* Maroochy Research Station, Queensland Horticulture Institute, Department of Primary Industries, PO Box 5083, SCMC, Nambour 4560, Queensland, Australia.1. INTRODUCTION
Avocados are recorded growing in Australia as early as the mid-eighteenth century. However, the modern-day industry dates from 1928 with the first importation of named varieties from California. Small quantities of fruit were first offered for sale on major metropolitan markets during the mid-1930s where they were accepted for their novelty value.
The industry initially developed on the subtropical eastern coast of Australia, in southern Queensland and northern New South Wales (latitude 25°S - 30°S). Production has since spread to all other mainland states where the crop is grown from latitudes 17°S to 34°S. During the early years of development, the avocado industry could only be considered as a 'backyard' enterprise. Individual growers had small numbers of trees and inevitably were engaged in some other form of horticultural pursuit for their major income source.
During the late 1960s and early 1970s, the fruit became popular in up-market restaurants and hotels where it was seen as a luxury item. In 1974, abnormally high summer rainfall affected the major production areas of Queensland and New South Wales, decimating the industry. It was estimated that 50 per cent of all bearing trees in these two states died by drowning or Phytophthora root rot, while most of the survivors went into severe root rot decline. Subsequently high market prices resulted in large numbers of trees being planted during the mid to late 1970s. These continued until 1987 when prices fell in response to a fully supplied market, and the previous optimism was replaced by a concern of over-production. Growth in the industry has been more moderate during the 1990s.
2. PRESENT SITUATION OF AVOCADO CULTIVATION
2.1 Production areas
Avocado production in Australia enjoys a wide geographic distribution growing between latitudes 17°S (500 m asl) and 34°S. This environmental diversity, combined with selected varieties, gives fruit supply to markets on a year-round basis. Production peaks from June to November with lighter supplies during the summer months. Queensland and New South Wales are the largest producing states with an estimated 60 and 30 per cent, respectively. The balance of the crop is grown in Victoria, South Australia and Western Australia where plantings are more recent but tree numbers are rapidly increasing, buoyed by higher prices for summer production.
2.2 Production statistics
The total Australian avocado production of 1996/97 was estimated at 18,000 tonnes with the fruit mainly sold in domestic markets. There is no developed export of avocados from Australia at present, though a programme to send fruit into New Zealand to complement their out-of-season time of the year is underway. Farm size is variable and ranges up to 100 hectares with most growers controlling between 5 and 15 hectares of trees. The better growers in the more favourable environments average about 20 tonnes/hectare while the highest recorded commercial yield for one season is 50 tonnes/hectare. An industry average for bearing trees is estimated at 7 tonnes/hectare.
The industry is based on Mexican/Guatemalan race cultivars thereby differing from the tropics where avocado production is based on West Indian race cultivars. The main cultivars grown in Australia in order of maturity are:
'Shepard': a cultivar developed in California. It is a precocious, heavy-cropping, semi-dwarf tree with green pear-shaped fruit. The flesh is pale yellow when ripe with a smooth, buttery texture. It has a B-type flower pattern and is sensitive to the environment during flowering with a requirement that night temperatures do not fall below 12°C. The fruit has moderate disease and pest resistance and it is the earliest maturing cultivar. Its major disadvantage is the large seed size.
'Fuerte': the main cultivar in Australia from the 1960s through to the late 1980s, but today of lesser importance and gradually being replaced by other cultivars. Developed in California it is a green-skinned, pear-shaped fruit maturing shortly after 'Shepard'. It has a B-type flower and its cropping is more reliable in warmer climates.
'Sharwil': selected as a new cultivar in Queensland and thought to be predominantly Guatemalan with some Mexican race genes. The tree is vigorous with the potential to set heavy crops of smooth, green, pear-shaped fruit. Flesh quality is excellent and seed size typically 13 - 15 per cent of the fruit. The cultivar has a B-type flower pattern with a sensitive reaction to environment (temperature x flowering). 'Sharwil' is a mid season-maturing variety coming in after 'Fuerte. It has become the most important cultivar of Hawaii, and is more disease and pest resistant than Fuerte.
'Pinkerton': a Mexican/Guatemalan hybrid. It was bred in California and crops heavily and reliably in most districts in Australia. It has an A-type flower and the tree is semi-dwarf in habit. Fruits are green-skinned with a slight pebbly texture and a long neck.
