to evaluate and compare University of Florida rootstocks and other recently released rootstocks grafted with grapefruit, navel orange, and mandarin scions by measuring tree growth and HLB tolerance. Three independent large-scale field trials were established in September 2019 in Fort Pierce, FL, USA. Trial 1 (T1) included 36 rootstocks with ‘Ray Ruby’ grapefruit as the scion; Trials 2 and 3 (T2 and T3, respectively) included 30 rootstocks with ‘Glenn’ navel orange F-56-11 and ‘UF-950’ mandarin as the scion, respectively. Tree canopy volume, trunk diameter, CLas titer, HLB severity index, and leaf nutrient concentrations were evaluated during 2020 and 2021. Significant differences among rootstock-scion combinations were found in each trial for most of the assessed traits. In T2, UFR-15 consistently developed the largest ‘Ray Ruby’ grapefruit trees during both years. In T3, ‘Glenn’ navel orange F-56-11 trees were larger on C-22, and US-802. Similarly, US-802 and US-942 generated the largest ‘UF-950’ mandarin trees. Overall, trees had optimum levels of macro- and micronutrients except for calcium. CLas infection and HLB visual index varied among scion-rootstock combinations, especially during the first year of growth when intensive flushing was produced. Generally, trees grew vigorously with WGFT+50-7 and Willits inducing the lowest HLB symptoms in all evaluations. Production and fruit quality need to be evaluated to determine the suitability of potential scion-rootstock combinations that can confer consistent economical and biological advantages under the current HLB scenario in the Indian River District.

Huanglongbing (HLB, a.k.a. citrus greening disease) has reduced Florida citrus production and acreage substantially since its discovery in 2005. Most commercially important citrus scion cultivars such as sweet oranges are susceptible to the disease and no cure is available at present. This has increased the demand for HLB-tolerant rootstocks to retain tree health and maximize productivity through higher-density plantings. Polyploidy in citrus has been associated with reduced tree size and enhanced biotic and abiotic stress tolerance. This study compared the potential of tetraploid rootstocks with diploid rootstocks for inducing dwarfing of the grafted 'Valencia' orange scion and for enhancing productivity and tolerance to HLB. Two trials, one at a poorly drained site in south-east Florida and another at a well-drained sandy site in central Florida, were established in 2015. Tree size, yield, yield efficiency, fruit quality, leaf nutrients, and canopy health were assessed over three production seasons from 2018/19 to 2020/21. The different rootstocks varied significantly in their effects on tree size, yield, and yield efficiency. Tree size induced by the diploid rootstocks, which included the Florida industry standards Swingle, sour orange, X-639, and US-897, was 55% larger on average compared to the tetraploid rootstocks. Cumulative yields were 32% higher on diploid rootstocks compared to tetraploid ones. However, tetraploid rootstocks induced a 27% higher yield efficiency on average. A significant rootstock effect on canopy health and nutrient status was found, but this effect was not consistent across locations and production years. Tetraploid rootstocks did not display higher HLB tolerance than diploid rootstocks, including the industry standards. However, effective tree size control, high yield efficiency, and higher fruit quality induced by many of the tetraploid rootstocks revealed their potential to be used in high-density plantings.

Most of the commercially important citrus scion cultivars are susceptible to Huanglongbing (HLB), which is the most devastating disease the citrus industry has ever faced. Because the rootstock can influence the performance of the scion in various ways, including disease and pest tolerance, use of superior rootstocks can assist citrus growers with minimizing the negative effects of HLB. The objective of this study was to assess rootstock effects on the horticultural performance and early production potential of ‘Hamlin’ sweet orange (Citrus sinensis) trees in commercial field settings under HLB-endemic conditions. Two field trials were conducted in different locations in Central and Southeast Florida. The trials were established in 2015 and included 32 diverse diploid and tetraploid rootstock cultivars and advanced selections. One trial was performed in Highlands County, FL, on a poorly drained flatwoods-type site. Another trial was performed in Polk County, FL, on a well-drained sandy Central Florida Ridge site. Horticultural traits including tree height, canopy volume, trunk diameter, canopy health, leaf nutrient content, yield, and fruit quality were assessed during the 2018–19 and 2019–20 production years. Significant differences were found among trees on different rootstocks for most of the measured traits, particularly tree vigor and productivity, but rootstock effects also varied by location. Rootstocks that induced large tree sizes, such as the diploid mandarin × trifoliate orange hybrids ‘X-639’, ‘C-54’, ‘C-57’, and ‘C-146’, also induced higher yield, but with lower yield efficiency. Most of the tetraploid rootstocks significantly reduced tree size, among which ‘Changsha+Benton’, ‘Green-3’, ‘Amb+Czo’, ‘UFR-3’, and ‘UFR-5’ induced high yield efficiency. Therefore, these rootstocks have the potential to be used in high-density plantings. However, trees on some of these small size-inducing rootstocks had a higher mortality rate and were more vulnerable to tropical force winds. This study provides important information for the selection of rootstocks with the greatest production potential in an HLB-endemic environment, especially during the early years of production.

