Abstract
‘Georgiagem’, a new low-chilling cultivar. is the first highbush blueberry (Vacciniwn corymbosum L.) cultivar developed from the cooperative blueberry breeding program of the Georgia Agricultural Experiment Station and the USDA. Although ‘Georgiagem’ originated mostly from tetraploid highbush blueberry parentage, it obtained adaptation to the southern U.S. environment from the diploid species, V. darrowi, selected from the wild in Florida (2). ‘Georgiagem’ is early ripening, moderately productive, and has fruit of medium size with good color, small stem scar, firmness, and pleasant flavor.
Abstract
Native highbush blueberries (Vaccinium corymbosum L.) from the flatwoods of Alachua County (North-central), Florida and from Highlands County (Central peninsula) were surveyed for chromosome number and crossability with breeding lines derived from northern highbush cultivars. The Alachua County population was predominately tetraploid; a diploid component differed in leaf serration and glandulation. Tetraploid plants were fully cross-fertile with highbush cultivars and breeding lines. Diploid plants from the Alachua County population were cross fertile with both V. elliottii and V. darrowi. V. corymbosum from Highlands County was diploid.
Diploid plants in Vaccinium section Cyanococcus, including plants of V. darrowi Camp, V. atrococcum. Heller (diploid V. corymbosum L.), a V. atrococcum × V. darrowi F1 hybrid, and a V. atrococcum × V. elliottii Chapmn. F1 hybrid, were hand pollinated in a greenhouse with pollen from diploid V. arboreum Marsh. (Section Batodendron). The resulting seeds were germinated and the seedlings were transplanted to a high-density field nursery. Forty of these F1 intersectional hybrids were selected after 2 1/2 years and transplanted to a 1.5 × 4-m spacing. Most of these plants were vigorous and flowered heavily in subsequent years, but only a small percentage of the flowers produced fruit. In 1990, however, >4000 berries were harvested from the 35 surviving plants. Open-pollinated seed from a much smaller number of berries was planted in Dec. 1987; these seeds produced ≈200 seedlings, some of which had moderate to high fruit set in a field nursery in 1989. Six of these seedlings, which were selected for high vigor, high fruit set, and characteristics intermediate between section Cyanococcus and section Batodendron, had fruit set ranging from 19.4% to 92.7% when pollinated with pollen from tetraploid V. corymbosum cultivars. One of the six seedlings was highly self-fruitful, and some intercrosses among the six seedlings produced much viable seed. Large-scale introgression of V. arboreum genes into tetraploid highbush cultivars likely will be possible by the methods used in this study.
Morphological characteristics of many derivatives from Vaccinium arboreum Marsh × Vaccinium section Cyanococcus crosses were studied. The purpose of the study was to determine if V. arboreum traits were being inherited and expressed in hybrid progeny and to identify characteristics that would enable hybrid field identification. This study focused on the F1 hybrids of V. darrowi Camp × V. arboreum (F1 hybrids) and the open-pollinated progeny of the F1 hybrids [mother is known (MIK)]. Also included in the study were the parents: V. darrowi, V. arboreum, and V. corymbosum L. (pollen parent of the MIKs). Many leaf, flower, and fruit characteristics were measured for all five taxa. Leaf characteristics included length, width, and presence or absence of stalked glands, pubescence, and marginal bump glands. The floral characteristics measured were corolla length and width, corolla aperture, pedicel length, peduncle length, bracteole length and width, and the presence or absence of anther awns and bracteoles. Berry and seed mass were the fruit characteristics investigated. Four unique V. arboreum traits were found to be expressed in the F1 and MIK hybrid populations. These were the presence of anther awns, large seed size, bracteole shape, and marginal glands. These traits should permit field identification of hybrid plants.
`Gulfcoast' southern highbush blueberry (Vaccinium corymbosum × V. darrowi) plants were placed in 3 × 6 × 2.5 m net cages with one colony of honey bees per cage and one of three pollinizer treatments: “self (other `Gulfcoast' plants), “cross/highbush” (other southern highbush cultivars), or “cross/rabbiteye” (various rabbiteye blueberry cultivars). In addition to unlimited pollination, bee foraging was controlled on individual flowers by placing small bags over corollas after 0, 1, 5, or 10 visits. Fruit set, fruit weight, fruit development period, and seed number data were taken, as well as data to relate floral morphology to duration of bee foraging. All measures of fruiting increased significantly with increased bee visitation; the threshold for significant gains in production occurred between 1 and 5 visits. Ten visits generally provided a good approximation of unlimited pollination. Set, weight, and earliness of ripening was as good, or better, for fruit derived from rabbiteye pollen compared to fruit from self- or cross/highbush-pollination.