'Hass': currently the most important cultivar and grown in all districts in Australia. 'Hass' was selected in California. The tree is upright and vigorous with an A-flower type. It produces ovate fruit with a pebbly skin that changes from green to black when ripening. 'Hass' is a late-maturing variety with some tolerance to fruit diseases and pests. Crops are heavy and reliable as long as fruits are not hung too late. Fruit size tends to be small in the warmer areas of production.
'Reed' and 'Wurtz': two late maturing cultivars of minor importance. 'Reed' is a prolific-cropping cultivar with large, round, green-skinned fruit of excellent quality. It is a cultivar of the Guatemalan race and trees are upright and semi-vigorous. 'Wurtz' has a pear-shaped, green-skinned fruit which it bears on a densely foliaged, semi-dwarf tree with a weeping habit. Cropping can be strongly biennial.
The Australian avocado industry has been developed on seedling Guatemalan rootstocks. This was by way of convenience rather than choice, as a significant population of seedling trees existed as a relic from the days of unregulated importation. Nevertheless, for the most part they have served the industry well. However, with the increased sophistication in orchard management, there is an interest to narrow the genetic diversity to a few elite lines. Mexican race rootstocks were introduced into the industry during the early 1980s and these are serving as a comparative basis for evaluation and selection.
3. PRODUCTION OF PLANTING MATERIAL
3.1 Accreditation scheme
To reduce the spread of Phytophthora cinnamomi, the root rot fungus, and avocado sunblotch viroid, the Australian avocado industry developed a disease-free nursery scheme for the production of trees. The scheme is known as the Avocado Nursery Voluntary Accreditation Scheme (ANVAS) which is operated under strict hygiene. It is a voluntary scheme but carries the prestigious endorsement of the Australian Avocado Growers Federation. Strict guidelines are adhered to which include pasteurisation of potting media, all plants grown on raised benches, dust suppression in the nursery and a disease-free water supply. Plants are inspected and tested twice each year by government inspectors and nurseries not reaching the required standard are expelled from accreditation until they correct their problems. Nurseries participating in the scheme offer trees for sale that are certified free of Phytophthora root rot and avocado sunblotch viroid and that are true-to-type with respect to the variety offered for sale.
Avocado production in Australia is based on named varieties so that all trees sold are grafted. Rootstocks used are either seedlings of the Guatemalan (Velvick) or Mexican (Duke 7) races; however, there is increasing interest in using vegetatively cloned rootstocks of elite selections. Cloning is based on the mother-seed principle where a vigorous seedling is grown and the rootstock scion grafted as soon as practical. Once growth begins on the rootstock scion the plant is moved to a dark room and the shoot etiolated in darkness. A metal band is loosely fitted around the etiolated shoot that is then covered with soil, allowing the new growth to develop in full sunlight. Once the new growth is large enough the desired variety is grafted. As the plant continues to grow, the metal band cuts into the etiolated rootstock material and roots begin developing above the band. Eventually the mother seed is starved and cut off from the plant.
4. ESTABLISHMENT OF ORCHARDS
4.1 Land preparation
Avocados are grown on a wide range of soil types from free-draining, red basaltic loams, previously supporting rain forest, to eroded coastal sand dunes. Where soil depth is marginal (less than one metre), mounding along the row is employed to increase the effective root zone and improve drainage. Prior to planting a new orchard it is recommended that any tree stumps or large roots be removed to reduce the risk of Armillaria root rot developing and spreading to young trees. The pH of the soil should be checked and adjusted to about 5.5 with lime or dolomite if necessary. Liming materials should be applied before the final cultivation to ensure that they are well incorporated into the soil. Deep ripping is recommended to improve sub-surface drainage. Zinc is the most common trace element deficient in Australian soils and if required is also incorporated during final soil preparation.
Individual tree sites are prepared by hand about 3 months prior to planting. At each planting site 200 to 300 grams of superphosphate along with 10 to 15 litres of poultry manure are dug in to a depth of 300 millimeters over a one square metre area. The Pegg philosophy - cover-cropping and mulching as an integral part of root rot control - is practised by a significant proportion of growers in subtropical areas during the establishment phases of the orchard. Invariably these properties have higher productivity.
4.2 Planting time
Planting time varies depending on the district as it is tied in to climatic conditions. In subtropical Australia the most-favoured planting time is the autumn when temperatures are warm enough for establishment but not so hot that trees stress and sunburn. Root growth continues through the winter and trees grow away strongly in spring. However, at more southern latitudes the risk of frost damage to young trees through the winter is high and planting is favoured during spring.
