Kim D. Bowman and Greg McCollum
Ute Albrecht and Kim D. Bowman
Huanglongbing (HLB) is a destructive disease of citrus in most citrus-producing countries worldwide. The disease, presumably caused by phloem-limited bacteria of the genus Candidatus Liberibacter, affects all known citrus species and citrus relatives with little known resistance. Typical disease symptoms are the production of abnormal-looking fruit and chlorosis or blotchy mottle of the leaves followed at advanced stages by tree decline and death. Trifoliate orange (P. trifoliata L. Raf.) and some of its hybrids reportedly lack distinct disease symptoms despite infection with the pathogen. US-897 is a hybrid of trifoliate orange and ‘Cleopatra’ mandarin (C. reticulata Blanco), the latter being highly susceptible to HLB. This study investigated whether field-grown, naturally infected trees and greenhouse-grown, graft-inoculated seedlings of this genotype display tolerance or resistance to HLB. It was shown that naturally infected US-897 trees exhibited no distinct disease symptoms commonly associated with HLB, except for the occurrence of few mottled leaves in a small percentage of trees. Analysis of fruit and seed from infected trees did not detect any growth reduction or otherwise negative impact on development. Graft-inoculated US-897 seedlings became polymerase chain reaction (PCR)-positive for the pathogen but exhibited a superior performance compared with ‘Cleopatra’ mandarin seedlings, which displayed severe disease symptoms soon after inoculation. Despite infection, most US-897 seedlings did not develop leaf symptoms typical for HLB. Foliar symptoms observed in a small number of plants at later stages of the disease were faint and difficult to discern. Contrary to ‘Cleopatra’ seedlings, growth in stem diameter was only moderately reduced or unaffected in infected US-897 seedlings. The superior performance of US-897 plants in greenhouse and field locations suggest tolerance of this genotype to Ca. L. asiaticus.
Greg McCollum and Kim D. Bowman
The objective of this experiment was to compare fruit-quality parameters of ‘Ray Ruby’ grapefruit grown on seven rootstocks. Four recent releases from the United States Department of Agriculture (USDA) rootstock breeding program, ‘US-852’, ‘US-897’, ‘US-942’, and ‘US-812’ (all Citrus reticulata × Poncirus trifoliata hybrids), ‘x639’ (C. reticulata × P. trifoliata), along with industry-standard ‘Sour Orange’ and ‘Swingle’ citrumelo were evaluated in a commercial orchard trial in Indian River County, FL. Fruit-quality data were collected in 2011–12 (eight harvests), 2012–13 (five harvests), and 2014 (single harvest). In each season, rootstock effects on fruit size, total solids, and solids acid ratio were significant. ‘Sour orange’ and ‘Swingle’ produced the largest fruit, whereas ‘US-897’ (a semidwarfing rootstock) produced the smallest fruit. Peel thickness (measured only in the 2011–12 season) was greatest in ‘Sour Orange’ early in the season, but not toward the end of the season. Misshapen (“sheep-nosed”) fruit occurred more frequently on ‘Sour Orange’ than on other rootstocks, although the incidence of sheep-nosing was minor. Analysis of variance (ANOVA) for fruit-quality data collected in January of each of the 3 years confirmed that ‘Sour Orange’ and ‘Swingle’ produced the largest fruit and ‘US-897’ produced the smallest fruit. Total solids were the highest in ‘US-897’ and the lowest in ‘x639’ and ‘US-852’. Taken together, our data indicate that ‘US-942’ and ‘US-897’ rootstocks produced fruit with quality characteristics that equaled or exceeded ‘Sour Orange’ and ‘Swingle’, the two most common rootstocks used in the Indian River district.
Kim D. Bowman and Ute Albrecht
Modern citrus nursery production makes use of potted-tree propagation in greenhouses. Supplemental lighting is one method by which nursery tree growth and profitability may be significantly improved, but limited specific information is available. Five replicated experiments were conducted to determine the utility and effects of increasing daylength during the winter months by supplemental illumination from light-emitting diode (LED) or high-pressure sodium (HPS) lights in citrus nursery propagation. Studies used ‘Valencia’ sweet orange scion, the most common citrus cultivar grown in Florida, and the commercially important rootstocks sour orange, ‘Cleopatra’ mandarin, ‘US-812’, ‘US-897’, ‘US-942’, and ‘US-1516’. Comparisons used the three common types of citrus rootstock propagation: seed, stem cuttings, and micropropagation. Six responses were measured in the lighting experiments, including vegetative growth before budding, scion bud survival, and scion bud growth after budding. Supplemental HPS or LED light to extend daylength to 16 h in the citrus nursery during short-day winter months was observed to be effective in increasing unbudded rootstock liner growth and ‘Valencia’ scion growth on all rootstocks and propagation types. Generally, the positive effect on vegetative growth from an increased daylength was stronger with the HPS light than with LED light, while increasing daylength with LED light, but not HPS light, provided some increased bud growth initiation. Use of HPS or LED supplemental lighting to extend daylength offers significant growth advantage for the citrus nursery industry in winter.
Kim D. Bowman and Frederick G. Gmitter Jr.
