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The extent of self-fertility and self-fruitfulness was studied in Vaccinium arboreum Marsh, V. darrowi Camp, and in seedlings, termed MIKs, from open-pollination of V. darrowi (section Cyanococcus) × V. arboreum (section Batodendron) F1 hybrids. The open pollinations that produced the MIKs occurred in a field containing tetraploid southern highbush selections (based largely on V. corymbosum L.), and the pollen parents of the MIKs are believed to be southern highbush selections. The MIKs that were studied had been selected for high fruit set after open pollination in the field. Both V. arboreum and V. darrowi exhibited very low self-fruitfulness and self-fertility when hand-pollinated in a greenhouse; the former produced no seedlings from more than 600 selfed flowers, and the latter produced only 13. By contrast, southern highbush clones averaged 70 seedlings per 100 pollinated flowers when selfed and 230 when crossed. Self-fertility and self-fruitfulness of the MIKs were higher than those of V. arboreum and V. darrowi but lower than those of southern highbush selections. MIK × MIK crosses gave fewer seedlings per 100 pollinated flowers (84) than highbush × highbush crosses (230), probably reflecting their hybrid ancestry. Although introduction of V. arboreum genes into southern highbush blueberry gives plants of excellent vigor and adaptation to north Florida, several generations of breeding will be needed to obtain cultivars with high fertility and berry quality.
Efforts are underway to develop genetic linkage maps for two interspecific blueberry populations (Vaccinium darrowi × V. elliottii and V. caesariense-derived populations). To date, 72 RAPD markers have been mapped, and another 200 markers have been identified as suitable for mapping in the V. darrowi × V. elliottii-derived population. Inheritance of 40 RAPD markers has been followed, and additional 40 RAPD markers have been identified as suitable for mapping in the V. darrowi × V. caesariense population. These two populations are comprised of individual plants that should have a wide range of chilling requirements. At a later date, plants will be classified according to their chilling requirements to identify RAPD markers that cosegregate with chilling requirements. Presently, a bulked-segregant analysis is being performed on a tetraploid breeding population (primarily V. corymbosum) to identify RAPD markers linked to chilling requirement genes.
A genetic linkage map for blueberry has been constructed from over 60 RAPD (random amplified polymorphic DNA) markers that segregated 1:1 in a testcross population of about 40 plants. Prior to map construction, polymerase chain reaction conditions were optimized and RAPD marker reliability was confirmed. The mapping population was derived from a cross between diploid blueberry plants: Fl interspecific hybrid, `US388' (V. darrowi, `Fla4B' × V. elliottii, `Knight'), and another V. darrowi, `US799'. The map currently comprises 12 linkage groups corresponding to the basic blueberry chromosome number and covers a total genetic distance of over 800 cM, with a range of 2-30 cM between adjacent markers. Interestingly, a few pairs of marker loci behaved differently from both linked and unlinked loci, being found at a much higher frequency in the recombinant configuration than the parental configuration possibly suggesting selection for certain combinations of alleles.
Abstract
Ten colonies of Vaccinium darrowi Camp were sampled at each of 9 sites in the Florida panhandle and 6 sites in and around the Ocala National Forest in the central Florida peninsula. The colonies averaged 53 cm tall in the panhandle, with leaves 9.8 mm long and 4.1 mm wide. By contrast, colonies in the peninsula averaged 136 cm tall—well outside the range described for the species—with leaves 12.7 mm long and 5.7 mm wide. The species was diploid and entirely evergreen in both regions. In the central Florida peninsula, natural hybrids between V. darrowi and a 3-m tall, deciduous, diploid race of highbush blueberry (V. corymbosum L.) are common where streams and lakes border the dry scrub habitat of V. darrowi. The robust form of V. darrowi in the Florida peninsula may have evolved from the petite form in the panhandle as a result of introgression from the highbush coupled with selection for characteristics that enhance survival on the deep, xeric sands of the peninsula. V. darrowi from the central peninsula has characteristics that make it valuable in breeding blueberry cultivars.
Inbreeding coefficients were calculated for highbush blueberry (Vaccinium corymbosum L.) cultivars based on a tetrasomic inheritance model. This model yielded lower inbreeding coefficients than previous calculations based on a disomic tetraploid inheritance model. Recent trends in breeding have resulted in significant use of V. darrowi Camp as a source of low-chilling germplasm for use in the southern United States. There is also a trend toward increased inbreeding in several crosses from which recently released cultivars have been derived. Increased inbreeding coefficients do not represent a detrimental situation in blueberry per se.
To determine if blueberry shoestring virus (BBSSV) is absent in the southern United States due to resistance of cultivars, we mechanically and rub-inoculated 1-year-old rooted microshoots of nine cultivars representing southern rabbiteye (Vaccinium ashei Reade), southern highbush (hybrids of V. corymbosum and V. darrowi Camp), and northern highbush (V. corymbosum L.). Leaves were sampled from plants, and enzyme-linked immunosorbent assay screened for the presence of virus over 15 months. Only a few individuals were infected after aphid inoculation, but many northern and southern cultivars became infected after mechanical inoculation. Northern highbush `Elliot' (50%) and `Blueray' (46.3%) had the highest infection rates, followed by rabbiteye `Climax' (36.3%) and the southern highbush `O'Neal' (12.5%). The lowest rates of infection were found in southern highbush `Georgiagem' (2.5%), `Misty' (2.5%), rabbiteye `Brightwell' (0.0%), and northern highbush `Bluecrop' (2.5%). Since many southern cultivars were infected by the disease, resistance likely has not excluded BBSSV from the southern United States.
Abstract
V. myrsinites Lam. is most susceptible to canker among the blueberry species native to Florida and survives by resprouting from underground rhizomes. V. darrowi Camp suffers less canker damage than V. myrsinites, some mature colonies showing little or no damage but others being nearly destroyed by the disease. Most mature V. darrowi colonies in the State have some stems cracked and swollen by canker. Old native plants of V. ashei Reade and V. fuscatum Ait. show light to moderate canker damage in many parts of the State but both species are more resistant than the previous two. V. elliottii Chapm. shows no canker over much of its range in Florida, but localized populations have moderate to high infection. Only one cankered plant of V. arborewn Marsh, has been found, and no canker has been found on V. stamineum L.
Abstract
Adaptation to upland soils was investigated in a diallel study involving progeny of 4 blueberry (Vactinium) clones consisting of 1 highbush blueberry (V. corymbosum L.) clone and 3 interspecific hybrids. Both general and specific combining ability were significant. Seedling progenies of NJUS 11 (V. ashei × V. atrococcum) and US75 (V. darrowi × V. corymbosum) grew well on an upland soil. Although not included in the diallel crosses, NJUS 64 (V. myrsinites × V. angustifolium) progenies also grew well on upland soil.
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.
`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.