Depending on the value of land, either intensive or extensive planting densities are used by Australian growers. Intensive planting densities rely on the principle that close-planted trees will be thinned as they begin to crowd so the mature orchard will have a reduced number of trees compared with the initial number. Initial tree numbers planted are about 300 per ha but once thinned are reduced to about 170 per ha. Various spacing configurations are used including 6 x 6 metres and 7 x 5 metres. The advantage of intensive-planted orchards is that greater cash flow is generated in the early years of the orchard that more than compensates for the extra costs of establishment. In extensive-planted orchards, trees are planted in the position they will occupy for the life of the orchard. One of the most popular spacings in extensive-planted orchards is 9 x 7 metres which gives a tree population of around 150 trees/hectare.
It is normal prior to planting to install irrigation, allowing one sprinkler for each tree position. The site (approximately one square metre) is dug over, incorporating 100 grams of a complete analysis fertilizer and a hole dug to accommodate the root ball. After planting it is good practice to cover the one square metre planting site with an open mulch, which reduces water loss and suppresses weed growth.
5. CARE AND MANAGEMENT OF ORCHARDS
5.1 Training and pruning
There is little tree training practised on avocados during their establishment years apart from tipping the central shoot to encourage branching when trees are received from the nursery. Tipping growing points of trees in their second year to increase tree complexity is carried out by some farmers but there is little evidence of benefits.
Controlling tree growth in mature orchards remains the biggest challenge of orchard management. Being terminal flowering trees, avocados need to grow each year in order to remain productive, hence trees become larger. To maintain orchard access between rows, major limbs are periodically removed, cutting back to the main trunk. Similarly, to contain tree height major limbs are lopped from the top of the tree when necessary. In some orchards, trees are not pruned at all until their size is so great that they are uneconomic to manage. They are then stumped to approximately one metre high and allowed to regrow. After this treatment they are back in full production within two years.
Current research is investigating the use of mechanical pruning equipment to hedgerow trees at strategic times during the summer. Regrowth is then sprayed with a growth inhibitor (uniconazole) to suppress shoots and encourage floral initiation. Conclusive results from this research are not yet available.
5.2 Crop nutrition
Crop nutrition is based on critical leaf levels to predict requirements for the following season. Critical leaf levels established for avocado are presented in Table 1.
If required, most nutrients are applied in the spring or summer, usually by fertigation. Generally, however, nitrogen is applied after the spring fruit drop is completed. This is to reduce the risk of promoting too strongly spring shoot growth at the expense of the developing fruit. AVOMAN, a computerised orchard management programme, provides a customized nutrition programme for each block of trees in an orchard integrating the past cropping history of the trees, current crop load, rainfall and nutrient status.
Table 1. Critical leaf levels of nutrients for avocado production. Levels are determined from sampling fully matured leaves from non-fruiting shoots at the end of summer when trees are in a quiescent phase.
Most orchards establish a grass sward between rows that are kept mown. However, to reduce competition, grasses and weeds around trees are sprayed with Round Up. Mulches of straw, cane tops or sorghum hay are placed around trees to suppress weed growth and to improve root health. Research has shown that fruit size can be increased by as much as 15% by using a mulching programme around trees.
The avocado is a tree of rain forest origin and requires regular watering to maintain high yields. Most orchards in Australia have undertree mini-sprinklers to supplement rainfall and to apply nutrients to the trees (fertigation). Water monitoring is by either tensiometers or the more sophisticated electronic monitoring devices that provide a continuous picture of the water status of the root zone. When using tensiometers it is usual to maintain soil metric potential between -40 and -10 kPa at 300 mm depth.
Water use during the winter months falls away but there is a rapid increase in requirement as the trees come into flower during early spring. Water stress during the first eight weeks of fruit growth can lead to reduced fruit size as cell division is retarded. Also, water stress during the natural period of fruit drop at the end of spring, can increase fruit loss, thereby reducing final yield. Ring-neck, a natural cincturing of the pedicle, has been related to water stress during the later stages of fruit maturity, and can reduce fruit size and final yield.
