1532

2: Edible fruits and nuts

Persea americana Miller

Gard. Dict. ed. 8 (1768).

LAURACEAE

2n = 24

Persea gratissima Gaertn.f. (1807), Persea drymifolia Schlecht. & Cham. (1831), Persea nubigena L.O. Williams (1950).

Avocado (En). Avocatier (Fr). Indonesia: adpukat, avokad. Malaysia: avokado, apukado. Papua New Guinea: bata (Pidgin). Philippines: avocado. Cambodia: 'avôkaa. Thailand: awokado. Vietnam: bo', lê dâù.

The probable area of origin of avocado is a Chiapas-Guatemala-Honduras centre. The Spanish conquest found the avocado indigenous in Central America from Mexico to Peru and east to Venezuela. During the 17th Century the avocado became established on islands of the Caribbean but it was not until the 19th Century that it spread beyond Central America and reached South-East Asia (Indonesia, the Philippines). At present the avocado is grown in many tropical and subtropical countries.

The tree is grown for its nutritious fruit that has long been important in the diets of the people of Central America. Consumption is most often as an uncooked savoury dish mixed with herbs and/or spices, as an ingredient of vegetable salads, or as a sweetened dessert (Indonesia, the Philippines). However, its texture and colour can be used to enhance the presentation and consumption of many foods. The oil is used by the cosmetic industry in soaps and skin moisturizer products. The flesh is also used in traditional medicine.

Mexico has the largest production, most of the crop being consumed within the country. Other important producers are: Brazil, United States, Dominican Republic, Indonesia, Peru, Israel and Haiti. These countries account for about 80% of the world's avocado production, estimated to be 1.6 million t per year. Indonesia has the largest production in South-East Asia with 60 000 t from 15 000 ha, followed by the Philippines with 22 500 t per year from 5000 ha. Comparatively small quantities of avocados enter into international trade, these being produced predominantly in the subtropics of Israel, Spain, South Africa and the United States. The main importing countries are the United Kingdom and France, but new markets are opening in other EC countries and more recently in Japan.

The flesh represents 65—75% of the total fruit weight. The contents vary widely for different cultivars. The approximate contents per 100 g edible portion are: water 65—86 g, protein 1—4 g (unusually high for fruit), fat 5.8—23 g (largely mono-saturated and documented as an anti-cholesterol agent), carbohydrates 3.4—5.7 g (of which sugars only 1 g), iron 0.8—1.0 g, vitamin A 75—135 IU and vitamin B complex 1.5—3.2 mg. The energy value is 600—800 kJ/100 g. The high oil content of the mature fruit gives the flesh a buttery texture which is neither acid nor sweet. The flesh is rich in iron and vitamins A and B; it is easily digestible, providing a highly nutritious solid food for infants.

A dome-shaped, evergreen tree, conforming to Rauh's architectural model, up to 20 m tall. Primary anchorage roots penetrate to 3—4 m, but the tree is largely supported by a shallow (to 0.5 m) unsuberized secondary root system. Leaves spirally arranged, simple and entire, variable in shape and size; petioles 1.5—5 cm long; blade elliptic to lanceolate, ovate or obovate, 5—40 cm x 3—15 cm, reddish when young, turning dark green, waxy above, glaucous beneath, with prominent midrib and veins. Inflorescences panicles of cymes, produced at the end of twigs, mostly indeterminate, ending in a vegetative bud, many-flowered; flowers bisexual, 3-merous, fragrant, greenish; perianth of 2 whorls, 3 outer and 3 inner tepals, about 5 mm long, densely tomentose; stamens 9 in 3 whorls, plus 1 whorl of 3 staminodes; inner whorl stamens each bear 2 orange nectaries at the base; pistil with 1-celled ovary, slender style and simple papillate stigma. Fruit a large fleshy berry, single-seeded, pyriform or globose, 7—20 cm long, yellow-green to maroon and purple, weight ranging from 50 g to 1 kg; exocarp 1—3 mm thick, smooth to warty; mesocarp yellow-green and of butter-like consistency. Seed large, globose, with 2 seed coats and 2 large fleshy cotyledons enclosing a small embryo.

