HORTSCIENCE 55(11):1788–1793. 2020. https://doi.org/10.21273/HORTSCI15279-20 a nonperenniating floral buds as a character of V. padifolium. The occurrence of non- perenniating buds has implications for the Field Performance of Backcross (BC1) season of flower bud development, the dif- ferentiation time of flowers within a bud, and Blueberry Hybrids of Vaccinium the location of such buds (often as adventi- tious buds on older wood). Our V. padifolium padifolium (Section Hemimyrtillus) with parental clone is notable for its profuse and continuous flowering and its high number of flower buds located on both young and old V. corymbosum/V. angustifolium wood. Continuous/repeat flowering is a trait Mark K. Ehlenfeldt, Joseph Kawash, and James Polashock that, if introgressed into V. corymbosum, may P.E. Marucci Center for Blueberry and Cranberry Research and Extension, allow multiple crops or continuous cropping. Other useful traits of V. padifolium include its Genetic Improvement of Fruit and Vegetables Laboratory, U.S. Department general vigor, large mature plant size, excel- of Agriculture, Agricultural Research Service, Chatsworth, NJ 08019 lent fertility, good fruit size, and general self- fruitfulness. Its evergreen nature may be a Additional index words. Vaccinium cylindraceum, V. arctostaphylos useful trait under some conditions but might Abstract. There is ongoing interest in transferring new characteristics into commercial also present management problems in terms blueberry from other blueberry species. Vaccinium padifolium is a species distantly of physiology and/or allowing the plants to related to commercial blueberry that has traits of notable value to conventional act as insect or pathogen reservoirs under blueberry development. Among these traits are upright structure, strong growth, field conditions. abundant flowering and fruiting, superior self-fertility, fruit-cluster structure suited to mechanical harvesting, and repeat/continuous flowering. Previously produced F1 hy- Utilization Value of the Hemimyrtillus brids of V. padifolium 3 V. corymbosum were used in crosses with a variety of Species conventional blueberry selections to generate 13 backcross (BC1) families. The backcross families were evaluated under field conditions to determine their performance and to Ballington (2001) reported several inter- guide further use of this germplasm. The offspring varied considerably, but most families sectional crosses of V. cylindraceum with produced several individuals of acceptable commercial quality. The recovery of V. both 2x and 4x germplasm of section Cyano- padifolium characteristics varied. Many plants bore evidence of V. padifolium plant coccus and other sections. Ballington gener- structure, but none showed indications thus far of continuous or repeat flowering. The ated BC1 hybrids with V. corymbosum, best selections from these families have been saved and will be intermated to generate the several of which were grown under field next cycle of this material. These clones will also be crossed to hybrid combinations of V. conditions to observe their performance. Of arctostaphylos and V. cylindraceum to further diversify and recombine this germplasm. the limited number of hybrids produced, the majority failed to thrive; however, a few BC2 and F2 selections continued to be evaluated on Blueberries (family Ericaceae, genus the secondary gene pool have also contrib- an ongoing basis (Ehlenfeldt and Ballington, Vaccinium, commonly section Cyanococcus) uted small amounts of germplasm to named 2012). These hybrids used only V. cylindra- are a diverse taxonomic group, and blue- blueberry cultivars—among them, 2xV. ceum and V. corymbosum germplasm. berries currently in commercial production elliottii,6x V. constablaei, and 2x V. tenel- A significant challenge in breeding with represent three major Vaccinium species and lum. exotic species hybrids is the recovery of the two ploidy levels: 4x V. angustifolium (low- necessary horticultural plant type, and espe- bush blueberry), 4x V. corymbosum (high- Section Hemimyrtillus and Its Species cially deriving plants with fruit morphology bush blueberry), and 6x V. virgatum (syn. V. that is neither too small nor too dark to meet ashei; rabbiteye blueberry). As such, these Vander Kloet and Dickinson (1992) con- commercial standards. In a previous paper, three types may be considered the primary sidered the species of section Hemimyrtillus we reported on the generation of fertile 4x gene pool of blueberry. Two other commer- to be the remnants of a once widely distrib- hybrids between V. padifolium and V. corym- cial types of blueberry, half-high blueberry uted taxon. Today, six species comprise sec- bosum and subsequent backcrosses to culti- cultivars and southern highbush cultivars, are