The History of Hop Breeding and Development

Grower selections, pre-1900

Although there is documented evidence of hop cultivation in Europe from the ninth century, with cultivation in Britain from at least 1524, the exact origins and the husbandry methods used are obscure. However, the division of a rootstock or the cutting of underground stems for the vegetative multiplication of hop plants is well described from the earliest records. ³ Thus, hop growers knew how to increase the stocks of plants identified as superior. By 1669, different types of hops with differing attributes were being noted. ⁴ It is most probable that these would be plants that were better adapted to the soils or the climatic conditions in a particular locality. They would tend to become the commonest type of hop grown in that area and be known by the name of the area, or a distinctive feature of their appearance. Thus, Percival ⁵ describes hop varieties such as the Farnham Hop, the Canterbury Whitebine, and Golden Tips. The same practice elsewhere in the World produced distinct varieties known by their production areas such as the Saaz region of the Czech Republic, or the Hallertau or Tettnang areas in Germany.

In addition to local adaptability, variation could arise through natural clonal changes or by the unintentional establishment of a seedling. If these plants were selected, they were often attributed to the grower who recognised their potential. Thus arose such varieties as Mr Golding's hop or Mr Fuggle's hop, later contracted to Goldings and Fuggles. The traditional hops in the USA, the Clusters were almost certainly chance seedlings which have arisen through natural hybridisation between European hops introduced by the Massachusetts Company in 1629 and wild US hops. ⁶

After several centuries of grower selection, very many different varieties existed. By 1900 more than 20 varieties were known in England and at least 60 recorded in mainland Europe. ⁷ Modern methods of analysing essential oils have been used to distinguish which of the older varieties are clones of each other and which are distinct varieties. Varieties such as Rodmersham, Mercers, Mathon, East Kent, Canterbury, Cobbs, Bramling, Eastwell and Early Bird have been shown to be variants of the same variety, the Golding, selected in 1790. The Golding clones vary in their ripening date, yield and susceptibility to diseases. In contrast, Fuggle, reputed to be selected in 1861 as a chance seedling, has indeed been shown to be a distinct variety. However, oil analysis indicates that Fuggle and Savinjski Golding are clonal variants of each other.

Most of the varieties from pre-1900 have since been discarded but this period has given some varieties that are still in production today and that are still highly prized for their quality.

Breeding programmes during the twentieth century

As early as 1763, brewers had noted that the quantities of hops required to produce a certain level of bitterness varied slightly between different varieties, and that there were also differences in the keeping qualities of beers brewed with different hop varieties. ⁸ However, with increasing international trade in hops, European brewers in the late 1800s were noting that some of the imported varieties produced markedly different levels of bitterness in their beer in comparison with local hops. Improved chemistry and analytical abilities soon identified this as a reflection of the resin content, with the Oregon Clusters from the USA providing the highest bitterness levels. Furthermore, these imported varieties imparted different preservative properties to the beer. As today, bitterness and shelf-life were such important qualities in beer that brewers were soon demanding improved hop varieties, able to impart greater bitterness or longer shelf-life.

At the same time growers, too, were demanding improved varieties. There were no effective chemical treatments for pest and disease control and crop yields could vary dramatically between seasons, depending on the weather and the level of pest or disease attack. Indeed, hop crop production was so variable and unpredictable that it was used as the basis for gambling. ⁹ Improved resistance of hops to pests and diseases was sought by growers. In response to these demands by brewers and growers, and armed with the new science of genetics, hop breeding programmes were started in most of the hop-producing countries.

The hop plant is very sensitive to daylength and local soil conditions and, as a result, varieties usually do not perform well outside the original area where selected. Although exchanges of varieties between growers in different countries had been practised for centuries, hop improvement programmes needed to be in the country in which the variety would be grown.

