Tetraploid blueberry cultivars with a 100- to 400-h chill requirement (mean temperature of the coldest month 13 to 16 °C) are being grown in central and northeast Florida, in parts of Georgia and California, and in Argentina, Chile, Australia, and Spain. These cultivars are used to produce berries for the export market that can be harvested 1 month or more before high-chill cultivars grown in colder areas. Some low-chill highbush blueberry cultivars are also adapted to evergreen production in areas where no winter freezes occur or, where only light freezes occur, under plastic tunnels. Evergreen production gives off-season fruit, which ripens from late fall through early spring depending on the environment.
The southern highbush cultivars were bred by recurrent selection in areas with warm winters. Breeding started with hybrids between northern highbush cultivars and low-chill wild blueberries from the southeastern United States, notably V. darrowi (Sharpe and Darrow, 1960; Sharpe and Sherman, 1971). Vaccinium darrowi is a low-growing fire-adapted evergreen species that has no chilling requirement. It produces small berries and has a relatively long bloom-to-ripe berry development period. The recurrent selection process that produced cultivars had numerous selection objectives, some related to the plant and others to the berry (Lyrene, 2005; Lyrene and Moore, 2006).
Plant characteristics emphasized in breeding southern highbush cultivars include low chilling requirement and tolerance to blueberry diseases (Caruso and Ramsdell, 1995) associated with hot, wet summers and semievergreen production, including phytophthora root rot (Phytophthora cinnamomi Rands), stem blight [Botryosphaeria dothidea (Moug.:Fr.) Ces.& De Not], stem canker [Botryosphaeria corticis (Demaree & M.S. Wilcox) Arx & E. Muller], and numerous fungi that attack the leaves, including blueberry leaf rust [Pucciniastrum vaccinii (G. Wint.) Jorst]. Other plant characteristics that required selection were upright growth habit, elimination of the extreme “twigginess” that is characteristic of V. darrowi, the ability to leaf strongly during or shortly after the time of flowering, and high male and female fertility. The long flowering-to-ripening period of V. darrowi was undesirable in cultivars intended for the early market and had to be selected against.
Lyrene, P.M. & Moore, J.N. 2006 Blueberry breeding 38 48 Childers N.F. & Lyrene P.M. Blueberries N.F. Childers Horticultural Publications Gainesville, FL