A diverse population of grapefruit-like Citrus growing in Saint Lucia (West Indies), called forbidden fruit, was examined as a potential germplasm source for Citrus genetic improvement. Four clones from this population were studied by leaf isozyme analysis, and a distinct resemblance between forbidden fruit and grapefruit (C. × paradisi Macfady.) was observed at several loci, including identical banding patterns for peroxidase, phosphoglucose mutase, phosphohexose isomerase, and shikimic acid dehydrogenase. These results support morphological and historical indications of a close taxonomic relationship between modern grapefruit cultivars and Caribbean forbidden fruit. Comparison of isozyme allele segregation among seedlings of several forbidden fruit clones and grapefruit cultivars demonstrated a much higher degree of zygotic embryony in the former. Morphological diversity and zygotic embryony in the Caribbean forbidden fruit population may make it a useful genetic resource for breeding grapefruit and other Citrus species.
Kim D. Bowman and Frederick G. Gmitter Jr.
Chinotto is a selection of sour orange (Citrus aurantium L.) with short internodes and small leaves and fruit. Mature fruiting trees of Chinotto grafted on standard rootstocks produce healthy, but dwarf, trees. Most seedlings recovered from Chinotto fruit are of nucellar (maternal) origin and will faithfully grow to reproduce the Chinotto genotype and phenotype. Vigorous greenhouse-grown nucellar seedlings of Chinotto have internodes 5 to 10 mm in length and leaves 30 to 40 mm in length, about 30% the dimensions of the corresponding organs on standard sour orange nucellar seedlings. Sexual hybrids with Chinotto have been produced by controlled crosses with several other parents. Some hybrids with shortened internodes and small leaves were recovered among all hybrid progenies, regardless of whether Chinotto was used as seed or pollen parent. In some cases, segregation among Chinotto hybrids was about 1 normal: 1 dwarf. In other progenies, some intermediate forms were recovered along with normal and dwarf plants.
Kim D. Bowman, Lynn Faulkner, and Mike Kesinger
Four new citrus rootstocks developed by USDA, ARS and released between 2001 and 2010 have gained considerable commercial popularity in Florida and have been used for propagation of more than 2 million trees over the last 2 years. Results from three new field trials are presented comparing these rootstocks with other important rootstocks, and discussion is presented to summarize field performance in these trials and from numerous other sources. The rootstocks vary widely in their effect on tree vigor. When used as a rootstock for sweet orange, ‘US-802’ typically supports strong vigor and development of a large tree, ‘US-812’ and ‘US-942’ provide moderate vigor, and ‘US-897’ induces a relatively dwarf tree. Other characteristics and attributes of the four rootstocks are also discussed, including effects on cropping, fruit quality, disease and pest resistance, and tolerance of abiotic factors. Of special relevance, relative tolerance of these rootstocks to huanglongbing (HLB) disease can be surmised from a combination of data from several sources, with ‘US-942’ and ‘US-802’ typically providing the best yields per tree in trials containing many rootstocks and affected by HLB. ‘US-812’ and ‘US-897’ have appeared to have most utility in locations where HLB is not present or effectively managed, or where trees are being planted at high density to optimize production per unit area. To facilitate continuing expansion of commercial use, information on seed production, and the relative ease of nursery propagation is also presented.
Kim D. Bowman and Robert E. Rouse
Jacob B. Bade, Frederick G. Gmitter Jr., and Kim D. Bowman
Volatile oils were extracted from aqueous leaf suspensions of sweet orange [Citrus sinensis (L.) Osb.] cultivars Hamlin, Navel, and Valencia and grapefruit (Citrus paradisi Macf.) cultivars Marsh and Ray Ruby. Pressurized air was used as the sparging gas, and volatile oils were collected in a C-18 cartridge. Gas-liquid chromatography was used to separate and quantify 17 volatile components. Significant quantitative differences for individual components made it possible to distinguish sweet orange from grapefruit (four components), `Marsh' from `Ray Ruby' grapefruit (two components), `Hamlin' from `Valencia' or `Navel' orange (six components), and `Valencia' from `Navel' (three components). The simplicity and sensitivity of the procedure suggest potential use for Citrus taxonomic, genetic, and breeding research.
Kim D. Bowman, Jeffrey P. Shapiro, and Stephen L. Lapointe
Commercially used citrus rootstocks can all be seriously damaged by larvae of the sugar cane root weevil, Diaprepes abbreviatus (L.). Six greenhouse challenge experiments were used to compare the resistance of 24 selections of citrus rootstocks and related species to this weevil by measuring root damage, and larval growth and survival. The commercial rootstocks tested were found to be susceptible, while at least seven other species within the subfamily Aurantiodeae were observed to be significantly more resistant. The species Balsamocitrus dawei Stapf. was most resistant to weevil larvae, exhibited less root damage than commonly used rootstock cultivars, and significantly depressed larval growth and survival. The species Glycosmis pentaphylla (Retz.) Correa, Microcitrus australis (Planch.) Swing., Eremocitrus glauca (Lindl.) Swing., Severinia buxifolia (Poir.) Tenore, Triphasia trifolia (Burm. f.) P. Wils., and Citrus hystrix DC. suffered as much damage from the weevil as common rootstock cultivars but significantly depressed growth of larvae feeding on them. One new hybrid rootstock, HRS-801, also significantly depressed D. abbreviatus larval growth, but this effect has not yet been verified as having significance in a long-term or field situation. Several strategies are discussed for developing citrus rootstocks resistant to D. abbreviatus.