ABSTRACT

The breeding and development of new hop cultivars for New Zealand growing conditions is described. Since the programme was initiated in the early 1950s, ten cultivars have been released with a further three advanced selections currently undergoing extensive evaluation.

Historically both North American and European germplasm has been used in the hybridisation programme. There have been three distinct phases in the breeding programme. Initially the main aim was to breed for resistance to Black Root Rot caused by Phytophthora citricola. Three resistant cultivars were released in 1961-62. The aim of the second phase of the programme beginning in 1962 was to produce high alpha-acid types with low seed content by developing triploids cultivars. Two triploid cultivars were released in 1972. In total five high alpha-acid triploids and one medium alpha triploid with a low cohumulone content have now been released. The third phase was initiated in 1975 with the objective of developing low seeded triploid aroma cultivars. Essential oil profiles similar to European "Noble Aroma" cultivars coupled with improved agronomic performance under New Zealand growing conditions were aimed for. One aroma cultivar has been released to date.

Characters of currently grown cultivars and advanced selections are described. Breeding procedures currently being employed to develop high alpha and aroma types are presented. Possible future directions of the programme are discussed.

INTRODUCTION

Hops are grown in the Nelson province, at the top of the South Island of New Zealand. The climate can be described as temperate oceanic with the Nelson region being well protected from the prevailing weather patterns by a chain of mountains surrounding the alluvial flat land adjacent to the sea where hops are grown. Latitude of the region is 41-42°S. Mean temperatures range from 7°C in winter up to 17.5°C in mid summer. Rainfall is fairly evenly spread throughout the year, totalling 1400mm.

Hops were introduced to New Zealand in the mid 1800’s by early European settlers. Before 1900 the most common hops grown were Fuggle and English Golding². However, these cultivars were not well adapted to New Zealand growing conditions and much of the hop land was converted over to the American cultivar, Late Cluster. Records show that this cultivar was imported from the Russian River Valley of Northern California in 1897. It was widely known in new Zealand as "Cali".

Cali was the predominant cultivar grown in New Zealand then from early 1900s to the 1950s. Cali was a good yielding cultivar. During that time it became apparent that Cali suffered from a root rot disease caused by Phytophthora citricola, commonly known as black root rot. This disease was threatening the existence of the New Zealand industry and in some areas up to one quarter of the plants had to be replanted annually.

In the late 1940s the industry approached the New Zealand Government, more specifically the Department of Scientific and Industrial Research (DSIR), to help with this disease. In 1949 a hop research programme was initiated with the main thrust being plant breeding.

BREEDING PROGRAMME

Since 1949, there have been 3 distinct phases of the breeding programme.

(1) Breeding for disease resistance

The disease resistance breeding programme began in earnest in 1952/53 season when the root rot susceptible Late Cluster cultivar was crossed to resistant males that were derived from the root rot tolerant cultivar Fuggle.

This cross proved immediately successful and in 1960, DSIR bred cultivars First Choice and Calicross were released, followed in 1961 by Smoothcone³.

These cultivars quickly became predominant having good yields, alpha acids of 8-10%, and good tolerance to Phytophthora root rot.

(2) Triploid breeding

The second phase of the breeding programme began in 1960. Characters considered for selection were high alpha acid content, seedlessness, a range of harvest dates and maintaining good yields⁵.

To achieve these goals the work concentrated on the development of triploid cultivars, which are naturally seedless due to their altered chromosome number.

in the first stage of the triploid breeding process, colchicine was used to obtain tetraploid females. The first tetraploids were produced in 1961/62. They were First Choice and Smoothcone.

In the New Zealand programme, the above mentioned tetraploid females were crossed with diploid males and the first resulting triploid seedlings from these crosses were grown in 1965/66.

Four high alpha acid and naturally seedless cultivars Green Bullet, Harley’s Fulbright, Sticklebract and Super Alpha came out of these initial crosses and were released in the early 1970s. The programme has continued since that time with a further 2 releases in the 1980s; Alpharoma and Pacific Gem.

A further 2 high alpha selections are currently in advanced stages of testing. They are 77-60 and 67-11-58.

(3) Aroma breeding

Phase 3 of the programme began in the mid 1970s and involved the breeding of aroma cultivars adapted to New Zealand growing conditions.

The starting point for this programme has been the use of European cultivars Hallertauer, Tettnanger and Saazer.

Aroma breeding selection criteria has largely centred on obtaining good yielding seedless selections with chemistry profiles similar to their European parents. In general triploids have been selected with alpha acids of less than 6%, alpha/beta ratios of approximately 1 and a cohumulone content of <30%.