The Wye programme started by Salmon in 1906 laid the foundations for all hop breeding during the twentieth century. Other programmes followed the methods and procedures that he used, the objectives he set, and the varieties that he produced. Programmes were started in 1908 in USA (although soon discontinued and restarted in Oregon in 1931) and, in 1911, a programme was started at the Carlsberg Laboratory in Denmark. Hop breeding started in Hokkaido in Japan in 1912 and was later incorporated into the Sapporo Brewery. In 1924, hop breeding began at Zatec in the Czech Republic, although the programme, led by Osvald, was mostly concerned with clonal selection amongst the Saazer variety. A programme was started in 1926 at Hüll in Germany in response to the need for varieties resistant to downy mildew in the Bavarian Hallertau region. Since the end of World War II, publicly-funded programmes were started in Slovenia, the Ukraine, Poland, China, India and New Zealand. In addition, brewing companies started programmes in Japan, Australia, South Africa, and further programmes in the USA.

Although the reasons for initiating each programme differed, there are several broad objectives that they have all aspired to achieve :-

Breeding for increased resin content

Throughout the twentieth century, hop breeding has succeeded in steadily increasing the resin content of hop cones, and the alpha-acid content in particular. Few varieties being cultivated around 1900 exceeded 4% alpha-acid content but, by the end of the century, varieties were available producing more than 16-18% [Fig. 1]. This increase is often considered to be the main achievement of hop breeding programmes during the twentieth century. The quest to obtain even higher alpha-acid contents continues in most current programmes.

Figure 1. Comparison of the resin content of hop cones between a variety bred at the end of the twentieth century (left) and a grower selection from pre-1900 (right). (EMR)

Breeding for increased resin content was started by Salmon at Wye who introduced wild North American hops into his breeding programme, recognising that higher resin contents were a feature of USA hops rather than European hops, although the aroma of USA hops was inferior. The concept of using wild accessions in a programme was highly innovative; breeding of most crops at the time used only commercial varieties as parents. Salmon claimed that the undesirable ‘American’ aroma segregated independently from the resin content and that it would be possible to produce a variety with high resin content and an improved aroma. However, the first two successful varieties from this programme, Brewer's Gold (released in 1934) and Bullion (1938) retained a distinctive ‘American’ aroma and owed their success to an alpha-acid content of about 8% and a good commercial yield. Northern Brewer (1944), a seedling of Golding and a male derived from the ‘American’ line, was the first to combine high resin content and good ‘European’ aroma. These three varieties were grown widely in mainland Europe and USA, as well as in England.

These three varieties bred by Salmon have been used as the starting point for all high-alpha breeding programmes throughout the world and there have been only two bitter-type varieties released that do not include one of Salmon's varieties in their immediate or close parentage. These exceptions, Talisman and Comet, were also derived from wild American breeding material. It can be estimated that more than 50% of the world's current area comprises varieties derived from Salmon's three principal varieties.

Since the release of Northern Brewer, there has been a steady increase in the alpha-acid content of varieties across the world. The alpha-acid content of Northern Brewer has been exceeded by many varieties including Pride of Ringwood in Australia (1953), Atlas and Apollo (known as the ‘Super-Styrians’) released in Slovenia in 1971, Wye Northdown in the UK (1971), and Galena and Eroica in the USA (1979 and 1982 respectively).

Wye Target, released from Wye in 1972 was the first variety consistently to exceed 10% alpha-acid in commercial production and rapidly became the predominant variety in the UK. However, the development of these bitter hops, at more than twice the resin content of the traditional varieties, caused alpha-acid to start to be traded as a commodity with hops sold on the basis of kilograms of alpha-acid. With the advantages of the economies of scale, the USA became the main producer-country for alphaacid. Programmes at Oregon and Washington State Universities produced further successful alpha-type varieties; Nugget (1983), Olympic (1983) and Chinook (1985).

Major developments in hop processing since the mid-1980s have increased the importance of breeding for higher alphaacid contents. Over 85% of the world hop crop is now processed into pellets, extracts or further downstream products [Fig. 2]. These operations reduce the bulk density of raw hops and allow new packing technologies to improve and extend storage. However, the economics of such operations are determined largely by the alpha-acid content of the hops. This has led to the newest generation of high-alpha varieties, released 1990 - 2000 and known as ‘super-alpha’ types, and distinguished by alpha-acid contents greater than 15%. These include Admiral from Wye, Taurus and Magnum from Germany, Eastern Gold and Kitimadori from Japan, Pacific Gem from New Zealand, Victoria and Opal from Australia, and Columbus, Tomahawk, Zeus, Warrior and Millennium from USA.