Fresh seed held at 25°C day/ 15°C night germinates within 3 weeks of sowing, the radicle emerging first. Seedlings grow quickly and continuously under warm, moist conditions. Later extension growth occurs in more or less synchronous flushes. Both temperature and crop load affect flushing, and bearing trees in the subtropics usually produce two major flushes, in spring and summer. More frequent flushes occur in moist tropical regions. In the subtropics floral induction occurs during a period of quiescence following summer growth and is enhanced by cool weather (20°C days/ 10°C nights). However, with cultivars adapted to the tropics (33°C days/ 25°C nights), induction and flowering may occur during any quiescent period, resulting in successive flowering and fruiting throughout the year. The time between floral induction and anthesis is about 2 months, less for cultivars adapted to high temperatures. Anthesis in the tropics lasts only 2—3 weeks, but can extend to 2—3 months in cool subtropical environments.
The avocado has a unique dichogamous flowering behaviour. The dichogamy is protogynous as each flower opens twice, the first day as functionally female (pistil receptive) and the next day as functionally male (pollen shedding). This behaviour is synchronous daily because all open flowers on a tree are either functionally female or functionally male. The flowering behaviour is complementary as cultivars (or seedlings) are categorized into type 'A' where flowers are female in the morning and male the afternoon of the following day, and type 'B' where flowers open as female in the afternoon and male the following morning. Hence self-pollination is only possible when male and female flowering within a tree overlap, which occasionally happens during rapid changes in weather.
Fruit development from setting to maturity follows a sigmoidal curve and takes 6—12 months. As in other fruits, the early period of growth is characterized by rapid cell division, but in avocado, cell division in the mesocarp continues; it can make a significant contribution to fruit growth almost up to maturity. Ripening is inhibited while the fruit is on the tree, but when picked, fruit softens to eating condition in 4—14 days.

Within Passiflora americana 3 ecological races are distinguished, sometimes considered as botanical varieties. In order of increasing tropical adaptation they are designated Mexican (Passiflora americana Miller var. drymifolia (Schlecht. & Cham.) Blake), Guatemalan (Passiflora nubigena L.O. Williams var. guatemalensis L.O. Williams) and West Indian (Passiflora americana Miller var. americana). The Mexican and Guatemalan race fruits contain more oil (10—30%) than the West Indian fruits (3—10%). Guatemalan fruits generally mature later than the other two races; Mexican race fruits have the thinnest skins. The West Indian race fruits are more tolerant to saline conditions. The races hybridize freely, giving a range of genotypes with adaptation from cool dry 'Mediterranean' to hot humid 'tropical lowland' climates. Types with predominantly Mexican x Guatemalan germplasm are more suited to the subtropical regions, whereas West Indian or West Indian x Guatemalan hybrids are successful in tropical climates. Many elite selections have been made and these are vegetatively propagated. In subtropical regions 'Hass' has become the dominant cultivar and the standard for world trade. Other important cultivars are 'Fuerte' and 'Pinkerton'. Cultivars of the tropics include 'Cardinal', 'De Leon No. 1', 'Calma' (all 3 grown in the Philippines), 'Booth 8', 'Lula', 'Peterson' and 'Waldin'. There are no reports of named cultivars being grown in other South-East Asian countries.

