Blueberry cultivars were treated with either soil drenches or foliar applications of paclobutrazol. Soil drenches of 25 mg·L-1 inhibited shoot elongation and stimulated earlier and greater flower bud production on `Bluetta', `Bluecrop', and `Jersey'. The treatments increased bud numbers 359% to 797%, and stimulated compound bud formation, while reducing formation of vegetative buds. This resulted in overcropping and reduced fruit size. Foliar applications at concentrations of 5, 10, 50, and 100 mg·L-1 increased bud set. Treatments did not significantly alter time to 50% flowering in `Bluecrop' or `Duke', but hastened flowering up to 5 days in `Blueray' at 200 ppm. Fruit ripening was significantly delayed at 100 and 200 ppm in `Bluecrop' due to overcropping, but no delays were observed in `Blueray' or `Duke'. Plant size and vigor appeared to be a determining factor in plant response. Chemical name used: PP333 or (2RS,3RS)-l(4-chlorophenyl)-4,4-dimethyl-2-(l,2,4-triazol-1-yl)pentan-3-ol (paclobutrazol).
Mark K. Ehlenfeldt
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.
Mark K. Ehlenfeldt
Self- and cross-fertility was evaluated in the highbush blueberry cultivars Bluegold, Duke, Legacy, Nelson, Sierra, Sunrise, and Toro, all released since 1987, by comparing them to standards of `Bluecrop' and `Rubel'. Percent fruit set increased with cross-pollination in all cultivars except `Bluecrop', which decreased by 13%. The average increase in the recently released cultivars was 43%. Fruit weight also increased in cross-pollinations for all cultivars except `Rubel', which showed a decrease of 2%. Average increase in fruit weight on cross-pollination in the recently released cultivars was 27%. Fruit set and fruit weight measurements suggest that `Duke', `Legacy', and `Nelson' could perform well in solid stands, but `Sierra' and `Toro' are more likely to need cross-pollination for best yields. Investigations were also made on a group of 10 cultivars, to evaluate whether ripening time of the pollen source cultivar had any effect on the ripening time of the fruiting parent. No single pollen source had consistent general effects on ripening, although specific combinations of females and males appeared to either hasten or delay ripening. The largest deviations were seen in delays of ripening, suggesting that poor pollination may have been the greatest factor contributing to the observed variation in ripening times.
Mark K. Ehlenfeldt
Self- and cross-fertility were evaluated in the highbush blueberry (Vaccinium corymbosum L.) cultivars Bluegold, Duke, Legacy, Nelson, Sierra, Sunrise, and Toro, all of which have been released since 1985, by comparing them with `Bluecrop' and `Rubel', which were used as standards. Cross-pollination increased fruit set in all cultivars except `Bluecrop', in which set was reduced 13%. The average increase among recently released cultivars was 43%. Cross-pollination also increased fruit weight for all cultivars except `Rubel'. Average increase in fruit weight with cross-pollination of the recently released cultivars was 27%. Fruit set and fruit weight measurements suggest that `Duke', `Legacy', and `Nelson' would perform satisfactorily in solid stands, and that `Sierra' and `Toro' probably need cross-pollination for maximum yields.
Mark K. Ehlenfeldt
Mark K. Ehlenfeldt and Nicholi Vorsa
Forty-one half-sib families (>3000 individuals) segregating for parthenocarpic fruit production were evaluated under field conditions. Within these families, ≈280 parthenocarpic individuals were identified. In general, three categories of segregants were observed: normal-seeded types, small/low-seeded types, and parthenocarpic types. Inheritance patterns suggested that the trait is recessive, but did not fit simple tetrasomic recessive or incomplete dominance models. Lack of fit may be attributable to environmental interactions or incomplete penetrance of this trait. Further evaluations are underway with small/low-seeded types with the expectation that offspring of these plants may express the trait at higher frequencies than in F1 × F1 crosses. The reduced vigor in many of the parthenocarpic segregants suggests that this germplasm will be most useful, initially, in improving fruit quality in the intermediate expression types, which have better fruit set and reduced seed development.
