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A special issue of Agronomy (ISSN 2073-4395). This special issue belongs to the section "Horticultural and Floricultural Crops".

Deadline for manuscript submissions: closed (15 December 2020) | Viewed by 35628

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Dr. Fred Gmitter

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Guest Editor

Citrus Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Lake Alfred, FL 33809, USA
Interests: citrus breeding and genetics for variety improvement; genomic research to develop efficient ways to select improved plants and to enable cloning and manipulation of important genes; seedless variety development by mutation breeding and tissue culture methods; citrus germplasm acquisition and characterization; applied genetic transformation research

Dr. Ute Albrecht

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Guest Editor

Southwest Florida Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Immokalee, FL 34142, USA
Interests: citrus; citrus production; citrus diseases; rootstocks; rootstock propagation; root traits; rootstock-scion interactions; huanglongbing
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Special Issue Information

Dear Colleagues,

Citrus is the most widely grown and economically significant fruit crop worldwide. Sweet oranges are the most ubiquitous cultivar group, but the diversity of citrus also includes mandarins, grapefruit, pummelos, lemons, limes, and several other minor types. Although there is substantial genetic diversity within Citrus, these most widely known citrus fruits represent cultivar groups, which diversified predominantly through accumulated somatic mutations and are not products of sexual hybridization. The narrow genetic diversity in cultivated types has made citrus very vulnerable to damaging diseases such as Huanglongbing (HLB, or citrus greening), which has recently spread rapidly through most of the world’s citrus producing regions. Further, the market-defined phenotypes specific to sweet orange, grapefruit, etc. impose limitations on the breeding techniques that can be utilized. Nonetheless, citrus breeding programs have been active in developing and releasing new cultivars. As these new cultivars find their way to commercialization, new techniques for production and orchard management are being developed and utilized especially in regions where HLB is endemic. This Special Issue focuses on the following topics: (i) innovative scion/rootstock breeding strategies; (ii) molecular or genome-based techniques; (iii) unique and new phenotype creation; (iv) consumer driven breeding; (v) cultivar evaluation; and (vi) orchard design, water and nutrient management, and protected structure production.

Dr. Fred Gmitter
Dr. Ute Albrecht
Guest Editors

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Keywords

citrus breeding
molecular breeding
genomics
breeding techniques
citrus production

Published Papers (9 papers)

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Research

16 pages, 4254 KiB

Open AccessArticle

Optimizing Recovery of Hybrid Embryos from Interspecific Citrus Crosses of Polyembryonic Rough Lemon (Citrus jambhiri Lush.)

by Jagveer Singh, Harvinder Singh Dhaliwal, Anirudh Thakur, Gurupkar Singh Sidhu, Parveen Chhuneja and Frederick G. Gmitter, Jr.

Agronomy 2020, 10(12), 1940; https://doi.org/10.3390/agronomy10121940 - 10 Dec 2020

Cited by 6 | Viewed by 2671

Abstract

The prevalence of rough lemon rootstock in India is presently threatened by the spread of Phytophthora root rot. Developing an alternative Phytophthora-resistant rootstock that also withstands other abiotic constraints is a priority for a sustainable citrus industry in India. Complementary progenitors can be found in citrus germplasm to combine the desired traits, particularly between Citrus and Poncirus, a closely related genus. The production of hybrids accumulating the desired dominant traits, irrespective of the heterozygosity level of the parents, is an effective way to develop new candidate rootstocks. Rough lemon was hybridized with Swingle citrumelo, X-639 citrandarin, and GouTou Cheng. A reliable method was developed for greater recovery of hybrid seedlings from these crosses on the basis of morphology and simple sequence repeat (SSR) markers. The greatest proportion of hybrid seedlings resulted from embryo rescue at 95 days after pollination (DAP) (48.5% in rough lemon × X-639 citrandarin and 56.7% in rough lemon × Swingle citrumelo) on the basis of multifoliate leaves. The proportion of hybrid seedlings decreased with increase in embryo development stage after 95 DAP until the last stage of observation (125 DAP). Significantly higher proportion of multifoliate seedlings was recovered by in vitro culture compared with in vivo germination from the mature seeds (26.9% vs. 5.2%, respectively). Swingle and X-639 citrandarin are not homozygous for the multifoliate allele from Poncirus, and GouTou Cheng lacks the multifoliate allele; thus, unifoliate hybrids were selected from all crosses on the basis of SSR marker genotypes. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

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12 pages, 261 KiB

Open AccessArticle

Rootstock Influences on Health and Growth Following Candidatus Liberibacter asiaticus Infection in Young Sweet Orange Trees

by Kim D. Bowman and Ute Albrecht

Agronomy 2020, 10(12), 1907; https://doi.org/10.3390/agronomy10121907 - 02 Dec 2020