In terms of essential oils, seedlings with high humulene and low myrcene contents in particular have been selected.

Other characters selected for have been good yield coupled with a seed content of 2% or less.

There has been one cultivar released; called NZ Hallertauer. This cultivar has alpha acids of 8-10% and is currently being used commercially both in New Zealand and overseas for its aromatic and bittering properties.

A new selection, 77-01, has an essential oil fingerprint somewhat similar to that of Hallertauer Mittlefreüer. 77-01 is currently being evaluated in brewing trials.

CHARACTERISTICS OF NEW ZEALAND HOP LINES

Table 1 summarises cultivars currently grown commercially in New Zealand along with their main chemical attributes. There are 5 grown commercially; Super Alpha represents 35% of the planted area, Green Bullet and Sticklebract together make up 20% with the newer cultivars released from the programme, Pacific Gem, grown on 25% of the area and NZ Hallertauer making up 20%.

Table 1:
Characteristics of New Zealand bred cultivars and selections : 6 year means

Cultivar	Yield	Alpha	Beta	Cohumulone	Oil
	(kg/ha)	acids (%)	acids (%)	content (%)`	content (%)
Super Alpha	3060	13.7	8.5	38	1.47
Green Bullet	3090	13.6	7.2	42	1.18
Sticklebract	3210	14.1	7.6	45	1.26
Pacific Gem	3220	14.9	8.3	42	1.36
NZ Hallertauer	2590	9.5	7.7	37	0.92
Southern Cross	2730	14.1	7.0	28	1.24
Pacific Hallertauer	1950	6.0	7.2	27	1.32

Cultivar	Humulene	Caryophyllene	Farnesene	Myrcene	Hydro carbons	Oxidation products	Floral estery	Citrus-piney
Super Alpha	20.9	6.1	0.1	50.6	78.3	1.18	1.65	2.59
Green Bullet	18.0	5.8	0.1	56.2	80.8	1.02	1.74	1.88
Sticklebract	10.3	5.3	4.9	52.6	73.7	1.03	1.95	7.18
Pacific Gem	16.5	5.7	0.1	56.2	78.8	0.94	1.51	1.97
NZ Hallertauer	12.4	6.2	4.6	46.5	70.3	1.15	2.11	4.88
Southern Cross	13.1	4.0	4.4	58.0	80.3	0.74	1.73	1.94
Pacific Hallertauer	30.6	10.9	0.0	36.9	79.0	1.62	1.38	3.95

(a)   Pacific Hallertauer figures are for one season only.

(b)   Oxidation products of humulene and caryophyllene; caryophyllene oxide,
       humulene monoepoxides I,II,III humulol, humulenol II, humulene diepoxides

(c)   floral-estery components; linalool, gernayl acetate. geranyl isobutyrate and geraniol

(d)   Citrus-piney component; limonene, -cadenine. y-cadenine, muurolene, selinene

(e)   Subsequent to the publication of this paper selection 77-60 was released 1994 as "Southern Cross"
       and selection 77-01 was released 1994 as "Pacific Hallertauer"

Yield-wise, the seedless triploids perform very well. The figures presented here are not commercial yields but 6 years averages from 200 hill plots at our Research Centre. In general commercial yields are less than those presented in Table 1. NZ Hallertauer has a yield of approximately 80% of the other commercial cultivars. The alpha acid figures are also 6 year averages presented on a dry weight basis. The 4 high alpha acid cultivars generally have values of 13-14% while NZ Hallertauer is 9%. Cohumulone contents of the 5 cultivars range from high 30’s to mid 40’s.

The main feature of the essential oil profiles is the variation in humulene and farnescene levels between the cultivars while caryophyllene and myrcene levels are much the same.

The performances of the 2 advanced selections still undergoing evaluation are also presented in Table 1.

The main feature of 77-60 is that it is a high alpha acid type with a cohumulone content of 28%.

77-01 is bred from Hallertauer MF, and has a yield of approximately two-thirds that of the high alpha types. It can be considered an aroma type with reasonably low alpha acids, a cohumulone content of 27%, a high humulene content, no farnesene and comparatively low myrcene levels.

CURRENT BREEDING OBJECTIVES

Breeding seedless triploid types both for high alpha acids and also for aroma constituents remain the current breeding objectives.

Under the favourable New Zealand climate triploid seedlings are able to be evaluated successfully in their year of field establishment for yield, alpha and beta acids, seed content and cohumulone content.

