was the most important everbearer from 1940 to the modern period. ‘Fairfax’ was widely planted in the middle of the century from southern New England to Maryland and westward to Kansas. It was particularly noted for its outstanding flavour but may have been more important as a breeding parent, finding its way into the pedigree of numerous European, USA, Canadian and even Japanese cultivars.
In the late 1940s and early 1950s, several more cultivars achieved importance including J. Clarke’s ‘Sparkle’ or ‘Paymaster’ (New Jersey, 1943), E. Morrow’s ‘Albritton’ (North Carolina, 1945), H. Thomas and E. Goldsmith’s ‘Lassen’ and ‘Shasta’ (California, 1945), D. Scott and G. Darrow’s ‘Pocahontas’ (Maryland, 1946) and C. Schwartze’s ‘Northwest’ (Washington, 1949). ‘Sparkle’ dominated in the north-east and mid-west in the 1950s and 1960s, due to its high flavour, attractive appearance and resistance to red stele. ‘Northwest’ was the most planted variety in the USA in the 1960s, even though all of the acreage was in Oregon and Washington (Darrow, 1966). It was particularly noted for its lateness and tolerance to virus diseases. ‘Shasta’ was widely grown in the central coast of California in the 1950s and 1960s because of its large size, firmness and long season. ‘Lassen’, grown extensively in southern California about the same period, was prized for its short rest period and high productivity. ‘Pocahontas’ was widely grown in the lower mid-west and south in the 1960s and 1970s, and even became important in Italy in the late 1970s. It was known for its productivity, large, attractive, firm berries and good freezing quality.
The middle decades of the 20th century saw the release of D. Scott’s ‘Surecrop’ (Maryland, 1950) and ‘Midway’ (Maryland, 1960), R. Bringhurst and V. Voth’s ‘Tioga’ (California, 1955), P. Hawthorne’s ‘Headliner’ (Louisiana, 1957) and ‘Dabreak’ (Louisiana, 1961), L. Spangelo’s ‘Redcoat’ (Ontario, 1957), H. Thomas and E. Goldsmith’s ‘Goldsmith’ (California, 1958) and A. Brook’s ‘Florida 90’ (Florida, 1952). ‘Surecrop’, which was important in the upper south and lower mid-west during the 1960s and 1970s, was noted for its high disease tolerance both to leaf and soil pests. ‘Midway’ replaced ‘Robinson’ in the 1960s in the mid-western USA, primarily because of its firmer berries and resistance to red stele. ‘Redcoat’ dominated eastern Canada in the 1960s and 1970s. It was known for its high yield, appearance, earliness and shipping qualities. ‘Headliner’ and subsequently ‘Daybreak’ became important in the south in the 1960s and 1970s, replacing the earlier varieties, due to their larger size, higher productivity and an earlier season. ‘Tioga’ replaced ‘Lassen’ in California in the late 1960s because of its greater size, attractiveness, firmness and productivity. ‘Florida 90’ became popular in Florida in the late 1950s due to its very long, large, early berries and high flavour (Darrow, 1966). ‘Goldsmith’ was the first important private variety in California, noted for its productivity and shipping quality.
DIVERGENCE OF NORTH AMERICAN BREEDING PROGRAMMES
As previously described, strawberry breeding began in North America by hybridizing F. × ananassa cultivars derived from Europe with native genotypes of the founding species of F. chiloensis and F. virginiana that came from diverse geographic and subspecies origins (Darrow, 1966). Breeding programmes subsequently cropped up all across the continent in a broad range of climates ranging from the cold temperate climates of the eastern seaboard to the mild Mediterranean ones of coastal California. Over time, considerable genetic differentiation occurred among the various breeding programmes through hybridization and selection. The wild progenitor ecotypes would have provided an almost unlimited source of genes to generate unique regional group structures (Hancock and Luby, 1993).
Over 50 years ago, Darrow (1966) identified a series of 18 traits that separated F. chiloensis from F. virginiana and then rated five eastern varieties for the proportion of their phenotype that appeared to be based on F. chiloensis. According to his rating system, ‘Fairfax’ expressed 57% of the characteristics of F. chiloensis; ‘Earlidawn’, 28%; ‘Blackmore’, 31%; ‘Howard 17’, 31%; and ‘Missionary’, 27%. From these values he suggested that his eastern breeding programme had been selecting for more F. virginiana characteristics than F. chiloensis characteristics. He further asserted that ‘Fairfax’ with the most F. chiloensis characteristics would be more broadly adapted than the others and that ‘greater use of F. chiloensis in breeding for superior varieties seems indicated’ (Darrow, 1966).
Much more recently, Hardigan et al. (2018) analysed genome-wide DNA profiles of 1300 octoploid individuals, including wild species, historic varieties (1814–present) and the University of California germplasm collection. They evaluated 16,492 polymorphic and subgenome-specific markers. They found that the cultivars bred for California were substantially differentiated from the temperate-climate cultivars developed in eastern North America and they did indeed carry a much higher proportion of genes from F. chiloensis. Selection over the last 20–30 years in California had restructured genetic diversity similar to the restructuring that occurred during the first 200 years of breeding, and coastal F. × ananassa has diverged further from the temperate F. × ananassa than the latter from their wild progenitors (Fig. 2.8). They also showed that introgression of the day-neutral flowering trait from non-ancestral F. virginiana ssp. glauca into coastal California led to a transition to a different gene source of late-season flowering.
Fig. 2.8. Population divergence [Weir and Hill F-statistic (F) arrow labels] of wild progenitor species (South American F. chiloensis and F. virginiana), temperate F. × ananassa (TM-F×a), California coastal (CC-F×a) pre-1970, and CC-F×a post-2000. Internal values reflect average marker diversity (π) within the populations. (From Hardigan et al., 2018.)
AMERICANIZATION OF EUROPEAN BREEDING PROGRAMMES
As previously described, strawberry breeding began in Europe in the early 1800s, with a collection of New World representatives of F. chiloensis and F. virginiana. In the mid-1800s, the early hybrid cultivars from Europe found their way across the Atlantic and were used by the first North American breeders to initiate their programmes. Over the next 100 years, the European and North American breeding programmes tended to operate in isolation with only limited germplasm exchange. However, this separation dramatically changed in the 1960s, when European breeders began to widely employ improved North American cultivars from California in their breeding programmes.
The appearance of US germplasm in European breeding programmes has been documented in a couple of recent studies using diagnostic DNA markers. Gil-Ariza et al. (2009) used simple sequence repeat (SSR) markers derived from expressed sequence tags to determine similarity relationships of 92 mostly US and European cultivars from the mid-1800s to present. They identified three clusters: (i) a clade of 26 old accessions (most of them released before 1969) of European or American origin with little California genetics incorporated; (ii) a clade of 44 later-released accessions from both continents represented by ‘Camarosa’ and carrying a goodly proportion of genetics from the Strawberry Breeding & Research programme at the University