Evolution of the avocado has occurred in the rain forests of the humid subtropics and highland tropics of Central America, yet commercial production has extended to the lowland tropics and the cooler semi-arid regions of the world. The success in such diverse climates is undoubtedly due to the different tolerances of the three ecological races. The West Indian race is most sensitive to cold and may be damaged when temperatures fall below 1—2°C. However, trees of this race are adapted to high temperatures (28—36°C with a 4—5°C diurnal range). Mexican and Guatemalan race trees can withstand —4°C for short periods; optimum daytime temperatures for growth are 25—33°C with an 8—10°C diurnal range. Established trees will tolerate temperatures to 40°C, but prolonged exposure with low relative humidity results in severe stress and loss of productivity.
The leaves have a high stomatal density (40 000—73 000 cm^-2, only on the underside) and the limited vascular network typical of rain forest species. Photosynthetic rates are low (characteristically 5—9 µmol CO₂ m^-2 s^-1) and leaves reach light saturation at about 25% of full sunlight. Trees do not respond to variations in daylength. The trees and fruit are susceptible to wind damage and in exposed sites shelter should be provided.
Avocado requires a well-drained aerated soil because the roots are intolerant of anaerobic conditions; waterlogging for more than 24 h can kill trees. Root growth is shallow and a low frequency of root hairs limits the uptake of water and nutrients. Roots are most active at soil temperatures of 18—28°C; growth and water relations in the tree are disrupted when soil temperature falls to 13°C. Avocados are successfully grown where annual rainfall is as low as 300 mm (supplemented with irrigation) and in excess of 2500 mm. Year-round maintenance of soil moisture is necessary for high yield. Critical periods of water demand are during flowering and fruit set, 3 months later when early fruit drop occurs, and during the following period of fruit filling and maturation. Prolonged wet periods lead to fruit losses from anthracnose. Avocados are sensitive to salinity; electrical conductivity and chloride content of irrigation water should not exceed 550 µmhos and 80 mg/kg respectively. When roots have been damaged by root rot, chloride-based fertilizers should be avoided. A pH of 5.0—5.8 is optimum for growth and fruit yield.

In many tropical countries trees are seedlings giving rise to heterogeneous populations. Budding, and increasingly grafting, are the preferred methods of clonal propagation. Seed for rootstocks should be extracted from mature fruit from trees free of sun-blotch viroid which is seed-transmitted. Seed quickly loses viability and should be sown within 7 days. Viability may be prolonged for several months by dusting seed with a copper fungicide and storing in damp sawdust or peat in polythene bags at 4—6°C. Seed may be directly sown into planting positions, usually 2—3 seeds per site and the strongest seedling is selected for field grafting. They may also be pre-germinated in sand or sawdust and then transferred to containers where they are grafted and grown to a suitable size for field planting. With intensive care, seedlings grow big enough for grafting within 2—3 months from sowing.
Nurseries have been a major source of dissemination of root rot and the strictest hygiene is practised to keep the nursery free from this feared disease. Fruit for seed is picked from the tree (fruit gathered under the tree may be contaminated), potting media are pasteurized or sterilized, water comes from a protected source or is disinfected, plants are raised on through-draining benches, only regular staff has access to the nursery plants, etc. Rootstocks have been selected which are less susceptible to root rot; to eliminate seedling variation, these are cloned by air layering or by rooting of etiolated cuttings.
During orchard establishment field crops or vegetables may be grown between the trees. Bananas are sometimes used as a companion crop in Australia and the Canary Islands. Ten litres of farmyard manure and 300 g of phosphorus should be incorporated to 50 cm depth at each plant position and the pH should be raised to 5.0—5.8 with lime or dolomite, if necessary, before planting. Spacing of trees varies from 6—12 m on the square (280—69 trees/ha). Close spacings improve yields for the first 6—8 years but thereafter tree removal becomes necessary. Hedgerows are an alternative, but avocados respond poorly to pruning and the planting pattern should allow for thinning to accommodate full-grown trees (e.g. 9 m x 6 m thinned to 12 m x 9 m).

During the first year the trees require special protection against wind. Maintaining a 10 cm thick fibrous mulch (e.g. wheat, sorghum or rice straw) around young trees suppresses weed growth, conserves soil moisture and protects and encourages the development of the root system. Avocados have a rapid turnover of leaves and after a few years the fallen leaves afford a measure of self-mulching. In humid areas cover crops of sorghum or maize raise soil organic matter and inhibit root rot.
Growth of young trees is promoted by fertilizing regularly with complete nutrient mixes and/or farmyard manure high in nitrogen. Excessive nitrogen in bearing trees stimulates vegetative growth at the expense of fruiting. The foliar levels of nitrogen before flowering should not exceed 2.0% in lower yielding cultivars and 2.6% in prolific cultivars. All nitrogen should be applied in split dressings during the growth of the summer flush. Zinc and boron deficiencies may require specific attention in many areas.
Where irrigation is required, soil tensiometer readings at 30 cm should not fall below 0.25—0.3 MPa in sandy soils and 0.3—0.4 MPa in clay soils.
Pruning is restricted to shaping the trees during the first few years and the removal of lower limbs which obstruct access as trees grow larger. Timely thinning of the orchard is very important, but inevitably growers tend to reason that it can wait for another year. Once trees become too large to manage they are rejuvenated by scaffold pruning to 1.5—2.0 m. Exposed bark is painted with an acrylic paint to prevent sunburn. Scaffold pruning cannot replace tree thinning, because in unthinned orchards the canopy closes again before substantial crops have been produced.