Cited by 16 | Viewed by 2181

Abstract

Health, growth, fruit production, and fruit quality of citrus crops are severely affected by tree infection with Candidatus Liberibacter asiaticus (CLas) and subsequent development of the disease huanglongbing (HLB). The use of HLB-tolerant rootstocks is one strategy that is used to ameliorate the effects of HLB in commercial production. Although there is a clear long-term rootstock effect to improve tree performance, such field trials take many years for establishment and evaluation, and this long time-period is very limiting to expeditious evaluation of new rootstocks. In this study, we have conducted a 50-week greenhouse experiment to evaluate rootstock influences on Valencia sweet orange tree response to CLas infection. The infection of trees with CLas reduced scion and rootstock growth, increased leaf yellowing, and reduced the number of leaves per tree and leaf area, regardless of rootstock. There were clear rootstock influences on some traits during the 50-week study. In general, infected trees on US-942 rootstock had lower CLas root titers, less reduction of the number of leaves, less reduction of leaf area, and less leaf yellowing, as compared with some of the other rootstocks. The 50-week greenhouse evaluation method provided results that corresponded well with results from long-term field testing, indicating this may be a useful tool to accelerate evaluation and selection of new rootstocks, as well as in testing other HLB management strategies. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

21 pages, 5367 KiB

Open AccessArticle

Disease Resistant Citrus Breeding Using Newly Developed High Resolution Melting and CAPS Protocols for Alternaria Brown Spot Marker Assisted Selection

by Carmen Arlotta, Angelo Ciacciulli, Maria Concetta Strano, Valeria Cafaro, Fabrizio Salonia, Paola Caruso, Concetta Licciardello, Giuseppe Russo, Malcolm Wesley Smith, Jose Cuenca, Pablo Aleza and Marco Caruso

Agronomy 2020, 10(9), 1368; https://doi.org/10.3390/agronomy10091368 - 11 Sep 2020

Cited by 12 | Viewed by 3786

Abstract

Alternaria alternata is a fungus that causes a serious disease in susceptible genotypes of citrus, particularly in mandarins. The Alternaria citri toxin (ACT) produced by the pathogen induces necrotic lesions on young leaves and fruits, defoliation and fruit drop. Here, we describe two methods of marker-assisted selection (MAS) that could be used for the early identification of Alternaria brown spot (ABS)-resistant mandarin hybrids. The first method is based on a nested PCR coupled to high resolution melting (HRM) analysis at the SNP08 locus, which is located at 0.4 cM from the ABS resistance locus, and was previously indicated as the most suitable for the selection of ABS-resistant hybrids. The method was validated on 41 mandarin hybrids of the CREA germplasm collection, and on 862 progenies generated from five crosses involving different susceptible parents. Four out of five populations showed Mendelian segregation at the analyzed locus, while a population involving Murcott tangor as male parent showed distorted segregation toward the susceptible hybrids. The second method is based on a cleaved amplified polymorphic sequences (CAPS) marker that was developed using the same primers as the nested PCR at the SNP08 locus, coupled with BccI restriction enzyme digestion. To verify the reliability of the two genotyping methods, in vitro leaf phenotyping was carried out by inoculating A. alternata spores onto young leaves of 101 hybrids, randomly chosen among the susceptible and resistant progenies. The phenotyping confirmed the SNP08 genotyping results, so the proposed method of selection based on HRM or CAPS genotyping could be routinely used as an alternative to KBioscience competitive allele specific polymerase chain reaction (KASPar) single nucleotide polymorphism (SNP) genotyping system to improve citrus breeding programs. While the study confirmed that the SNP08 marker is a reliable tool for MAS of new citrus hybrids with different genetic backgrounds, it also identified a small group of genotypes where the resistance mechanism requires further investigation. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

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12 pages, 2857 KiB

Open AccessArticle

Rootstocks Influence Yield Precocity, Productivity, and Pre-Harvest Fruit Drop of Mandared Pigmented Mandarin

by Marco Caruso, Alberto Continella, Giulia Modica, Claudia Pannitteri, Riccardo Russo, Fabrizio Salonia, Carmen Arlotta, Alessandra Gentile and Giuseppe Russo

Agronomy 2020, 10(9), 1305; https://doi.org/10.3390/agronomy10091305 - 02 Sep 2020

