AVOCADO PRODUCTION
1. Introduction
It is an important crop in the term of production and trade. World production stands at 1,500,000 tons / year. Tanzania produces about 55,500 tons / year.
2. Origin and Distribution
2.1 Origin
The fruit has its origin in Central America, under varying climatic conditions as influenced by altitude.
2.2 Distribution
Currently the cultivation of avocado has extended beyond the limits of its zone of origin. It is grown in all tropical and subtropical areas of the world within latitudes 43 0 N and S. Major producing countries are Mexico, Brazil and USA in California. In Africa major producers are: South Africa and DR Congo. In East Africa, Kenya is the leading producer.
3. BOTANICAL DESCRIPTION
3.1 Botanical Classification
Family: – Lauraceae: – Includes important plants such as Cinnamon.
Genus and species: Persea americana Miller.
The species is divided into three distinct races:
– Guatemalan – (origin 1500 – 2350 m asl)
– Mexican – (1400 – 2700 m asl)
– West Indian (American) – (100 – 450m asl)
Guatemalan : – Fruit have thick skin with (grit cells), medium to high oil content.
– Size of fruit medium to large.
Mexican : – Very thin skin
– Flesh (buttery) with high oil content.
– Fruits small to medium in size
West Indian : – Moderately thin, leathery skin
– Watery flesh with low oil content
– Fruit are large
3.2 Plant characteristics
3.2.1 The plant
· The avocado tree is an evergreen subtropical spp.
· Grows to 10 – 15 m in height
· The form / shape of the tree is very variable depending on variety and Method of multiplication
– Seedling trees are slender due to strong apical dominance.
– Grafts are variable in shape depending on cv. They are erect to cylindrical.
3.2.2 Leaves
– Are arranged in spirals, formed in flushes.
– The new growths vary in color from light green (young) to shiny dark green (when adult).
– The leaves are persistent and entire.
– They contain a chemical: abacatine, that as a bitter test with diuretic properties
3.2.3 Flowers
– Are perfect, pale yellow in color
– Are borne in terminal and axillary panicles at the end of a branch
– The panicle (inflorescence) has a single central axis with branches which each carry 1, 2 or 3 flowers.
The plant can flower once or twice a year depending on the climate. The flower is hermaphrodite (complete) but is characterized by a diurnally synchronous dichogamy.
Ø That is, the flowers have both female and male organs, but they do not function at the same time (simultaneously). Each flower is female when it first opens, ie:
Ø Its stigma will receive pollen from other avocado flowers, but its stamens do not shed pollen at this first opening.
Ø This first or female stage lasts for 2 to 3 hours. The flower then closes and remains so for the rest of the day and that night. The following day, it opens again. Now the stigma will however no longer receive pollen. It is no longer functional. Instead the flower is now shedding pollen; the flower is now male.
Ø After being open for several hours during the second day, the flower closes again. This time it closes for good. If it had been successfully pollinated at first opening, under favorable conditions, a fruit will develop.
Thus, avocado is essentially cross-pollinated. Bees and other large insects are the most efficient. It can be said that nature has provided for avocado cross-pollination by creating varieties of 2 kinds known as: –
– Type A: – In which flowers are female in the morning of the first day and male in the afternoon of the second day.
-Type B: -Plants that behaves in the reverse of type A. Flowers are female in the afternoon (of the first day) and male in the following morning.
Consequently, the 2 types complement each other. A variety of one type provides pollen when a variety of the other type needs it.
With hundreds of flowers of both types opening day after day, the daily situation becomes: –
Morning Afternoon
Type “A” Female Male
Type “B” Male Female
Thus the necessary pollination of trees is obtained through this natural way of synchronization between varieties of type A and B.
What is the implication of this floral behavior for isolated trees or a monoclonal orchard?
Theoretically; in respect to these principles of the biology of the avocado flowers, an isolated tree or a monoclonal / mono-varietal orchard cannot be fertilized and cannot therefore produce fruits, due to lack of pollination.