The Third Edition of the Florida Citrus Rootstock Selection Guide was recently released online. This website is a valuable resource containing updated information on rootstock options. The site is unique in that several technologies there complement the rootstock table, including: 1) an interactive online version of the table; 2) an extensive bibliography containing over 100 rootstock references; and 3) an expert system to help focus on the best rootstock candidates given certain user-selected criteria. The expert system is a backward-chaining platform that interviews users about their planting and site requirements. The system uses artificial intelligence technology to infer the best candidate rootstocks based on those criteria. The results are presented in an ordered list from top to bottom showing rootstocks that might be considered. The expert system is built on the Exsys Corvid® Core for Mac OS® X platform, which has the advantages of providing robust development features at a reasonable cost. This paper presents the expert system, provides details on the development process, and discusses the results of a focus group presentation for real-world user feedback.

First published in 1989 as Rootstocks for Florida Citrus, the work of Dr. William Castle and his colleagues remains relevant 30 years later. The purpose of the Florida Citrus Rootstock Selection Guide (FLCRSG) is to provide timely and useful citrus rootstock information to help Florida citrus growers make well-grounded, practical decisions. New problems with abiotic factors, pests, and diseases make the FLCRSG a standard document for the Florida citrus industry. Initially published as a book, it was integrated into an informative wheel and is now a web-based expert system with an interactive table with 21 characteristics of 48 rootstocks. Much has changed within the Florida citrus industry since the discovery and spread of the vector-transmitted disease huanglongbing (HLB). Rootstocks were not initially part of the discussion related to managing HLB, but that has changed, particularly given the accumulating evidence that trees on various rootstocks may differ in the incidence or tolerance of the disease. The authors have prepared this timely and necessary update of the former editions and have expanded the list of rootstocks. We revised the information on the UFR series and added three new U.S. rootstocks (US SuperSours) that have not yet undergone the usual extensive field evaluation in Florida. These new rootstocks offer improvements regarding HLB tolerance and several other meaningful traits, such as tree size, high yield, and juice quality that appear essential to the future of our citrus industry.

This updated 4th edition of the Florida Citrus Rootstock Selection Guide (FLCRSG) is a revision of the 2016 publication. The guide is a convenient, easy-to-use reference to 21 characteristics of 49 rootstocks. Of those, 12 are time-honored commercial rootstocks (highlighted in blue), which are the most reliably characterized. The next 13 rootstocks are minor commercial ones (highlighted in green) that are less frequently used today in Florida but may have been prominent at one time. The third group consists of the most recently released 24 rootstocks (highlighted in yellow) for which there is limited commercial experience. The new addition includes three new USDA rootstocks and updates information on a few traits. Written by William S. Castle, Kim D. Bowman, Jude W. Grosser, Rhuanito Soranz Ferrarezi, Stephen H. Futch, and Steve Rogers, and published by the UF/IFAS Horticultural Sciences Department.https://edis.ifas.ufl.edu/hs1260

After the arrival in 2005 of citrus greening disease or Huanglongbing in Florida, making a profitable rootstock decision became more complicated. New rootstocks are being developed and released for commercialization at an accelerated pace. Regardless of these changes, there remains a time-honored framework for selecting rootstocks. This publication provides guidance in 4 aspects of scion selection: site history, objectives, sources of information on rootstocks, and choosing a rootstock that matches site and grower objectives.https://edis.ifas.ufl.edu/hs178 This is a revision, original publication: Castle, William, and James Ferguson. 1. “Considerations for Choosing the Right Rootstocks”. EDIS 2003 (13). https://journals.flvc.org/edis/article/view/108974.

Anthracnose fruit rot and leaf blight caused by Colletotrichum species are important diseases of pomegranate in the southeastern United States. In this study, 26 isolates from pomegranate were identified based on pathological and molecular characterization. Isolates were identified to species based on multilocus sequence analysis with the internal transcribed spacer region, glyceraldehyde-3-phosphate dehydrogenase, β-tubulin, and chitin synthase genomic genes. Pomegranate isolates grouped within the C. acutatum and C. gloeosporioides species complexes, with more than 73% belonging to the latter group. Three species were identified within the C. acutatum species complex (C. nymphaeae [n = 5], C. fioriniae [n = 1], and C. simmondsii [n = 1]), and three other species were identified within the C. gloeosporioides species complex (C. theobromicola [n = 11], C. siamense [n = 6], and C. gloeosporioides [n = 2]). Inoculations of pomegranate fruit showed that isolates from the C. acutatum species complex were more aggressive than isolates from the C. gloeosporioides species complex. Interestingly, opposite results were observed when leaves of rooted pomegranate cuttings were inoculated. In addition, Colletotrichum isolates from pomegranate, strawberry, blueberry, mango, and citrus were cross-pathogenic when inoculated to fruit. This is the first study identifying six different species of Colletotrichum causing pomegranate leaf blight and fruit anthracnose in the southeastern United States and the potential cross-pathogenic capability of pomegranate isolates to other commercially important crops.