Figure 2. Hop cones and products including pellets, extracts and essences (Botanix)

Increasing the alpha-acid content has been the main factor increasing the productivity from hop fields. In most other crops, plant breeding has produced markedly higher yields. In hops up until the late 1980s, however, the yields obtained from newer varieties in any particular country have not increased greatly over the levels found in the varieties selected by growers from pre- 1900. Productivity measured as the yield of alpha-acid, therefore, has increased in proportion to the alpha-acid content. This has been of benefit mainly to brewers, particularly those who base their buying on the cost of alpha-acid, regardless of variety or place of origin. For growers the acreage required for production has fallen in directly inverse proportion to the increase in alpha-acid levels.

Improving analytical capabilities, notably the use of HPLC techniques, have enabled the composition of the resins to be determined as well as the total content. ¹⁰ This has led to an emphasis on the quality of the resin. Following isomerisation during wort boiling or hop processing, there are six different isomers of the alpha-acids and these are perceived as providing different forms of bitterness. Based on experience with different varieties, many brewers dislike the bitterness derived from cohumulone and prefer to purchase alpha-acid from varieties with low cohumulone content. Thus, selection in most breeding programmes has been for high alpha-acid content with low cohumulone. The characteristic is highly heritable ¹¹ and cohumulone contents have reduced considerably. Whilst an older bitter-type variety such as Brewers Gold has a cohumulone content of 45%, recent varieties such as Magnum have cohumulone of only 25%.

With the increasing use of hop processing since 1980, and the importance of the economics of these operations, the stability of the alpha-acid has also become a desirable quality consideration. In general, it is considered desirable for high alpha-acid varieties to have good storage so that there is minimal loss of alpha-acid during the first few months of storage before processing. However, reviewing progress in increasing the storage stability of varieties shows that the increases in the total alpha-acid content have generally preceded improvements in storage. Thus, Brewers Gold (1934) stored poorly but was followed by Northern Brewer which had improved storage. Similarly, Wye Target (1972) has very poor storage but Nugget (1983) stores well. Columbus (1998) has, perhaps, the poorest storage of any bitter-type variety but Millennium (2000) has good storage.

Breeding of aroma varieties

The breeding of new aroma varieties has been much less successful than that of alpha varieties. Many brewers have continued throughout to use traditional varieties to impart aroma and hop flavour qualities to their beer. Unfortunately for brewers, many traditional varieties were abandoned following the introduction of static mechanical picking machines between 1945 and 1965. The older varieties tended to have large cones, often growing in clusters, that could be picked in handfuls. Such cones were unsuitable for machinery because they would breakup or they failed to roll down the inclined belts [Fig. 3] used to separate leaves from cones. ¹² Those traditional varieties that did remain, although desired by brewers, often had shortcomings for the grower.

Figure 3. Separation of hop cones from leaves within a hop picking machine, using a series of inclined belts. (Magadan, Spain)

Breeders have aimed to improve the agronomic features of specific favoured traditional varieties whilst retaining an aroma as close as possible to the original. Despite using pedigree breeding methods, the complexity and subjectivity of this task, together with the lack of scientific knowledge of the contribution of hop aroma to beer aroma and flavour, means that it is very difficult to produce a seedling which approaches the aroma of the mother variety. New varieties have only gained acceptance when production of the traditional variety has been severely threatened, often by a new disease.

Wilt disease, caused by Verticillium alboatrum, appeared in the UK in the 1920s and was recognised in continental Europe in the 1950s. ¹³ There are no chemical means of controlling this soilborne disease and virulent forms will kill the hop plant and destroy production. All traditional varieties are fully susceptible and it has threatened the continued production of Fuggles and Goldings in the UK and Hallertau Mittelfrüh in Germany. Production has continued when infection has been slight through isolation and strict phytosanitary precautions. However, when the disease is endemic in the soil, resistant varieties provide the only means of continued production. Fortunately, the wild USA germplasm introduced by Salmon carried resistance to wilt disease. Letters from Salmon to local hop growers indicate that USA-derived males were also being used on Kent farms as pollinators and, in 1911, a grower noted an interesting hybrid amongst the variety Bates Brewer. He established a small area on his farm at Beltring, Kent under the code 1147. The farm was subsequently bought by the Whitbread brewery and it suffered a serious outbreak of wilt in the early 1930s. Hybrid 1147, however, clearly showed resistance and, now known as Whitbreads Golding Variety (‘WGV’), still remains in production today as a wiltresistant aroma-type alternative to Goldings.

Wilt disease was studied in detail by Keyworth at East Malling, Kent. ¹⁴ He undertook the field screening of advanced selections from the Wye programme which were often obtained from crosses between wild USA hops and well-regarded European aroma hop varieties. A number of resistant selections were identified and became established as aroma-type varieties including Early Choice (1948) and Bramling Cross (1951), both seedlings of Goldings and both still in production today. A joint programme between Wye and East Malling aimed specifically to develop a wilt-resistant alternative to Fuggles and several varieties were released of which Progress (1967) has been the most successful. It continues in current production.

Northern Brewer, also a seedling of Goldings, was not identified as resistant to the virulent UK strains of wilt but it was found to be resistant to strains of Verticillium wilt present in the Hallertau region of Germany. The success of Northern Brewer in that area was attributable to its combination of high alpha-acid, good aroma and resistance to wilt. The German breeding programme has continued to breed for wilt resistance in its aroma-type selections and more recent such varieties include Tradition (1993) as a resistant replacement for Hallertau Mittelfrüh and Select (1993) as a replacement for Spalter.

In 1924, downy mildew, which had been recognised for the first time only a few years earlier, became epidemic and devastating throughout all hop growing areas in Europe, threatening production. ¹⁵ In response, a breeding programme was initiated at Hüll in Germany with the sole objective of developing varieties resistant to the disease. As with Salmon and alpha-acid content, the German programme looked to wild accessions as a source of resistance to downy mildew. It took nearly 40 years before the programme yielded its first varieties, Hüller Anfang (1962) and Hüller Fortschritt (1966). Resistance to downy mildew is now incorporated into all new varieties originating from the Hüll programme. Of these, the dual-purpose variety Perle (1978) has been the most successful, combining resistances to wilt disease and downy mildew with good aroma and bitterness. As such, it has largely replaced Northern Brewer in the Hallertau region. The dual-purpose UK variety Wye Challenger (released in 1971) is also strongly resistant to downy mildew and owes its resistance to the resistant germplasm developed at Hüll during the 1950s.

In the USA, the threat from downy mildew was avoided by moving production from the wetter east coast states to the drier west coast; initially California, and then Oregon, Idaho and Washington State. ¹⁶ However, the disease inevitably followed the production and traditional Clusters were particularly susceptible, proving impossible to continue in production in the more humid Oregon. Several introduced varieties, notably Fuggle, showed some resistance but were too ill-adapted to produce economic yields. The Oregon State University hop breeding programme, therefore, used Fuggle as a parent to produce downy mildewresistant aroma cultivars suitable for the Oregon area. Cascade (1972) and Willamette (1976) continue to be the most successful releases from this programme.

Archaeological evidence found hops to be a cargo on the Graveney Boat, dated to c. 950 AD, ¹⁷ indicating that hops have been traded between countries for centuries. Brewers have been forced to import from countries where varieties grew well when it has not been possible to source from the home country. Illadaptation of a variety, most commonly due to changes in latitude and, hence, daylength, are usually seen as poor, uneconomic yields. This has been a particular feature of European aroma hops in the USA. To reduce dependence on imports, breeders have endeavoured to produce varieties with aromas similar to those of imports, but suitable for growth in the USA. Initial selection has been on the composition of the major essential oils [Fig. 4] followed by subjective organoleptic evaluation and brewing trials. Thus, as already described, Fuggle replacements include Willamette and Cascade. Hallertau-types include Mount Hood (1989), Liberty (1992), Crystal (1993) and Vanguard (1998). Sterling (1989) was bred as a substitute for Saaz, and Ultra (1995) reputedly combines aroma properties of both Saaz and Hallertau. Similarly, Santiam (1997) has an oil profile similar to Tettnang, and Glacier (2000) resembles Strisselspalt and Hersbrucker Spät.