Fruit-spotting bug causes fruit blemishes and is active in subtropical areas of Australia. As it migrates into orchards from surrounding cover it is difficult to control and requires a protective insecticidal programme for the duration of the fruiting season. The red-shoulder or monolepta beetle can cause substantial damage to leaves and fruit if it swarms into orchards. Also migratory, it is difficult to control and daily inspection of trees with spot spraying of swarms is the most effective method to contain this pest. Queensland fruit fly in eastern Australia and Mediterranean fruit fly in west Australia can damage fruit, but are of minor importance. Several species of leaf-webbing caterpillar can cause damage to fruit in the autumn months and can be controlled by strategic applications of targeted insecticides. Integrated Pest Management systems developed for citrus orchards are gradually being introduced into avocados.
Phytophthora root rot caused by Phytophthora cinnamomi is a major international disease of avocado and is present in all countries producing this crop. All areas in Australia producing avocados are affected by this disease. Phytophthora root rot is successfully controlled by trunk injection with potassium orthophosphonate using a 20% a.i. solution which is injected at the rate of 15 ml per metre of tree diameter. Despite the spectacular success of phosphonate treatments, an integrated approach of biological and chemical control is still recommended in the high-risk subtropical areas of northern New South Wales and Queensland. Anthracnose, a serious fruit problem in most environments, is controlled by regular spraying with copper-oxychloride and a post-harvest treatment of prochloraz (Sportak). Verticillium wilt is a disease of lesser significance. It is a soil-borne disease which attacks the vascular system but seldom kills plants, and there is no effective fungicidal treatment.
6. HARVESTING OF FRUIT AND ORCHARD YIELDS
6.1 Maturity standards
Fruit of all cultivars must reach a minimum of 21% dry matter and ripen naturally without shrivelling before it can be harvested legally and sold.
Fruit is hand-harvested from trees when mature using an array of picking aids. On the larger orchards, trees are harvested with the aid of hydraulic ladders (cherry pickers) while on the smaller properties picking poles are used to reach fruit. For most cultivars, fruit needs to be clipped with a button retained on the pedicle end. This reduces the risk of stem-end rot invading the fruit as it ripens. Fruit is picked into large bins usually mounted on trailers to facilitate their movement to the packing shed. It is important to keep the fruit out of direct sunlight after picking to prevent it from heating.
For early-maturing cultivars and those with a fruit size problem, fruit is normally picked over two harvests removing the larger fruit first hence allowing the smaller fruit to grow. Generally, late hanging of fruit, which is often practised to market when prices are higher, drives the trees into biennial bearing. Due to the number of cultivars and the wide range of latitudes at which the crop is grown, avocados can be harvested year-round in Australia although the peak of production is during the winter and early spring months.
Yields are variable depending on the district and cultivar. Growers following 'best commercial practice' can sustain yield in mature orchards at 18-23 t/ha. However, the Australian industry as a whole has an average yield of only 7 t/ha though this figure incorporates production from young trees as well as mature orchards.
On reaching the packing shed, fruit is passed through a spray treatment of Sportak (prochloraz) to control anthracnose. Fruit is then size and quality graded, packed into 6 kg trays and pre-cooled prior to transportation. Fruit quality is determined by its freedom from insect, wind or hail marks. Most fruit is shipped in refrigerated trucks by road to markets in major cities.
Traditionally, growers have marketed their fruit through agents/merchants. In major cities, however, over the last 10 years there has been an increase in direct marketing to the big chain stores which now sell about 80% of Australias fresh fruit and vegetables. This style of marketing is best suited to big growers who can deliver a consistent long line of fruit. To gain market share, smaller growers are now coming together in cooperative packhouses where they can offer the same supply as the larger producers.
Market research has shown that more avocados are sold if they are offered to the consumer in a ready-to-eat condition. This has led to a system of pre-ripening fruit prior to stocking retailer shelves. Fruit is gassed with ethylene at a central location and held at 21°C until it is sprung, then taken to the retail point of sale.
7.3 Quality assurance
Food safety has become an important issue in Australia with strict guidelines now being set by the food industry. The major chains invest heavily into promoting brand names and cannot afford adverse publicity that might arise from product exceeding the legal limits set for pesticide residues. Hence, growers with direct marketing to the major chains are required to maintain a production schedule documenting all procedures carried out, from field operations to delivery to the point of wholesale. This level of documentation is also useful to the grower, who is able to check the success of the production and postharvest handling strategies implemented, and make changes where necessary.
Along with a compulsory R, D & E levy, the Australian avocado growers pay a compulsory promotional levy of 3 cents/kg. The levy is collected by the Australian Horticultural Council and their marketing experts put together a promotional budget each year that is agreed by industry. Promotion is mostly limited to the printed media, leaflets and point-of-sale demonstrations on ways to use the product.
