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Cultivating Avocados

By goGreen | September 29, 2011
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Avocados are rich in protein, carbohydrates, vitamins and minerals while the oil content consists of polyunsaturated fatty acids. One half of a Fuerte fruit supplies a considerable percentage of the daily nutrient requirements and makes an important contribution to a balanced diet.

Buyer’s guide

The quality of the trees grown in the nursery determines the success of an enterprise. Trees that received poor or incorrect treatment in the nursery will lag behind in the orchard, no matter how carefully the buyer tends them, and may die easily.

Container and root development

Growth medium

Foliage

Graft union

Climatic requirements
The 3 best-known avocado races each has specific climatic requirements as a result of adapting to their original environment.

West Indian cultivars originated in the humid, tropical lowlands of Central America and are best adapted to continuous hot, humid conditions with a high summer rainfall. Like all avocado cultivars they are, however, extremely sensitive to drought and do not tolerate frost well (minimum temperature of 1,5 °C). The optimum temperature for growth is 25 to 28 °C. The humidity should preferably be above 60 %.

The Mexican races originated in the cool, subtropical highland forests of Mexico and mature trees can withstand temperatures of –4 to –5 °C. They should not be planted in areas prone to frost in August and September, because flowers are damaged easily by frost. A humidity range of 45 to 60 % should suffice. The optimum temperature for growth is 20 to 24 °C.

Guatemalan cultivars originated from the tropical highlands of Guatemala and require a cool, tropical climate without any extremes of temperature or humidity. The trees can withstand light frost, down to –2 °C, but the flowers are very sensitive to frost. High temperatures of about 38 °C, especially if combined with low humidity, could cause flower and fruit drop. A humidity level of 65 % or higher is required.

Temperature

Humidity

Rainfall
All avocado cultivars grown commercially in South Africa are sensitive to water stress. An annual rainfall exceeding 1 000 mm is desirable, and it should be well distributed, with the only dry period in June and July. However, most of the suitable areas in South Africa experience a dry period during flowering, necessitating supplementary irrigation.

Wind
Avocados tend to have brittle branches that are damaged easily by wind. The majority of blemishes causing a downgrading of fruit most probably also result from wind damage. From a climatological point of view, the best areas for commercial avocado production are therefore the cool, subtropical parts of Mpumalanga and the Limpopo Province as well as KwaZulu-Natal where the rainfall is fairly high and mist occurs frequently.

Soil requirements
A healthy avocado tree has a root system that can penetrate the soil to a depth of 1 m. Root rot (Phytophthora cinnamomi) can develop fairly quickly in poorly-drained soils. It is therefore essential to determine in advance the suitability of the soil for avocado production.

Methods of soil examination
Soil can only be examined by digging profile holes at least 1,5 m deep in areas where there are different soil characteristics. Even if the soil on the surface appears to be fairly homogeneous, it is still advisable to dig at least one profile hole per hectare.

In hilly areas holes must be dug in different positions along the slope to get an indication of the drainage properties of the area. For example, it may be necessary to dig drainage furrows in the lower-lying areas to prevent water from accumulating above the restricting layers.

Aspects of concern when digging a profile hole are colour, texture, structure, patches, concretions and stones, as well as soil depth.

Colour

Texture
Avocados do best in soils with a clay content of between 20 and 40 %. If the clay content is below 20 %, the soil has a limited water-retention capacity and unless optimum irrigation is applied, the trees will sometimes suffer temporarily from drought.

A too high clay percentage makes irrigation difficult because overirrigation and high rainfall lead to oversaturation of the soil. This means that water drains away relatively slowly, which promotes root rot.

Structure
In soils with a moderate to strongly developed block structure, such as soils that can be broken into hard clods when dry, root development will be restricted. Ideal avocado soils display only small, fine cracks when a dry profile wall is examined.

Patches
If a light-coloured layer with many patches occurs within 1,8 m below the soil surface, root problems can be expected, especially with irrigation. Such a soil can be regarded as a moderate to high-risk avocado soil.

Concretions and stones
The same requirements concerning depth apply to black concretions (iron and manganese) in light-coloured soil. If concretions and stones occur as a type of gravel and form more than 30 % of the volume of a soil layer, the water-retention ability of that layer will be adversely affected, and irrigation practices will have to be adjusted accordingly.

Chemical soil properties

Cultivars
Avocado growers should produce high yields of good-quality fruit, acceptable to the consumer. There is, however, no single cultivar that can fulfil all the requirements of the grower, the packer, the retailer and the consumer at the same time.

Fuerte
Good production potential

Hass
Good production potential in cool areas. Fruit is smaller in warm areas

Pinkerton
Consistent heavy bearer

Ryan
Good production potential; bears heavily and fairly consistently

Soil preparation
It is important to examine the soil for suitability regarding depth, drainage and compacted layers (see Methods of soil examination).
A representative sample of the proposed orchard must be taken for soil analysis. It is desirable to take the soil sample at least 9 months, but preferably 12 to 24 months prior to planting. This gives the farmer enough time to prepare the soil thoroughly, particularly if large quantities of lime are required.

Soil sampling
It is important that a sample represents a soil of homogeneous characteristics, i.e. where no visible differences in the soil occur. If there are differences regarding colour and texture in such a land, the land must be subdivided accordingly and separate samples taken of the different parts.

A soil auger or spade can be used for taking samples.

Depth of sampling
This should be from 0 to 0,3 m for topsoil and 0,3 to 0,5 m for the subsoil sample.

Number of samples
A sample must be made up of at least 10 subsamples (preferably more). The area represented by the complete sample should not exceed 3 ha.

Mixing and packaging

The analysis results will supply valuable information regarding fertilisers to be applied before planting. If required, lime or phosphate should be thoroughly worked into the soil before planting.

Method of soil preparation
The soil must be loosened as deep as possible before planting. In this case it will not be necessary to make large planting holes.

Layout of orchard
An avocado orchard should be profitable within 7 to 10 years.
There are 3 patterns according to which trees can be arranged in an orchard:

If trees are spaced in such a way that no thinning will be necessary during the lifespan of the orchard, only slightly more than 50 % of the land is utilised. Effective land use therefore, means that the trees are initially spaced close together, to be thinned systematically and selectively at a later stage. There is, however, no proof that any specific layout is the best. The choice of planting distance and the pattern of planting depends on the following factors:

The final decision must be based on economic principles, because each of the aspects mentioned has an influence on the ultimate economic value of an orchard.

Planting distance and planting pattern
The choice of a planting pattern (rectangular pattern discussed here) depends on the management practices followed.

Interrow spacing
Economic considerations and access for implements determine interrow spacing of trees. Final distances of less than 10 m will necessitate thinning before the orchard is 10 years old. High-density plantings can therefore be planted at less than half the “final” distance on the understanding that trees in the semipermanent rows are removed timeously.

Planting and early care

Planting hole

Irrigation

Shade

Protection against animals

Cover crops

Leaf analysis
Leaf analysis indicates the nutrient status of an orchard. The trees can be fertilized accordingly. Soil analysis, in addition to leaf analysis, increases the reliability of the recommendations made.

The following aspects of leaf sampling must be followed strictly:

– Select approximately 20 healthy trees well distributed throughout the orchard, homogeneous in appearance and representative of the orchard.
– Exceptionally good or poor trees must not be sampled.
– The trees must be marked clearly, e.g. with paint, so that both the soil and leaf samples can be taken from the same marked trees every year.

Fertilisation

Time of application

Nitrogen and potassium
If fertiliser is applied in the first year, nitrogen (N) and potassium (K) must be divided into at least 4 applications and applied during the warm months. Thereafter the application times are as follows:

Zinc
Most soils are either naturally low in zinc or the zinc is not available. The following quantities should therefore be applied annually:

Boron
Many avocado orchards are also low in boron and the trees should be sprayed every year with 100 g borax or 75 g Solubor/100 l water.

Irrigation
Avocado trees are very sensitive to waterlogged conditions as well as to drought stress. The aim of irrigation is therefore to maintain the soil-moisture content between these two extremes, except in June and July when a drier period is required to stimulate flowering.

The purpose of avocado farming is to obtain optimum yield and reduced vegetative growth from an orchard.

Irrigation systems
Light, frequent irrigations are necessary. A flood irrigation system is therefore not suitable. A sprinkler system, particulary dragline, is more suitable, especially if capital is limited. The best type for avocado orchards is undoubtedly one of the daily-flow types, preferably microjet irrigation.

Weed control
Weed control can be done by either mechanical or chemical methods.

Mechanical
There are 3 types of mechanical weed control:
Hand or mechanical slashing
Care should be taken when hand slashing is used not to cause injuries to the tree trunk. In time mechanical slashing will lead to compaction and ripping will have to be considered to promote root growth.

Disking
This method is not recommended as feeder roots are continually damaged and soil is compacted just below the depth of cultivation.

Ploughing and ripping
Both methods loosen the soil, especially heavy soils. These should, however, not be practiced at intervals shorter than 2 years, because root growth will be arrested too often. Ploughing displaces the compacted layer to just below the depth of the plough while ripping can break up compacted layers if it is done when the soil is relatively hard and dry.

Chemical
Herbicides that control the entire weed spectrum in an orchard must be used. Therefore, the weed species and their relative abundance in the orchard should be determined before selecting a herbicide.

Cover crops and mulching
Cover crops are temporary crops, grown for the purpose of improving the soil, either as soil protection or to be disced in as green manure.

Kinds of cover crops
Cover crops can be divided into leguminous (nitrogen-fixing) and nonleguminous crops. Leguminous crops include various types of beans, while rye, oats, barley and buckwheat are examples of nonleguminous crops.

The following crops can be considered:

Pests
Avocado producers must be familiar with the insects that occur in orchards as pests. Most of these are controlled by natural enemies. The injudicious use of agrochemicals on avocado trees could, however, allow minor pests to develop into major economic risks.

Fruitflies
This pest has only recently gained economic importance in avocados. When the fruit is picked before it is ripe, the larvae never reach maturity. However, if the fruit remains on the tree for extended periods, as in the home garden, fruitflies may occasionally develop to maturity.

Symptoms

Control
The few natural enemies of the fruitfly cannot control the problem effectively. There are 2 methods of control, namely:

The Natal fruitfly also infests other host plants, such as bugweed or bugtree, bramble and wild-growing guavas which often constitute a significant proportion of the indigenous bush surrounding avocado orchards. These plants should be eradicated in an area of up to 200 m or more around the orchards. They should be cut down to 200 mm above the ground. A suitable herbicide should be applied as soon as the plants have regrown to about 0,5 m.

Using poisoned bait to lure the flies should be considered if damage exceeds 5 %. The chemicals used in this process are, however, detrimental to the parasitoid-predator balance in the orchards.

Heart-shaped scale

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