RAPD markers were used to determine the level of heterozygosity transmitted via 2n gametes from V. darrowi cv. Florida 4b (Fla 4B) to interspecific hybrids with tetraploid V. corymbosum cv. Bluecrop. The tetraploid hybrid US 75 was found to contain 70.6% of Fla 4B's heterozygosity, a value consistent with a first division restitution (FDR) mode of 2n gamete production. Crossovers during 2n gamete formation were evidenced by the absence of 16 dominant alleles of Fla 4B in US 75, and direct tests of segregation in a diploid population involving Fla 4B. RAPD markers that were present in both Fla 4B and US 75 were used to determine the mode of inheritance in a segregating population of US 75 × V. corymbosum cv. Bluetta. More than 30 homozygous pairs of alleles were located that segregated in a 5:1 ratio, indicating US 75 undergoes tetrasomic inheritance.
Resistance to blighting by Monilinia vaccinii-corymbosi (Reade) Honey was evaluated under greenhouse conditions in multiple populations of the diploid species Vaccinium boreale Hall & Aalders, V. corymbosum L., V. darrowi Camp, V. elliottii Chapm., V. myrtilloides Michx., V. myrtillus L., V. pallidum Ait., and V. tenellum Ait., as well as in accessions of the polyploid species 4x V. hirsutum Buckley and 6x V. corymbosum f. amoenum Aiton. Significant species differences were found in mean blighting levels averaged over 2 years, with values ranging from 3.5% for V. boreale to 49.2% for 2x V. corymbosum, compared with 27.5% for the resistant 4x V. corymbosum check, `Bluejay', and 64.3% for the susceptible 4x V. corymbosum check, `Blueray'. Wild Vaccinium species may serve as new sources of resistance to blighting, if resistance can be transferred easily and horticultural type recovered.
Mode of inheritance of cold hardiness (CH) in woody perennials is not wellunderstood. This study was undertaken to determine the mode of inheritance and gene action of CH in blueberry (Vaccinium section Cyanococcus). Two testcross populations (segregating for CH) derived from interspecific hybrids of V. darrowi (drw) × V. caesariense (csr) were used. Plants were cold-acclimated by a 4-week exposure to 4°C. Bud CH (LT50) was defined as the temperature causing 50% injury (visual) when subjected to controlled freeze–thaw. Results show that the drw and csr parents had an LT50 of –13° and –20°C, respectively. The F1 population exhibited mean LT50 of –14.7°C. The csr and drw testcross populations had a mean LT50 of –18° (39 individuals) and –14°C (33 individuals), respectively. Individuals of each population were distributed between parental values with center of distribution skewed toward the testcross parent. Since individuals having LT50s as same as the recurrent parents were present in each population of only 33–39 plants, data suggest that CH is determined by relatively few genes. To determine gene action, the estimates for various genetic parameters (calculated from joint scaling test) were used in generation means analysis to test various models. Results indicate that CH in blueberry can be best explained by simple-additive dominance model, whereas models including epistatic components did not satisfactorily explain the data.
A new southern highbush blueberry cultivar named `Camellia' was released in 2005 by The University of Georgia and the USDA–ARS. `Camellia' is a hybrid containing mostly Vaccinium corymbosum and a small amount of V. darrowi. The new cultivar was selected in 1996 at the Coastal Plain Experiment Station in Tifton, Ga. from a cross of MS-122 × MS-6, and was tested as TH-621 in plantings at Alapaha, Ga. beginning in 1998. `Camellia' has an estimated chill requirement of 450 to 500 hours (<7 °C). It is an early- to mid-season cultivar, having berries that are large, with a very light blue color, and a small, dry picking scar. Berry firmness is good and flavor is very good. `Camellia' flowers 5 to 8 days after `Star' and `O'Neal' in south Georgia, and ripens 4 to 9 days after `Star', and with `O'Neal'. Plants are highly vigorous, with strong cane growth and an open, upright bush habit and a narrow crown. Yields have been similar to `Star' and greater than `O'Neal'. `Camellia' should be planted with other southern highbush blueberry cultivars with a similar time of bloom for cross-pollination (`Star' and `O'Neal' suggested). It is recommended on a trial basis at this time. `Camellia' requires a license to propagate. For licensing information and/or a list of licensed propagators, contact the Georgia Seed Development Commission, 2420 S. Milledge Avenue, Athens, GA 30606; or visit their website at www.gsdc.com.
Genetic variation was surveyed within and among 5 Vaccinium species and several hybrid taxa for 6 aspects of flower size and shape. Sufficient variation was found to allow radical changes in flower architecture through breeding. The goal is a flower that pours pollen directly from the anthers onto the stigma without the assistance of pollinating insects. The flowers of V. elliottii had very short styles (mean in mm 5.3 compared to 10.2 for rabbiteye cultivars and 8.5 for highbush cultivars), and certain short-style recombinants from highbush cultivar × V. elliottii crosses came close to the desired positioning of stigmas relative to anthers. The distance (in mm) from the anther pore to the stigma averaged: V. ashei 2.7; V. corymbosum 2.4; V. darrowi 2.3; and V. elliottii 1.0. Compared to highbush cultivars, rabbiteye cultivars tended to have long corollas and narrow corolla apertures, two features believed to be related to poor honeybee pollination. These features were much more favorable in V. ashei × V. constablaei hybrids, with values averaging close to those for highbush cultivars.