Japan, also, has relied heavily on imported hops, particularly Saaz. Attempts to develop an equivalent Japanese variety were started in 1912 and produced Shinshu Wase from a cross with Saaz. It became the principle variety cultivated in Japan for the next 50 years. However, a new breeding programme started in 1967 attempted to combine the Saaz-type aroma with improved agronomic performance. Furano Ace was released in 1988 from this programme.

Figure 4. Gas chromatogram analysis of the essential oils of 'Hallertau Mittelfrüh'. 1= myrcene, 2= linalool, 3= beta-caryophyllene, 4= aromadendrene, 5= humulene, 6= farnesene, 7= betaselinene, 8 = alpha-selinene. (CMA, Germany)

In Europe, attempts to increase the yield of traditional aroma varieties through crossing and selection have been features of the programmes in Germany, Slovenia, and the Czech Republic. Opal (2001) and Saphir (2002) have been released from the German programme to join Perle, Tradition and Select as higher-yielding alternatives to the varieties used directly as their parents, Northern Brewer, Hallertau Mittelfrüh and Spalter respectively. In Slovenia, the A-series ‘Super-Styrians’ for alpha-acid content, described previously, were followed in 1979 by the aroma-type B-series Bobek, Buket and Blisk, and in 1990 by the aroma-type C-series Cicero, Cekin, Celeia and Cerera. With the exception of Bobek which is derived from Northern Brewer, all these Slovenian varieties have Savinjski Golding in their close parentage. The Czech programme for most of its history has been concerned with clonal selection within the traditional Saaz variety. However, recognising a long-term decline in the productivity of the variety in the Czech Republic, a limited hybridisation programme between Saaz and Northern Brewer breeding lines produced the aroma variety Sladek in 1994 which has the potential to yield up to twice the crop of Saaz, and dual-purpose varieties Bor (1994) and Premiant (1996).

Many of the hop improvement programmes have recognised that hybridisation would be very unlikely to reproduce the aroma of the prized traditional varieties and so have placed much emphasis on clonal selection. Most notable has been the Czech programme where collection and testing of local landraces of the Saaz variety from 1924 to 1987 has produced nine clones which progressively have outyielded each other: Lucan (1941), Blato (1952), clones '31', '72' and '114' (1952), Sirem (1969), Zlatan (1976), Podalsak (1989) and Blsanka (1993). In the UK, similar selection amongst clones of Goldings and Fuggles were made at East Malling between 1931 and 1957 to establish unified planting stocks for distribution to the industry ¹⁸ through a certification scheme started in 1943. This was combined, from 1955, with virus elimination strategies to provide high health status planting stocks of traditional varieties. ¹⁹ Similar clonal selection is recorded in Russia with the traditional Serebrianker variety, France with the landrace Strisselspalt, and the USA with Clusters.

As well as the essential oil composition, the resin quality and quantity is considered to be of importance in aroma-type varieties. Traditional varieties are distinguished by low alpha-acid content and, as previously described for bitter-type varieties, low cohumulone content. Breeding of newer aroma varieties has mostly aimed to match the characteristics of the older varieties although the cohumulone content has often been reduced. Some of the most recent aroma varieties have particularly low cohumulone contents with Glacier (2000) at 11-14% currently the lowest. Opinions are less clear on the value of good storage in the resin of aroma varieties. Recent aroma varieties, such as Mount Hood in USA and First Gold in the UK, store only moderately well.

Breeding for yield improvement

Until the late 1980s, attempts to increase the yield of hops by increasing the vigour of the hop plant to give greater cone numbers or cone weights have not proved very successful. Increased vigour has been associated with greater selfshading and yields have not increased. However, the development of triploid varieties has given greater potential for increases in yield and these have been realised in some programmes.

In particular, several of the new aroma varieties mentioned above are triploids. Examples include Willamette, Blisk and all of the Slovenian C-series. In contrast to the usual diploid hybridised varieties, where an equal genetic contribution is obtained from each parent, a triploid variety has twice as much genetic contribution from one parent. The technique, developed for hops in Japan, ²⁰ was applied to several traditional European aroma hops by Dark ²¹ at Wye. It is particularly suitable for aroma breeding because, theoretically, the desired flavour components of the female parent can be retained more easily. Trials of triploid varieties in England were unsuccessful because the increased vigour of such plants caused shading, disease problems, delayed maturity and reductions in yield. ²² However, where light intensities are greater and wirework higher, triploid varieties have produced greatly increased yields. For example, all varieties developed since 1970 in New Zealand have been triploids, including the most recent aroma-types such as Hallertau Aroma (1988) and Pacific Hallertau (1994), and alpha-types such as Pacific Gem (1987) and Pacific Sunrise (2000). In the USA, alpha-type Millennium is a triploid variety.