Once selections have been made on a single plant basis they are then evaluated in more detail in small plot replicated trials at one site usually over 2 seasons.

Any successful candidates from the small plot trials are then forwarded to large scale trials of 200 hills.

At both small plot and large scale trial stages, characters measured are yield, chemistry, machine harvesting properties and essential oil profiles. At the large scale trial stage the selections are also submitted for brewing trials conducted by New Zealand domestic breweries.

While most breeding efforts have naturally focused on females, triploid males for use as commercial pollenizors have also been selected, to ensure as low as possible a seed content in our commercial crops. Several males have been released to industry in the past 5 seasons.

Also, males are selected at the diploid level for use in our hybridization programme. Here selection has been for alpha acids, cohumulone content and humulene/caryophyllene ratio.

FUTURE DIRECTIONS

(1) Biotechnology

Areas of investigation that are being considered include (i) the fingerprinting of cultivars, (ii) the use of sex probes to eliminate male plants before crosses are planted in the field, and (iii) mapping to aid in the selection of quantitatively inherited characters.

(2) Population improvement

Recurrent selection (population improvement) procedures have not been widely utilised in hops. Studies conducted in New Zealand shown that, for a character such as alpha acids which has a heritability estimate of 0.5 on a single plant basis, good genetic gains can be expected by using phenotypic recurrent selection. Also recurrent selection in hops offers the possibility of selecting promising individuals at any point in the population improvement scheme.

To this end a broad based population has been set up and selection has been practised for the character alpha acids. In the cycle of phenotypic recurrent selection just completed there was a realised gain of 18% of the population mean for this character.

In future the broad based population will be expanded to include new germplasm acquisitions. Also, a selection index which will include yield as well as alpha acids is being considered.

(3) Triploidy alternatives

While the New Zealand programme has used colchicine as a means of doubling chromosomes over the last 30 years, other methods of obtaining tetraploids for use in our triploid breeding programme are now being investigated.

   (i) Endosperm culture

Because the endosperm tissue of seed is genetically triploid, the possibilities of using endosperm culture are being studied. Investigations are currently in their initial stages with techniques for successfully growing hop endosperm and subsequent plantlet formation being tested.

   (ii) Sexual polyploidisation

The use of non-reduced gametes, or sexual polyploidisation to obtain tetraploids is the second alternative currently being studied. This technique, successfully used in such crops as potatoes and lucerne (alfalfa), appeals as it does not have the same inbreeding problems associated with the asexual chromosome doubling methods. Basically the sexual polyploidisation relies on the identification of tetraploid females in seedling populations. It is hoped to use recurrent selection techniques at the tetraploid level to obtain commercially useful female parents.

(4) Gene pool expansion

In New Zealand there is an interest in expanding the germplasm base to meet the demands for new and improved cultivars. At present there are less than 70 H. lupulus cultivars in New Zealand and the acquisition of more genetic material from world collections for use in our breeding programme is being actively pursued. New Zealand is free of the major hop pests and diseases that afflict most other countries. Stringent quarantine regulations to prevent the accidental introduction of these diseases means that only a few importations each year are possible.

CONCLUSIONS

The New Zealand breeding programme has been very successful in meeting the needs of the New Zealand hop industry, which uses 100% New Zealand bred cultivars, and is now predominantly an export industry.

Ten cultivars have been released from the programme in the last 30 years. This has resulted in the introduction of disease resistance, increased yield performance, and some very high alpha acid types produced.

Currently new fields of research aimed at keeping pace with the demands of brewing science and technology are being embraced as the 21st Century approaches.

REFERENCES

(1) Beatson, R.A.; Brewer, V.R. 1990: Breeding and development of hop cultivars for New Zealand growing conditions. In: IOB/ANZ Section Proceedings of the 21st Convention. Pp. 109-111. Aust. Indust. Publ, Adelaide.

(2) Frost, A.A. 1983: Hops. In: Wratt, G.S. & H.C. Smith (eds). Plant Breeding in New Zealand. Pp. 145-154. Butterworths, Wellington.

(3) Roborgh. R.H.J. 1961: New varieties of hops tolerant to black root rot (Phytophthora spp). New Zealand Journal of Agriculture 103:169.

(4) Roborgh, R.H.J. 1964. Classification and botanical description of imported varieties of hop (Humulus lupulus) in Nelson, New Zealand. New Zealand Journal of Botany 2:10-18.

(5) Roborgh. R.H.J. 1969. The production of seedless varieties of hop (Humulus lupulus) with colchicine. New Zealand Journal of Agricultural Research 12:256