In most countries, also in South-East Asia, root rot (Phytophthora cinnamomi) is a serious disease causing loss of production and eventual death of the tree. The fungus lives in the soil and attacks the new tree roots when soil temperatures are between 13—32°C. Cheap and effective control is by trunk injection of the systemic fungicide potassium phosphonate, repeated annually. Other Phytophthora species which infect avocado are Phytophthora citricola, Phytophthora heveae and Phytophthora palmivora.
Anthracnose (Colletotrichum gloeosporioides) causes significant fruit loss in warm humid climates. Some cultivars have greater resistance, but under moist conditions copper sprays are required to produce marketable fruit. In some countries Cercospora spot of the fruit (Pseudo-cercospora purpurea) is a serious fungus disease and sunblotch viroid is widespread throughout the world. The viroid is transmitted through infected seed or scion material, pruning and propagation equipment, and also by root grafting between trees. It causes growth distortion of trees and fruit, giving significant economic loss. It is best controlled by destroying infected trees and using only clean sources of material for propagation.
In storage fruit may develop stem-end rot, which is usually caused by a complex of fungi, the most common ones being Thyronectria pseudotrichia, Colletotrichum gloeosporioides, and Dothiorella aromatica.
Various fruit flies attack the fruit causing superficial damage; although larvae rarely develop in fruit this can be a reason to refuse entry to some markets. Fruit-spotting bugs cause deep fruit lesions and fruit abortion, and various webbing caterpillars and loopers can also cause serious economic loss. In Indonesia trees may be stripped bare by caterpillars of Cricula moths. All pests can be controlled with contact insecticides if carefully timed.

Maturity is judged by the ability of the fruit to soften and become palatable without shrivelling or breakdown of the flesh once it is picked from the tree. Cultivars of Mexican and Guatemalan origin have fruit which will remain on the trees accumulating oil for 2—4 months after reaching maturity and 'on-tree-storage' is often used to prolong harvesting. However, this practice promotes alternate bearing. Maturity of West Indian or West Indian/Guatemalan hybrid cultivars is best defined by setting a picking date and a minimum fruit size each year, based on the readiness of fruit samples to ripen to a satisfactory eating condition. Fruits of these cultivars readily drop if not harvested when reaching maturity; they cannot be stored on the trees for long. Because of protracted flowering, the fruit matures over a period of time. Selective picking is often practised; this gives the remaining fruit the opportunity to grow out. Fruit is clipped from the tree, retaining a pedicel button which prevents the entrance of post-harvest diseases through the fruit stalk scar. There is room for improved harvest methods in South-East Asia where much fruit is simply shaken from the tree or thrown to catchers by boys climbing the trees. Picking from ladders into picking baskets, or with a picking pole fitted with a hook and a collection bag is recommended. After picking, the fruit should be shielded from direct sunlight to prevent the build-up of field heat.

From national figures on area and production over 1981—1985, mean annual yields as low as 5 t/ha in South Africa, 4.5 t/ha in California and 3.6 t/ha in Indonesia can be calculated. These figures improve if yield is expressed as a 'good commercial average' based on data from well managed orchards of productive cultivars: California 13.4 t/ha for some cultivars, Australia, Israel and South Africa 10—14 t/ha. For some orchards sustained yields of 20—24 t/ha have been reported. With improved cultivars and production technology it is estimated that sustainable yield is in the vicinity of 32 t/ha.