This 6-page fact sheet that was submitted for sunset review provides practical information and photographs to anyone interested in field identification of Australian Pine. Written by William S. Castle and Michael Andreu, and originally published by the UF Department of Horticultural Sciences, May 2008. Reviewed July 2017. HS1140/HS394: Field Guide to Identify the Common Casuarina (Australian Pine) Species in Florida (ufl.edu)

Information about citrus rootstocks has become an important part of understanding and managing citrus greening (Huanglongbing or HLB). This selection guide covers 20 characteristics of 45 citrus rootstocks and explains its methodology in detail. This revised 3-page fact sheet was written by William S. Castle, Kim D. Bowman, Jude W. Grosser, Stephen H. Futch, and James H. Graham and published by the UF Department of Horticultural Sciences, May 2015. SP248/HS1260: Florida Citrus Rootstock Selection Guide, 4th Edition (ufl.edu)

This document marks the official release of US Furr, a hybrid of Clementine x Murcott, and US Furr-ST, an irradiated variant of US Furr with apparent field tolerance to citrus scab (causal agent Elsinoe fawcetti Bitanc. and Jenk.). The hybridization creating US Furr and ultimately US Furr-ST was made at the USDA Horticultural Research Laboratory in Orlando, Florida, by Dr. Phillip Reece in 1953. Seeds were sent to the USDA Date and Citrus Station in Indio, California for hybrid evaluation. Dr. Joseph Furr identified this superior selection from the resulting seedlings. US Furr has been sufficiently promising that it has been introduced into at least five countries, under several different names. Budwood of US Furr was introduced into Argentina, Brazil, France (Corsica), Israel, and Spain in the 1980s and 1990s using material prior to irradiation and some possibly post-irradiation. 'US Furr' and US Furr-ST are high quality, December/January-maturing mandarins with excellent rind color, superior flavor, and moderate peelability (rind comes off in pieces similar to Sunburst and Murcott). US Furr and US Furr-ST are sexually self-compatible and fruit in mixed plantings average 12 to 24 highly polyembryonic seeds per fruit, and 6 to 12 seeds per fruit when planted in isolation from compatible pollinating varieties. Furthermore, very few to no fruit are set when flowers are bagged, indicative of low parthenocarpy. US Furr and US Furr-ST fruit average 150-215 g per fruit at maturity. US Furr and US Furr-ST trees are moderately vigorous, thornless, and spreading with fairly dense foliage. The diversity of names used for US Furr / US Furr-ST has reduced awareness that a single genotype has garnered wide attention, limiting impact and resulting in few US plantings. It is anticipated that this official release will garner increased interest in these cultivars leading to increased plantings. Many tasters report that US Furr and US Furr-ST are among the best tasting citrus they have eaten. These mandarin cultivars merit consideration for planting as a part of a mid-late season mandarin portfolio, and are released without any intellectual property restrictions.

An efficient in vitro regeneration system through direct shoot organogenesis was established for Murraya paniculata (L.) Jack (Orange Jessamine). Epicotyls, leaves, roots, and cotyledons from in vitro-germinated seedlings and several plant growth regulators (PGRs) were evaluated for their effects on plant regeneration. Longitudinally cut epicotyl segments were observed to be the optimal explants followed by uncut epicotyls (not longitudinally cut). Roots, leaves, and cotyledons were not suitable as explants as a result of little or no shoot induction. Adventitious shoot induction was enhanced by the addition of 6-benzyladenine (BA). The highest percentage of shoot induction (87%) and the greatest number of shoots per explant (12.7) occurred on Murashige and Skoog (MS) medium supplemented with 15 μM BA from longitudinally cut epicotyls followed by 5.2 shoots per explant from uncut epicotyls. Optimal concentration of gibberellic acid (GA 3 ) for shoot elongation was observed to be 15 μM. Eighty-five percent of the regenerated shoots produced roots with an average of three roots per shoot on MS medium supplemented with 5 μM indole-3-butyric acid (IBA). Our protocol for direct shoot organogenesis can potentially lead to the development of a robust method for production of transgenic plants of M. paniculata through Agrobacterium -mediated genetic transformation.