tion Hemimyrtillus, and three of these are vated germplasm (Ehlenfeldt and Polashock, mixtures using these species. Half-highs have found in limited localities. The species and 2014). The goal in working with these species been produced by hybridization of 4xV. their ranges are V. cylindraceum (Acxores, is to incorporate germplasm from this section corymbosum with 4x V. angustifolium and Portugal), V. padifolium (Madeira Islands, into cultivated germplasm and transfer the retain a significant but variable percentage Portugal), V. arctostaphylos (Caucasus Re- desirable traits that these species possess for contribution from each species. Southern gion), V. smallii (Korea, Japan, Russia), V. mechanical harvesting and commercial pro- highbush cultivars have been developed by yakushimense (Yakushima Island, Kyushu, duction. The objective we address here is the the introgression of low-chilling-requirement Japan), and V. hirtum (Japan, South Korea). evaluation of hybrids and BC1 derivatives species, primarily 2x V. darrowii and 6xV. V. padifolium, our primary species of under field conditions to understand strengths virgatum into 4x V. corymbosum at contribu- concern, is native only to the Madeira Islands and weaknesses and to determine where fur- tion levels that average around 25% in mod- and is typically found in subalpine land- ther effort needs to be applied. ern cultivars. Several additional species from scapes at altitudes of 1220 to 1700 m. Plants are bushy to treelike, and typically 1 to 4 m Materials and Methods tall; under native conditions, plants possess an evergreen habit. Under our conditions, V. Materials. We generated 13 families us- Received for publication 1 July 2020. Accepted for padifolium is insufficiently cold-hardy to ing US 1896 (= V. padifolium US 908 · V. publication 23 Aug. 2020. thrive outdoors. In general, plants have corymbosum US 1825) as either a female or a Published online 5 October 2020. tough, semiglossy leaves with a finely retic- male. The details of the origin of US 1896 J.K. is an ORISE Scholar. M.K.E. is the corresponding author. E-mail: mark. ulated texture. Plants produce creamy, pink- have been described previously (Ehlenfeldt [email protected]. tinged, bell-shaped flowers that give rise to and Polashock, 2014). Of the 13 families, two This is an open access article distributed under the medium-blue, ovate fruit (M.K. Ehlenfeldt, used US 1896 as a female and 4x V. cor- CC BY-NC-ND license (https://creativecommons. personal observations). Vander Kloet and ymbosum (‘Bluecrop’ and ARS 98-15, re- org/licenses/by-nc-nd/4.0/). Dickinson (2005) noted the development of spectively) as males. Eleven families were 1788 HORTSCIENCE VOL. 55(11) NOVEMBER 2020 generated using US 1896 as a male in crosses resumed growth no matter how poor was Neither proved to be winter-hardy; both using commercial cultivars and selections as considered a survivor. showed substantial dieback. After a year in females. These cultivars were Biloxi, Bluecrop, • Vigor was evaluated in mid-June. Rating the ground, US 1897 was no longer alive. US Brunswick (lowbush), Cara’s Choice, Duke, was done on a scale of 0 to 3, where 0 = 1896 continues to persist but is winter dam- Hannah’s Choice, Legacy, Northsky (half- struggling, 1 = fair vigor, 2 = average aged each year to a considerable extent. high), Sharpblue, Sweetheart, and TH 622. vigor, and 3 = above average vigor. Propagules of both of these selections were The details of these crosses were described • Height was determined at the end of the subsequently sent to U.S. Department of previously (Ehlenfeldt and Polashock, 2014). summer season (22 Aug. 2018) by mea- Agriculture, Agricultural Research Service All of the parents listed are named cultivars suring each plant to the nearest 5 cm. in Poplarville, MS, for field planting and except for ARS 98-15 and TH 622. ARS 98-15 • At the time of first inspection, bloom evaluation (still underway). is a breeding selection of the pedigree ‘Duke’ · density data were also taken. Flowering The BC1 generation seed, when first ger- ‘Nelson’. The selection TH 622 is a sibling of intensity was rated with subjective nota- minated and planted out to seedling flats, the southern highbush cultivars Camellia tions of no flowers, almost no flowers, light showed considerable segregation from weak (NeSmith and Draper, 2007) and Gupton flowering, average flowering, and vigorous to vigorous. It was expected that this level of (Stringer et al., 2012) and comes from the cross flowering, but for the purposes of this re- segregation might occur in the progeny of of MS 122 · MS 6. The cross MS 122 · MS 6 port, flowering intensity was reduced to a such wide hybrids, and it was additionally was notable for productive, vigorous plants binary (0–1)