Cited by 18 | Viewed by 3484

Abstract

Citrus fruit quality and scion productivity are influenced by the choice of rootstock. We aimed to evaluate the effect of rootstocks on yield and fruit quality of Mandared, a triploid pigmented mandarin. To do so, we established a rootstock field trial on a high pH soil (8.6) in which Mandared was grafted onto 11 rootstocks. These included some standard rootstocks, such as trifoliate orange ((Poncirus trifoliata (L.) Raf.), Troyer citrange (Citrus sinensis (L.) Osb. × P. trifoliata), Swingle citrumelo (Citrus paradisi Macf. × P. trifoliata), and C35 citrange (C. sinensis × P. trifoliata), as well as new releases from the Council for Agricultural Research and Economics (CREA, Acireale, Italy) and the University of California Riverside (UCR). The cumulative yield was measured over five consecutive years, while fruit quality was analyzed for two years. The trees on C35, C57 (Citrus sunki Hort. ex. Tan. × P. trifoliata), and C22 (C. sunki × P. trifoliata), started to set fruits one year earlier than the others. The trees on C57 provided some of the highest cumulative yields and canopy volumes. The production of Mandared grafted onto C57 was double that of Mandared grafted onto Troyer, while Mandared grafted onto C35 and C22 resulted in the best yield efficiency. The trees on Swingle and C57 significantly reduced the pre-harvest fruit drop, to which Mandared is particularly sensitive. However, grafting Mandared onto Swingle resulted in the highest variation among replicates, probably due to its high sensitivity to iron chlorosis. Most of the fruit quality parameters, such as fruit size, total soluble solids (TSS), and acidity were not significantly different among the rootstock treatments. However, fruits produced by Mandared grafted onto C22 had one of the highest rates of anthocyanin accumulation. The results indicate that C57, C35, and C22 were the most suitable rootstocks for Mandared in South-Eastern Sicily. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

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16 pages, 2778 KiB

Open AccessArticle

Fruit Yield and Quality of ‘Valencia’ Orange Trees under Long-Term Partial Rootzone Drying

by Amr Mossad, Vittorio Farina and Riccardo Lo Bianco

Agronomy 2020, 10(2), 164; https://doi.org/10.3390/agronomy10020164 - 23 Jan 2020

Cited by 21 | Viewed by 7285

Abstract

Climate, soil and tree water status, fruit yields and quality of ‘Valencia’ orange trees were monitored over five consecutive seasons (2007–2012) to study the effects of irrigation placement or volume. Three irrigation treatments were imposed: conventional irrigation (CI, 100% of crop evapotranspiration on both sides of the rootzone), partial rootzone drying (PRD, 50% of CI water only on one alternated side of the rootzone) and continuous deficit irrigation (DI, 50% of CI water on both sides of the rootzone). Yield parameters were generally not affected by PRD, and only yield per tree was lower in DI than CI trees. Fruit size and juice content were also reduced by DI, and not by PRD. Both PRD and DI increased juice soluble solids and acidity, vitamin C and carotenoid concentrations, as well as fruit, juice and sugar productivity per unit of irrigation water. Overall, the straight reduction of irrigation volumes by ~55% (DI) induced an average of 4.4% leaf dehydration, which caused significant fruit size reductions and a 3.4% reduction in juice yield, corresponding to a loss of about 2.6 tons of juice and 261 kg of sugars per hectare and year. On the other hand, water savings of about 2 mega liters per hectare and year with PRD caused a mild 2.3% leaf dehydration and mainly fruit quality improvements, indicating that PRD is a sustainable irrigation strategy for ‘Valencia’ orange. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

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11 pages, 2473 KiB

Open AccessArticle

Phospholipase D (PLD) Response to Water Stress in Citrus Roots and Leaves

by Fernando Alferez, Jian Wu and James H. Graham

Agronomy 2020, 10(1), 45; https://doi.org/10.3390/agronomy10010045 - 27 Dec 2019

Cited by 4 | Viewed by 2832

Abstract

The effects of drought stress on phospholipase D (PLD) gene expression and enzymatic activity were investigated in ‘Pineapple’ sweet orange. PLDs are tissue-specific, with overlapping functions, and in response to stress they may interact with ABA signaling. Tissue specificity for expression of PLDs and their regulation are unknown in citrus. To assess PLD response to water stress and gene expression/regulation in citrus, we subjected potted seedlings to increasing levels of soil drought. Evapotranspirational demand (ET) was estimated by measuring weight loss in pots, and water stress was further assessed by measuring ABA content. Three treatments were performed over a 3-week period: (a) Control treatment without drought stress (100% of daily ET); (b) mild water stress (50% ET); and (c) severe water stress (0% ET). ABA content increased during drought stress in both roots and leaves, being higher in leaves than in roots by the end of the experiment for any stress condition assayed. PLD enzymatic activity was monitored and expression of five PLD genes was studied. PLD activity increased linearly over time in response to increased soil drought and was around three times higher in roots than in leaves. PLD activation occurred initially in roots and then in leaves. PLD gene expression in response to stress soil drought differed between roots and leaves. These results show the potential of PLD as a suitable indicator of stress severity in citrus. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

