Partial to complete self-incompatibility is normal in most Vaccinium L. (Ericaceae) species. Wild blueberry plants of several Florida provenances and species were self- and cross-pollinated in a greenhouse free of pollinators. Fruit set of V. darrowii Camp (2x), V. corymbosum L. (4x), V. arboreum Marsh (2x), and F ₁ ( V. darrowii × V. corymbosum ) hybrids was higher after cross-pollination than after self-pollination. Partial to complete self-incompatibility was present in V. darrowii , V. corymbosum , and their tetraploid F ₁ hybrids. The three V. arboreum clones tested were fully self-incompatible. Intra- and interpopulation crosses in V. corymbosum , V. darrowii , and V. darrowii × V. corymbosum hybrids were highly successful, and self-pollination reduced all fertility parameters. Advanced selections of V. corymbosum were the most self-compatible clones tested, possibly because self-compatibility has been increased by breeders selecting for reliable fruit set in large fields planted with one or a few clones. One southern highbush selection and two F ₁ hybrids had fruit set of more than 70% after self-pollination. These plants could be potentially used to breed plants that could be planted in single blocks providing reliable yield.

Over several years, we obtained no hybrids after pollinating thousands of flowers of cultivated tetraploid highbush blueberry ( Vaccinium corymbosum L. hybrids, section Cyanococcus ) with pollen from diploid V. arboreum Marshall (section Batodendron , sparkleberry), a drought-tolerant blueberry relative native in the southeastern United States. In an effort to produce tetraploid V. arboreum that could be crossed with highbush blueberry, more than 30,000 seeds were soaked in aqueous colchicine (0.1% to 0.2%) for 24 h or more. The seeds were germinated, and putative tetraploid plants (selected based on morphological characteristics) were grown long enough to obtain pollen for microscopic examination. Twelve selected seedlings that produced unusually large pollen tetrads were used as pollen parents in crosses with more than 40 different tetraploid highbush cultivars and advanced selections. Eighty-six crosses, in which a total of 17,968 flowers were pollinated, gave 1,569 plants that were verified as hybrids after one growing season in the field. Hybrids varied from very weak to quite vigorous, some equaling highbush cultivars in vigor. A few vigorous hybrids were male-sterile, but most had at least some pollen fertility. Of the most vigorous F ₁ hybrids, 12 of the most fertile, based on the amount of pollen shed and on the microscopic appearance of the pollen, were backcrossed to highbush cultivars, and 3919 backcross seedlings were obtained. These varied widely in vigor but averaged higher in vigor than their F ₁ interspecific hybrid parents.

In Vaccinium L., most tetraploid hybrids between tetraploid cultivated highbush blueberry ( V. corymbosum L.) and diploid darrow's evergreen blueberry ( V. darrowii Camp) have been produced by exploiting V. darrowii 's tendency to produce 2n gametes and a strong triploid block, which greatly reduces the number of triploid hybrids produced. Colchicine-derived tetraploids offer an alternative method of producing V. darrowii plants that will easily cross with tetraploid Vaccinium species. V. darrowii (2n = 2x = 24) seeds were imbibed in 0.2% aqueous colchicine solution for 24 h. The seeds were germinated and seedlings whose morphology suggested colchicine effects were selected at various stages of development. No macromorphological changes were consistently associated with chromosome doubling. However, stomatal guard cells and pollen size increased substantially as a result of chromosome doubling. Several types of plants were identified after colchicine treatment: 1) plants with a doubled LI (epidermal tissues) and LII (internal tissues) plant layers; 2) periclinal chimeras with a doubled LI layer and a normal LII; and 3) periclinal chimeras with a doubled LII layer and a normal LI. Of ≈4000 seedlings that emerged from colchicine-treated seeds, 200 were selected for further examination based on leaf and stem morphology. Of the 200, five appeared to be tetraploid in LI, LII, or both layers based on stomatal guard cell size and pollen size. Crosses between colchicine-derived V. darrowii (4x) plants and southern highbush blueberry ( V. corymbosum ) cultivars (4x) were successful compared with 4x–2x and 2x–4x crosses using diploid V. darrowii and tetraploid southern highbush blueberry. Stomatal guard cells and pollen screening of the colchicine-treated plants were used as indicators of doubled V. darrowii plants and periclinal chimeras. The results from the crosses between colchicine-treated V. darrowii plants and tetraploid highbush blueberry cultivars confirmed the information obtained by stomata and pollen screening.