Gentle brushing of the fruit removes field bloom, scale insects and traces of fungicide, giving the fruit an attractive lustre. Shelf life of avocados is relatively short, the fruit reaching the eating-ripe stage within 4 to 14 days from picking, depending on its stage of maturity and the ambient temperature. Post-harvest life of Mexican and Guatemalan cultivars may be extended to 4—6 weeks with cooling (5—7°C) and modified atmosphere storage. West Indian cultivars have a shorter post-harvest life and in general should not be stored below 10°C.
First grade fruit is marketed in hand packed, single or double layer cartons containing 4—10 kg net weight. However, there is a move to larger, volume-filled containers for second grade fruit. There is little sophistication in post-harvest handling in South-East Asian countries, where cool storage is seldom practised and fruit is transported to local markets in open baskets or crates.

Germplasm of avocado is under threat because of the clearing of the rain forest in Central America, the large numbers of trees lost to root rot (accidentally introduced), and the narrowing range of cultivars grown. Material of genetic interest is being collected and held in a repository at the USDA field station at Homestead, Florida. The University of California at Riverside has a small collection of Mexican and Guatemalan lines as the basis of a large breeding programme, and Israel has numerous West Indian types being developed predominantly as rootstocks.

The cultivars grown result from selection of chance seedlings. The University of California at Riverside is currently the only organization maintaining a large formal breeding programme in which improved cultivars and rootstocks are being sought. Objectives for scions are precocity, regular heavy bearing over a wide range of environments, resistance to fruit diseases and post-harvest physiological disorders, and attractive appearance. Resistance to Phytophthora cinnamomi, salinity tolerance, and dwarf tree size are the major attributes being sought in rootstocks.

For many centuries the avocado has been a source of nourishment for the people of Central America. In the rest of the world people are not yet fully familiar with its rather unusual place — for a fruit — in the diet. The indications are that there is much scope for continued expansion of these relatively new markets. For South-East Asia much will depend on the success of avocado growing in orchards in the highlands as well as in the lowlands. If the fruit remains largely limited to home gardens it will be difficult to pay enough attention to fruit quality and presentation to win over large numbers of consumers.
The international markets are served by Mexican and Guatemalan cultivars, but the difficulties of moving large volumes of a very perishable fruit over large distances limit the pace of expansion. The West Indian cultivars are even more perishable and therefore limited to nearby markets. The available improved growing techniques need to be matched by improvements in post-harvest technology to facilitate market development.

Bergh, B.O., 1986. Persea americana. In: Halevy, A.H. (Editor): Handbook of flowering. Vol. 5. CRC Press Inc., Florida. pp. 253—268.
Bergh, B.O., 1987. Avocado breeding in California. South African Avocado Growers Association Yearbook 10: 22—24.
Gustafson, C.D., 1962. The salinity problem in growing avocados. California Avocado Society Yearbook 46: 100—105.
Harding, P.L., 1954. The relation of maturity to quality in Florida avocados. Proceedings of the Florida State Horticultural Society 67: 248—250.
Pegg, K.G., Whiley, A.W., Saranah, J.B. & Glass, R.J., 1985. Control of Phytophthora root rot of avocado with phosphorous acid. Australasian Plant Pathology 14: 25—29.
Scholefield, P.B., Walcott, J.J., Kriedemann, P.E. & Ramadasan, A., 1980. Some environmental effects on photosynthesis and water relations of avocado leaves. California Avocado Society Yearbook 64: 93—105.
Sedgley, M., Scholefield, P.B. & Alexander, D.McE., 1985. Inhibition of flowering of Mexican- and Guatemalan-type avocados under tropical conditions. Scientia Horticulturae 18: 21—30.
Whiley, A.W., Saranah, J.B., Cull, B.W. & Pegg, K.G., 1988. Manage avocado tree growth cycles for productivity gains. Queensland Agricultural Journal 114: 29—36.
Whiley, A.W. & Winston, E.C., 1987. Effect of temperature on varietal productivity in some avocado-growing areas of Australia. South African Avocado Growers Association Yearbook 10: 45—47.
Wolstenholme, B.N., 1987. Theoretical and applied aspects of avocado yield as affected by energy budgets and carbon partitioning. South African Avocado Growers Association Yearbook 10: 58—61.

A.W. Whiley

Whiley, A.W., 1991. Persea americana Miller. In: Verheij, E.W.M. and Coronel, R.E. (Editors): Plant Resources of South-East Asia No 2: Edible fruits and nuts. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea