From Plant Resources of South-East Asia No 2: Edible fruits and nuts, PROSEA Foundation
by A. W. Whiley
Taxon
Persea americana Miller.
Protologue
Gard. Dict. ed. 8 (1768).
Family
LAURACEAE
Chromosome Numbers
2n = 24
Synonyms
Persea gratissima Gaertn.f. (1807), Persea drymifolia Schlecht. & Cham. (1831), Persea nubigena L.O. Williams (1950).
Vernacular Names
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.
Origin and Geographic Distribution
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.
Uses
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.
Production and International Trade
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.
Properties
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.
Description
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.
Growth and Development
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.
Other Botanical Information
Within Persea americana 3
ecological races are distinguished, sometimes considered as botanical
varieties. In order of increasing tropical adaptation they are
designated Mexican (Persea americana Miller var. drymifolia (Schlecht. & Cham.) Blake), Guatemalan (Persea nubigena L.O. Williams var. guatemalensis L.O. Williams) and West Indian (Persea 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.
Ecology
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.
Propagation and planting
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).
Husbandry
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.
Diseases and Pests
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.
Harvesting
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.
Yield
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.
Handling After Harvest
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.
Genetic Resources
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.
Breeding
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.
Prospects
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.
Literature
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.
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