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17 pages, 1931 KiB

Open AccessArticle

Nutrient Status and Root Density of Huanglongbing-Affected Trees: Consequences of Irrigation Water Bicarbonate and Soil pH Mitigation with Acidification

by Kelly T. Morgan and James H. Graham

Agronomy 2019, 9(11), 746; https://doi.org/10.3390/agronomy9110746 - 12 Nov 2019

Cited by 13 | Viewed by 3099

Abstract

The Candidatus Liberibacter asiaticus bacterium, associated with Huanglongbing (HLB) disease of citrus trees, moves downward in the phloem and infects the roots soon after transmission by the Asian Citrus Psyllid (Diaphorina citri) vector into shoots. Before canopy symptoms appear, 30–50% of the roots are damaged. Without aggressive management to reduce abiotic and biotic stress, root loss increases to 70–80%. An extensive survey of HLB-affected groves in central and south-central Florida indicated that a greater decline in fibrous root health as well as a greater expression of HLB symptoms is observed where irrigation water is high in bicarbonates (>100 mg L⁻¹) and soil pH is >6.5. Over three seasons of survey, acidification of irrigation water in the central and south-central citrus growing regions of Florida reduced the decline in root density associated with HLB. Irrigation water treatment with sulfuric acid and soil amendment with elemental sulfur for 36 months to establish a soil pH range from 4.0 to 7.0 increased root growth, soil nutrient availability, and the uptake of Ca, Mg, Mn, and Zn in response to a gradual reduction in soil pH in young and mature Valencia orange groves on Swingle citrumelo rootstock. The reduction in soil pH increased yield and soluble solids in fruit and so would improve citrus production. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

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15 pages, 1267 KiB

Open AccessArticle

Varietal Change Dominates Adoption of Technology in Spanish Citrus Production

by Maria Dolores de-Miguel, Pedro Caballero and Maria Angeles Fernández-Zamudio

Agronomy 2019, 9(10), 631; https://doi.org/10.3390/agronomy9100631 - 11 Oct 2019

Cited by 7 | Viewed by 3289

Abstract

After describing the technology adopted and its influence on production and yield, the importance of varietal change, which is essential to meet demand, is underscored. The choice of a variety by growers depends on the expected price it will fetch along with the commercial and crop characteristics and the quality. Survival of a variety in the marketplace depends on the performance as expected and on the existence of other more competitive varieties. First, in Spanish citrus farming, the navel group predominate, with 41.10% of orange production, in which the main varieties are “Navelina” and “Lanelate.” The group of late white oranges is also important. The mandarin orange group includes clementines and mandarin hybrids; lemons also feature prominently. In sweet oranges, the navel group and late white group has often been renewed with varieties from the same group. In very early clementines varieties, the survival of which is usually short, the supply of new varieties is excessive. Because of its commercial characteristics and harvesting time, the group with the highest expansion possibilities are the mandarin hybrids which are in demand by hypermarkets. In the introduction of varieties, in recent years we have seen varietal clubs being established, with area control of each variety and very strict rules on cultivation and marketing. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

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15 pages, 2508 KiB

Open AccessArticle

Endophyte Bacillus velezensis Isolated from Citrus spp. Controls Streptomycin-Resistant Xanthomonas citri subsp. citri That Causes Citrus Bacterial Canker

by Muhammad Fazle Rabbee, Md. Sarafat Ali and Kwang-Hyun Baek

Agronomy 2019, 9(8), 470; https://doi.org/10.3390/agronomy9080470 - 20 Aug 2019

Cited by 27 | Viewed by 5620

Abstract

Citrus bacterial canker (CBC), caused by the plant pathogenic bacterium Xanthomonas citri subsp. citri (Xcc), is a devastating disease in many commercial citrus cultivars. Every year, CBC causes a substantial reduction in fruit quality and quantity that corresponds to significant economic losses worldwide. Endophytic microorganisms produce numerous bioactive secondary metabolites that can control plant pathogens. We investigated the antagonistic activities of 66 endophytic bacteria isolated from nine citrus cultivars to control streptomycin-resistant Xcc. The suspension of Endophytic Bacteria-39 (EB-39), identified as Bacillus velezensis, exhibited the highest antibacterial activity against three wild-type and six streptomycin-resistant Xcc strains, with the inhibition zones between 39.47 ± 1.6 and 45.31 ± 1.6 mm. The ethyl acetate extract of EB-39 also controlled both wild-type and streptomycin-resistant Xcc strains, with the inhibition zones between 29.28 ± 0.6 and 33.88 ± 1.3 mm. Scanning electron microscopy indicated the ethyl acetate extract of EB-39-induced membrane damage and lysis. The experiments using the detached leaves of a susceptible Citrus species showed that EB-39 significantly reduced the incidence of canker on the infected leaves by 38%. These results strongly suggest that our newly isolated EB-39 is a novel biocontrol agent against CBC caused by wild-type and streptomycin-resistant Xcc strains. Full article

(This article belongs to the Special Issue Recent Advances in Breeding and Production of Citrus)

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