Several hundred hybrid seedlings were produced by pollinating flowers of tetraploid highbush blueberry cultivars with pollen from 13 plants of Vaccinium stamineum that were selected as tetraploids following colchicine treatment. The hybrids were intermediate between the parents in many characteristics. They were less vigorous than the parents, but 46 plants flowered when 1.3 years old from seed. The F ₁ hybrids produced pollen abundantly, but only 30% of the microspores appeared potentially viable when viewed at 250X. F ₁ flowers that were pollinated with pollen from either parent taxon or with pollen from a different F ₁ hybrid produced thousands of well-developed seeds. The F ₁ hybrids were less successful when used as male parents in backcrosses to highbush, but 4790 well-developed seeds were obtained by pollinating 3250 highbush flowers. Flowers on F ₁ plants had long peduncles and pedicels, giving an open raceme. The flowers were open in the bud and had anther awns, two characteristics from V. stamineum. Berries on the F ₁ plants had black skins, and the ripe berries of 11 F ₁ plants had red to purple pulp like their V. stamineum parent. Berries on the hybrids were juicy. They had little or no bitterness typical of V. stamineum , and most had a pleasing balance of sugar and acid.

Diploid Vaccinium darrowii Camp has been used in breeding tetraploid southern highbush blueberry ( Vaccinium corymbosum L.) as a source of reduced chilling requirement, adaptation to hot, wet summers, and resistance to leaf diseases. V. darrowii in Florida is quite variable, but most crosses have involved only one V. darrowii clone, Fla. 4B. The use in breeding of a wider range of V. darrowii accessions would provide beneficial diversity in the blueberry cultivated gene pool. The purpose of this research was to determine the functional 2n gamete frequency of numerous V. darrowii genotypes when crossed with tetraploid V. corymbosum , and to study the pollen fertility and backcross ability of the interspecific ( V. darrowii × V. corymbosum ) hybrids to V. corymbosum . Crosses between diploid V. darrowii and tetraploid highbush blueberry cultivars had low fruit set compared with the V. darrowii × V. darrowii and highbush × highbush crosses. The unusually high number of hybrids per pollinated flower (HPF) in certain 4x-2x or 2x-4x crosses was attributed to high functional 2n gamete production in certain V. darrowii genotypes. Diploid Vaccinium fuscatum Aiton and diploid V. darrowii × V. fuscatum hybrids, when crossed with southern highbush blueberry cultivars, were equally productive of hybrids whether used as male or female parents. Variation in frequency of functional 2n gametes in V. darrowii , expressed as high HPF, was present within plants (megaspores vs. microspores) and among V. darrowii plants. Of the 114 interspecific ( V. darrowii × V. corymbosum ) hybrids studied, 106 had pollen stainability >50%. This indicated that most of these hybrids were tetraploid, because triploid blueberries, like most triploid plants, are highly sterile. Twenty-two V. darrowii × V. corymbosum hybrids were backcrossed to tetraploid highbush blueberry cultivars. Fruit set was variable, but large populations of vigorous hybrids were obtained. Lower fruit set was associated with hybrids that had lower pollen fertility. It should be possible to obtain plants of cultivar quality in a few generations of backcrosses.

‘Emerald’ is a low-chill tetraploid southern highbush blueberry hybrid that is well adapted to northeast and central Florida and to other areas receiving similar winter chilling (100 to 400 h below 7 °C). Emerald produces a vigorous bush with stout, semierect canes. It has medium to good survival in the field in north Florida. In northeast Florida, ‘Emerald’ flowers from mid-January to mid-February and ripens from mid-April to mid-May. ‘Emerald’ is capable of producing high yields of berries that are large, firm, and medium-dark in color with a small, dry picking scar and good flavor.