Consequently, this implies that, it will always be necessary to have in an orchard at least 2 varieties belonging to the complementary groups A and B and which have the same flowering period during the year.
In reality: –
· The belonging of varieties to either group A or B is genetically determined and independent of environmental conditions.
· However, the synchronization of the dichogamy (dianthesis) is not so rigid. As it is influenced by external environment, in particular the variations in temperature at flowering. These cause distortion in the flowering cycles of plants leading to overlapping of the female and male floral stages for 2 – 12 or even 100 hours.
The distortion enables entomorphillous auto pollination (self-pollination) of isolated tree and even cross-pollination in monoclonal orchards.
Temperature requirements for synchronized flowering cycles (In California):
Group A Group B
Min. temperature > 6.5 0 C > 10.5 0 C
Max. temperature < 19.0 0 C < 20 0 C
3.2.4 Fruit
· Is a berry with one loose or adherent seed that has a double seed coat.
· Shape and color of the skin depend on variety. Can be oblong, ovoid, piriform to spherical.
· Weigh between 50 – 900g depending on variety. The seed weighing 8 – 24% skin, 9 – 15% of the fruit by weight.
· In ancient times the seed was used in the production of indelible ink.
3.2.5 Root
Avocado is shallow rooted. The root system is composed of large, lateral roots that anchor the tree to a depth of 1.2 – 2.4 m. From the lateral roots emanate a profusion of feeder roots, which are superficially spread in the soil. These feeder roots can be seen when decaying leaves are raked aside.
3.2.6 Growth and development
· The avocado grows in flushes. Each flush matures in 4 – 6 weeks.
· As the new growth harden, the bark changes in color from green to gray.
· Apical dominance is very week, thus, branching always occurs at the end of a period of intensive elongation.
· The bearing surface of the tree is usually the outer portion of the foliage, which contains the fruiting wood.
4. ECOLOGY AND PHYSIOLOGY
4.1 Altitude and temperature
We have seen the wide range / areas of dispersion of avocado. It grows well from 43 0 S to 43 0 N. This implies a wide range of adaptation to climatic conditions particularly temperature. In the tropics it is grown from sea level to 2500 m asl
The existence of different races, contribute to the wide adaptation of the species.
– Mexican – low temperature adaptation (1400 – 2700m asl))
– Guatemalan – average temperatures (1500 – 2300m asl)
– American – high temperature (hot climate), low altitude (100 – 450 asl)
– Average temperature is 12 – 28 0 C.
4.2 Precipitation
Requirements are relatively low for:
– Mexican race – low
– Guatemalan – average; -1200 – 1600
– American – average; – well distributed
Bloom is not affected by rain (like with mango / cashew), unless it persists for a month or more. The protogynous => dichogamy of the flower is influenced by temperature. The succession of the male and female stages is more or less precise at constant temperature and very much disturbed by low temperature.
4.2 Light
Has high requirement for light (~ 23 to 25 hours)
4.3 Wind
Branches of avocado are very brittle and break easily, windbreaks are therefore important.
4.4 Soil
Well-drained soils are the most favorite. An impervious layer or water table within 90cm of the surface will lead to death of plants.
5. CULTIVARS
Cultivars (varieties) belong to the three races. Many commercial cultivars are hybrids between the races. The following are some of the most popular varieties
6. CULTURAL PRACTICES
6.1 Propagation
6.1.1 Use of seeds
The majority of plants in the tropics are seedlings.
– Unfortunately they are late bearing, variable, with an erect shape and therefore very tall.
– Have low productivity
Seeds can be stored in sealed polyethylene beds at 4 – 6 0 C & 85% RH for 5 months. Treatment with benomyl or thiram before storage is important. After one year viability is reduced to about 30%.
– After storage – treat with hot water (49 – 50 0 c) for 3 minutes followed by drying before sowing.