Mark K. Ehlenfeldt and Matthew Kramer
Rabbiteye blueberry hybrids that the U.S. Department of Agriculture, Agricultural Research Service (USDA-ARS) program has bred for northern adaptation are combinations of 6x V. ashei Reade, 6x V. constablaei Gray, 4x V. corymbosum L., and 2x V. darrowii Camp germplasm at the hexaploid level and are generally composed of 50% or greater V. ashei (rabbiteye) germplasm. Four northern-adapted rabbiteye (NRE) selections (US 1043, US 1045, US 1056, US 1057), four rabbiteye standards (‘Brightwell’, ‘Climax’, ‘Tifblue’, ‘Woodard’), two rabbiteye × V. constablaei derivatives (‘Little Giant’, ‘Snowflake’), and two highbush standards (‘Duke’, ‘Bluecrop’) were pollinated under greenhouse conditions with either self-pollen or a multicultivar, bulk-pollen mixture (appropriate to ploidy level and species) to determine the relative requirements for cross-pollination among NRE selections. Fruit set, berry weight, and seed set were subsequently evaluated. The results suggest that NRE selections, in general, exhibit cross-pollination needs intermediate to the parent types such that: rabbiteye > northern rabbiteye > highbush (i.e., rabbiteye has the lowest self-fertility and the greatest need for cross-pollination). Considerable variation existed among the NRE selections tested, which suggests that it might be possible to select clones with good levels of self-fertility, potentially equivalent to that of highbush blueberry.
Melvin R. Hall and Mark K. Ehlenfeldt
A rabbiteye blueberry selection, T-285, appears parthenocarpic when grown in the greenhouse and sparsely seeded when grown in the field. This semi-seedless character was analyzed to determine the nature and degree of its cross- and self-fertility in comparison to its parents, `Tifblue' and `Delite'. Ovule numbers from T-286 were similar to those of `Tifblue', but lower than those of `Delite'. Seed numbers of open-pollinated `Tifblue' and T-286 were similar, and lower than `Delite', but T-286 was notable in having fruit 60% larger than `Tifblue' and 10% larger than `Delite'. Both `Tifblue' and T-286 had a tendency to produce some open-pollinated fruit with few or no seed present. None of the selections produced significant amounts of fruit when self-pollinated. When cross-pollinated, all selections set more seed, but this was less pronounced with T-286 than with either of its parents.
Mark K. Ehlenfeldt and James J. Polashock
The primary gene pool of Vaccinium species used by blueberry breeders has traditionally been the North American Vaccinium species of section Cyanococcus. Blueberries in commercial production represent three primary Vaccinium species and two ploidy levels. Significant use has been made of the secondary gene pool of Vaccinium, especially in the development of southern highbush blueberry (Vaccinium ×corymbosum) cultivars. Section Hemimyrtillus species are distantly related and are best considered part of the tertiary gene pool of Vaccinium. Vaccinium padifolium, a member of section Hemimyrtillus and native to the Madeira Islands, Portugal, has features of notable value to conventional blueberry development, among these: upright structure, strong growth, abundant flowering and fruiting, good self-fertility, inflorescence structure suited to mechanical harvesting, and indeterminate/repeat flowering. Our objective was to incorporate germplasm from this section into cultivated materials and transfer the desirable traits these species possess for commercial production. We used V. padifolium as a female in crosses with V. corymbosum and generated two highly fertile hybrids. These hybrids are intermediate in morphology, phonological, and their hybridity has been confirmed through DNA testing. These hybrids were used in further crosses to a variety of section Cyanococcus selections and have generated numerous second-generation hybrids. We have also determined by flow cytometry the ploidy levels of the hybrids and several previously unevaluated section Hemimyrtillus species.