8. POTENTIAL FOR AVOCADO PRODUCTION
The greatest opportunity to increase Australian avocado production is to improve fruit quality on the domestic market. Currently, a significant quantity of fruit is thrown out at the consumer level due to an unacceptably high incidence of fruit rots that develop during ripening. Research has identified fruit calcium levels as a significant factor affecting fruit quality and reducing post-harvest rots, and methods are being researched to increase the level of this nutrient in fruit.
9. CONSTRAINTS IN AVOCADO PRODUCTION
Major constraints faced by the Australian avocado industry are poor fruit quality, the relative isolation from large export markets and the availability of suitable soil and water resources. The lack of confidence in the product by consumers is limiting opportunities to develop a larger domestic market, hence industry expansion will lead to an oversupply with a reduced return to growers. Avocado is currently not a favoured fruit in most Asian countries to which Australia could export fruit without major quarantine issues restricting entry. Fruit-fly disinfestation research is almost complete and if commercially successful will open some markets that can be exploited economically, e.g. New Zealand and Japan. Most of the suitable avocado soils along the subtropical coast of New South Wales and Queensland are currently growing sugarcane where farming traditions are entrenched. This limits the opportunities for expansion of the crop.
Human nutrition: the avocado is rich in vitamins C, E and beta-carotene. It is also a rich source of potassium and dietary fibre. The Australian avocado industry has been very aware of the value of avocado for human nutrition and has promoted this aspect of the fruit to consumers. Also the industry commissioned research by a leading cardiologist who found that the monounsaturated fats occurring in avocados reduce blood cholesterol while preserving the level of high-density lipoproteins which protect the blood vessel walls from atherosclerosis. This research has led to the Australian Heart Foundation recommending avocado as a dietary source of fat.
10. RESEARCH, DEVELOPMENT, EXTENSION AND GROWERS ORGANIZATION
The Australian avocado industry has a strong research and development programme funded by a national levy of 1.3 cents/kg collected at the wholesale point. Levies are administered by the Horticulture Research and Development Corporation (HRDC), a federal government agency established to support horticultural R, D & Extension programmes. For every dollar raised by the industry levy, HRDC contributes another dollar. Industry priorities are set at workshops held every five years where growers and government research and extension officers develop a forward plan. Generally, research projects are funded for a three-year term and may be renewed if required. The industry takes a proactive approach and identifies priority research areas when project applications are called each year. There are two major projects being funded currently by the avocado industry. These are 'Canopy health and management' and 'Field control of post-harvest diseases'.
The avocado industry has just finished funding a six-year programme on technology transfer at the cost of AUS$1.2 million. From this project two software packages have been produced: AVOMAN which is a highly customized orchard management programme and AVOINFO which is a bibliographic database. There has been a high uptake of these software programmes by the industry.
The Australian avocado growers are served by a national body, the Australian Avocado Growers' Federation (AAGF). This organization has delegate representatives from each state producing avocados. Representation is weighted in favour of Queensland and New South Wales, the two states which dominate production.
Within its limitations the AAGF coordinates promotional effort for avocados in the major Australian markets and acts as a national voice on factors important to the industry. It also assumes the responsibility to hold the biennial seminar and field days where the latest production trends, technology and marketing prospects are discussed. Organization structures differ between states, but in areas of significant grower numbers district associations are formed which serve to keep their members informed on new technology and marketing developments.
The Australian avocado industry has a strong political structure, with representatives from each of the producing states. It is member industry of the Australian Horticultural Council and the Horticultural Research and Development Corporation with mandatory levies collected for promotion and R, D & E by each of these bodies. Compared with other subtropical and tropical fruit crops in Australia, it is a technologically advanced industry funding innovative R & D programmes such as AVOMAN and AVOINFO. The industry has a strategic plan for the future and a R, D & E programme to assist it in reaching its goals.
Production is mostly targeted at the domestic market though research into disinfestation is assisting in overcoming quarantine barriers in the potentially lucrative markets of Japan and New Zealand. Like most primary producing countries of the western world, production costs are escalating faster than gross returns and to remain competitive growers have the need to increase production efficiency. To achieve this, investment into the development of new technology must continue, particularly plant breeding, rootstock selection, orchard management, disease control, IPM and post-harvest handling.