– Incisions made above and below the seed (at the apical and basal ends) hasten germination, which will occur in 3 – 5 weeks after sowing.
6.1.2 Vegetative methods
The commonest method is grafting on rootstock resistant to Phytopthora cinnamon .
6.2 Planting
Recommended orchard spacing varies with variety. It ranges from 5 – 6m x 6, 10 x 10m to 12 x 12m. These gives plant populations of 70 to 200 pl / ha. In order to promote good pollination 2 – 4 rows of an A group cv. should preferably be alternated with some rows of a B type cv. In other words, it is important to include a pollinator at every 5 th rows, in addition to installing at least two beehives per ha.
6.3 Soil management
The most important aspect is to avoid cultivation in orchard to avoid damaging the roots (are very superficial)
6.4 Irrigation
Irrigation is required whenever rainfall remains below the Ep (potential ETP).
6.5 Fertilization
Adequate quantities of nutrients from leaf analysis are the following:
N 1.6 – 2.0%
P 0.08 – 0.25%
K 0.75 – 2.0%
About 1.0 – 3.05
Mg 0.25 – 0.80%
Fe & B, 50 – 100pm
6.6 Training and pruning
Little training and pruning is required.
· For young plants, it is adequate to cut back some shoots or to pinch out the terminal buds of shoots growing straight up. This helps to keep trees low growing at a height of 5 – 8m. The practice keeps picking cost low.
· The first scaffold branch should never be permitted below 60cm above ground level, since the branch diameter will grow larger.
· Young plants 5 years old can be stimulated to rapidly produce fruiting wood by pruning tall central leader back to 10 – 30cm.
· Maintenance pruning should ensure cleaning of the plant while crowding and reduced plant heights are avoided.
· Skirting is the trimming of the drip line to avoid the foliage from touching the ground.
· Provision of props to support heavily laden branches.
7. HARVESTING AND YIELD
7.1 Harvesting
Time of harvest is variable depending on variety and climate. In Tanzania the peak harvesting time is between April and September.
Avocado can be harvested before ripeness, at a stage of maturity, which will permit the fruit to ripen off the tree, conserving good texture and organoleptic quality (characteristic).
Avocado is particular, among fruits in that the fruit can remain attached to the tree after ripening for a long time. For example, the fruits of the varieties Fuerte, Hass, Zutano and Nabal, can remain on the tree for several months after the attainment of their complete development. Varieties in the West Indian races do not however, dispose of this capability of auto conservation in situ .
However, if fruits are left on the tree for too long: germination of the seed will occur, immediately after harvest the fruit will enter the climacteric phase and therefore will rapidly deteriorate. If harvested too early (before its complete development), the fruit may fail to reach the climacteric phase, or do so after a very long period. This will lead to poor quality, wilting and become unfit for consumption. Fruits are harvested manually one by one.
7.2 Yield
Average yields range 5 – 10 tons / ha. Yield of 14 – 15 tons / ha have been recorded.
8. USES AND COMPOSITION
Avocado is a very nutrition fruit. It contains 3 – 3% oil, similar in composition to olive oil. It is rich in vitamins A and has reasonable quantities of vitamins B and C.
It has high calorific value, but low sugar content. It is therefore recommended as a high-energy food for diabetics. The fruit is eaten on bread, salads and lemon juice, vinegar and salt and pepper.
9. PESTS AND DISORDERS
9.1 Diseases
– Phytophthora cinnamomi is the most serious disease. It causes seedling blight, stem canker and root rot. It is controlled by:
– Spraying with cupper fungicides treatment to start with seeds.
– Avoid water logged conditions
9.2 Insects and mites – are not a serious problem.
9.3 Disorders
Effects of temperature
Very high temperatures greater than 36 0 C:
– On young foliage => rapid witting
– On fertilization => desiccation of pollen and floral parts
– On fruiting => Precocious fruit drop due to excessive water deficiency
– Production cycle => shortened.
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