Technological improvements in the USA in harvest machinery and oast house design and construction, particularly in the 1990s, allowed vigorous varieties such as Millennium to be harvested. In most US oast houses wooden construction has been replaced by concrete.

Figure 5. The glasshouse screening of hop seedlings for resistance to powdery mildew disease. (EMR)

These changes have facilitated a greater throughput of crop during harvest. Additionally, there has been a change in USA crop husbandry with a general move to widen the alley width in plantations to allow more light to penetrate the crop. Increased yields from new varieties with heavier cones have been achieved. Notable has been Columbus which can achieve yields of up to 3400kg/ha compared to 2400 kg/ha in Clusters. This is mainly due to a very heavy cone which, as a consequence, has proved difficult to dry sufficiently for prolonged storage.

The incorporation of pest and disease resistance

Of the major fungal diseases of hop known today, only powdery mildew was recorded before the beginning of the twentieth century ²³ where it was found throughout Europe. Until the late 1990s, it was not a serious enough problem to stimulate a breeding programme except in England. Fortuitously, Salmon was a mycologist and was employed at Wye College as such, not as a plant breeder. Many of the wild accessions that he introduced to the programme, notably those from Italy and the USA, carried resistance genes and he made a special study of the type of resistance which he designated as ‘blister’. ²⁴ He recognised resistance to powdery mildew in two of his varieties, Pride of Kent and Early Choice. Of the seven resistance genes for powdery mildew noted in hops to date, ²⁵ five of them derive from Salmon's accessions. His successor, Neve, introduced glasshouse screening of seedlings for resistance [Fig. 5] and incorporated resistance into all of his varieties. The first powdery mildew resistant variety that he released, Wye Challenger (1971), carried the ‘blister’ resistance which provided immunity at Wye. However, during propagation for commercial release in East Anglia it was exposed to different strains of the pathogen and a virulent form quickly emerged. Wye Target (1972) was resistant due to the action of a different gene, the R2 gene, and no virulent strains of the pathogen were immediately apparent. Wye Target remained effectively field immune to the disease in the UK for nearly 20 years. Wye Target was also used as the resistance source for the breeding of varieties in the Hüll programme. Following the introduction of Magnum in 1993 which proved to be particularly susceptible to powdery mildew, resistance breeding for this disease was given much higher priority. The R2 resistance from Wye Target was incorporated into Merkur (2000) and Herkules (2005) and remains effective in continental Europe, although no longer so in any part of the UK.

Powdery mildew appeared suddenly and devastatingly in Washington State, USA in 1997. It had been present in the east coast states but had not, until then, followed hop production to the west coast. Most US varieties were highly susceptible to the disease but Nugget showed complete immunity. Nugget carries the R6 gene for resistance and this resistance was quickly recognised in seedlings of Nugget at the advanced selection stages, including Millennium and Newport. These selections were accelerated through further trials and, within 3 years of the arrival of the disease, US breeders had registered new varieties resistant to the disease.

Breeding for resistance to wilt disease and downy mildew has been described for the development of aroma-type varieties, but is also a feature of bitter-types. Procedures for screening and selecting seedlings are now routine within several programmes and resistance to locally virulent forms of these two diseases is incorporated into all new varieties. ²⁶ However, resistance to UK strains of wilt disease has been associated genetically with poor agronomic and quality characteristics and the need to incorporate resistance to wilt has limited the productivity of the breeding programme at Wye. Wye Target suffered from agronomic problems during its farm trial stage including poor climbing ability and very small cones in some years. It was, however, the only variety able to withstand the more virulent forms of wilt disease that appeared during the 1970s in the Weald of Kent, and in the West Midland hop growing areas in the 1980s, and it became the principal UK variety despite its poor agronomy. Successively stronger resistance has subsequently been incorporated into Yeoman (1983), Phoenix (1996), and Pilgrim (2001).

Black root rot caused by Phytophthora citricola is only a sporadic disease problem in the Northern Hemisphere, but was a major cause of crop loss in New Zealand. Until the 1950s, up to 25% of the New Zealand hop area had to be replanted annually due to the susceptibility of the main variety, Californian (Late Cluster). A hop breeding programme was set up in 1949 specifically to improve resistance to this disease. The English variety Fuggle was found to be a source of resistance and was crossed with local material to yield First Choice and Calicross in 1960, followed by Smooth Cone in 1961. Tetraploid forms of these in turn were used to produce the triploid varieties Sticklebract, Super-Alpha and Green Bullet in the early 1970s. Screening of seedlings for resistance to black root rot became a routine part of the New Zealand hop breeding programme and subsequent varieties such as Alpharoma and Pacific Gem have all incorporated resistance to the disease. ²⁷

Although resistance to fungal diseases is found in most varieties developed during the twentieth century, none of these cultivars have resistance to insect or spider mite pests. The damson-hop aphid affects over 90% of the world hop area, necessitating at least one pesticide application each season. In much of Western Europe, including Britain, this pest can be devastating, leading to complete crop failure. Attacks from this pest were the main cause for crop variability referred to previously as a basis for gambling. Today in these areas, extensive pesticide applications are needed throughout the growing season because even very low levels of infestation cause staining which markedly reduces cone quality and value.

Screening of wild hop plants in Germany and the USA revealed some resistance, but the level was insufficient to prevent severe pest damage in the absence of other forms of control. At Wye, however, an accession of Japanese wild material in the germplasm collection was found to have strong, heritable resistance [Fig. 6]. ²⁸ The resistance was sufficient to withstand infestation without the need for pesticides and has been the basis of a new breeding programme. Initial work developed parental material by incorporating the resistance into locally adapted germplasm and the first crosses to produce genotypes with commercial attributes were made 1992-1995. The world's first aphid-resistant variety, Boadicea, submitted for variety rights registration in 2005, has remained free of aphids in its cones at harvest without any aphicide treatment throughout the farm trials stages.

Figure 6. The segregation of resistance to damson-hop aphid in field progeny. Susceptible individuals have been defoliated whilst resistant individuals retain their leaves. (EMR)

Differences in the degree of infestation to the other main pest of hops, the two-spotted spider mite, Tetranychus urticae have been discovered between genotypes. ²⁹ However, this pest can reproduce sexually through many generations within a single growing season, and it is unlikely that resistance in a variety would remain effective for very long. A resistance breeding programme is not in prospect.

Locally important breeding objectives

Dwarf varieties - The hop market is now fully international and unrestricted. Marketing boards and controlled national economies, present for much of the twentieth century, have disappeared and growers compete on a global basis. In the UK, hop farm size, at an average of about 23 ha, is too large to manage with family labour alone, as in much of continental Europe, but without the economies of scale of the large farms in the USA. The need to reduce labour costs, and dependence on the availability of suitable labour, has been a driving force in Britain for experimentation with simplified growing systems, particularly low-trellis production, to reduce costs of production and increase competitiveness.

In a low-trellis system, hops are grown as a continuous hedge 2.3 - 3 m high. Such systems are easier and less costly to erect, are more suitable for mechanisation reducing labour inputs [Fig. 7], and can be sprayed more efficiently to control pests and diseases. However varieties developed on a high trellis system of at least 5 m are not suited to lower trellis systems and yields can be reduced considerably. A breeding programme to develop suitable varieties was initiated at Wye by Neve in 1977. ³⁰ The programme aimed to develop new dwarf varieties that are shorter and less vigorous, yet produce a yield similar to conventional varieties.

Figure 7. The harvesting of dwarf hops. (EMR)

Again, the origins of this programme can be traced back to Salmon. Notes he made in 1911 indicate that he had observed dwarf plants in his breeding garden. One such he described as having ‘laterals of medium length, very closely placed, very fruitful, but of no direct promise’. His underlining of ‘direct‘ suggests that, although a productive plant, it was not suitable for the husbandry methods of the time. The dwarf characteristics arose from the wild US accessions in his programme and the crosses he made to investigate the ‘blister’ powdery mildew resistance. This combination of objectives was not present in any other programme and may explain why dwarf plants were found so readily at Wye when they have not been observed until recently in other programmes throughout the world.

Neve collected open-pollinated seed from dwarf plants in 1977 which were, in turn, used as parents in 1982. Analysis of the growth habits of segregating progeny from these crosses provided sufficient information that a programme to develop commercial dwarf varieties for the UK was started in 1985 by Darby.^{31, >32 & 33} The world's first dwarf varieties, First Gold, Herald and Pioneer were registered in 1996, followed by Pilot (2001) and Boadicea (2005).

Breeding programmes in Germany, Slovenia, Poland and the USA have now also reported starting to develop dwarf varieties as farm size increases ³⁴ or as labour becomes more costly in these countries. Daylength adaptability - The hop is a short-day plant and, when physiologically receptive, the initiation of flowering [Fig. 8] is in response to daylength of less than a critical length of about 15.5 hrs. Thus, almost all hop production is between the latitudes of 35° and 55°. In most countries outside this range, hops cannot be grown economically and breweries need to import their raw material. Breeding programmes in South Africa, Mexico and Columbia, and variety trialling programmes in India, Argentina and Kenya, have sought to select varieties adapted to much shorter daylengths.

Figure 8. Hop flowers initiated by shortening daylength. (EMR)

In South Africa, increasing political and commercial isolation of the country followed the victory of the National Party in South Africa's 1948 general election and the introduction of new legislation in the early 1950s. In response, South African Breweries Hop Farms Ltd. (then Union Hop Growers Ltd.) commenced a breeding programme in 1956 using seed from Fuggle and Hallertau breeding lines imported from Wye. The aim was to develop a more productive and locally adapted variety than the Golden Cluster being grown at that time. In the early 1960s, Golden Cluster succumbed to black root rot and production only continued through the introduction in 1968 of Pride of Ringwood from Australia. However, in 1972, the first variety emerged from the breeding programme. It was a Fuggle seedling named Southern Brewer and it quickly expanded in area to become the mainstay of the South African industry. Although better adapted, it still required additional daylength to achieve economic production, a problem which was overcome by artificially lighting the developing crop, and breeding continued to develop varieties able to tolerate shorter daylengths. The return of South Africa into the international community in the early 1990s dramatically accelerated the need for such varieties. ³⁵ Fortunately, several selections were at the advanced assessment stages and Outeniqua and Southern Promise were released in 1992 as the first varieties able to be grown without additional lighting. Southern Promise was also well adapted to the lack of winter chill experienced in the hop growing area of South Africa. An active breeding programme continues and Southern Star, a high-alpha seedling of Outeniqua, was released in 2001.

Future objectives of hop breeding programmes

The pioneering hop breeders in the early 1900s could not have predicted the advent of Verticillium wilt or downy mildew, nor could they have predicted the changes in hop husbandry or brewing technologies that have occurred during the twentieth century. Similarly, the future challenges facing hop breeders are unknown and, probably, unpredictable.

As discussed, many of the developments in the history of hop breeding have arisen through scientific progress, notably in genetics and chemical analysis. Much of current scientific progress is in the field of molecular biology. Many of these techniques and concepts will greatly increase understanding of the biology, chemistry and genetics of hops, from which many new developments in hop breeding are likely to flow. To date, however, no new varieties have been developed through such techniques but, when sufficiently developed, breeders may be able to respond more rapidly to changes in the requirements of growers and brewers so that the lead time in developing a variety is reduced. Similarly, an enhanced characteristic, such as pest or disease resistance to reduce pesticide treatments, could be incorporated more readily, or specific changes in existing varieties may be possible.

It is also probable that new characteristics will be sought and developed. Already in current breeding programmes, other constituents of the hop are being explored for potential for enhancement including the antioxidant xanthohumol, ³⁶ the phytoestrogenic properties of hops, ³⁷ or the antimicrobial beta-acid content. ³⁸ Furthermore, molecular techniques are being applied to examine the diversity found in wild hop germplasm pools. ³⁹ Thus, the innovative founding work of Salmon in using wild germplasm in a hop breeding programme is still finding relevance 100 years later in the age of molecular genetics.