Directorate Communication



Cultivation of avocados

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 Northern 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 pit 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 pit 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 b]2roken 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

Avocado 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

Flowering and harvesting months

 

              Warm regions                Cool regions

Flowering:        

June - September  July - October

Harvesting:       

March - August  May - November

Tree characteristics

General

Hass

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

Flowering and harvesting months

       

Warm regions    Cool regions

Flowering: 

July - September August - October
Harvesting:  June - October       August - December

Tree characteristics

General

Pinkerton

Consistent heavy bearer.

Flowering and harvesting months

 

Warm regions  Cool regions

Flowering:                                                                 

September - October  
Harvesting: June  July

Tree characteristics

General

Ryan

Good production potential; bears heavily and fairly consistently

Flowering and harvesting months

                            

Warm regions Cool regions

Harvesting:

September - December  November - February

Tree characteristics

General

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

Method of soil preparation

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 fertilised 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:

Sample the correct leaf

The sampling method is important:

- 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.

To obtain a meaningful fertilisation programme for a particular orchard, leaf samples must have been analysed for a period of at least 5 years.

It is not always possible to rectify deficiencies, excesses, or imbalances within one season because it is a gradual process.

Fertilisation

Quantity of fertiliser per year per tree according to age (guidelines)
Age (years)  LAN (28 % N)
(or equivalent) 
Superphosphate 
(11,3 % P)
Potassium chloride
(50 % K)
g

4 - 5 

6 - 7 

8 - 9 

10 - 12 

Maximum

150 

300 

450 

600 

800 

1 000 

1 200 

1 500 

200

400 

600 

900 

1 200 

1 500 

1 800 

1 800 

150

300

450

600

800

1 000

1 200

1 500 

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.

By applying half the calculated volume of water per tree after the moisture-stress period (June/July), new growth is retarded, competition with the young fruit is restricted and fewer fruit will abort.

After fruit-set full irrigation is applied to encourage a flush for fruit growth.

No fertiliser should be applied during flowering and early fruit-set, because it would also encourage vegetative growth, which competes with the young fruit for nutrients and water.

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 practised 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.

The summer crop reduces soil temperature, thereby promoting avocado root growth. It also produces a mulch for winter and controls weeds.

A cover crop prevents erosion and the loss of plant nutrients.

A leguminous cover crop enhances the nitrogen content of the soil.

Decayed cover crops increase the humus in the soil.

The time of planting a cover crop depends on the needs of the tree and the supply of available moisture. In seasons where good rains are experienced, the cover crop should be planted early, but in periods of drought the trees will need all the available moisture in the soil, and planting should therefore be delayed.

In young orchards it is possible to plough and prepare a seedbed between the tree rows, but as the trees grow older hand planting becomes the obvious method.

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:

Conditioning the soil before orchard establishment: velvet beans, soya-beans, sunnhemp, ration beans, lupins*, medics*, cowpeas.

Planting after orchard establishment: velvet beans, soya-beans, ration beans, cowpeas, lupins*, medics, wheat, rye, barley, teff, vetches, desmodiums, groundnuts, buckwheat.

* requiring winter irrigation

Avocado 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

The Natal fruitfly attacks both young and older fruit.

It lays its eggs just under the skin surface.

When the fruit is about golfball size, a sting lesion appears as a slight puncture mark surrounded by white powdery exudate. As the fruit develops the lesion becomes dry and distinct star-shaped cracks in the skin surface occur.

When the surface layer of skin around the lesion are removed with a pocket knife, the underlying tissue will appear bruised and, on further dissection using a 10 x magnifying lens, longitudinal white eggs, about 0,5 to 1 mm in length, should be found clustered together.

Control

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

Eradication of unwanted host plants

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.

Baiting

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

This insect grows to about 3 mm and has a reddish-brown colour. A white, woolly edge can be seen at the rear end of the adult female.

The female lays cream-coloured eggs which are kept underneath the body in the white, woolly secretion. More than 200 eggs have been counted underneath one female at a single count.

The young scales, known as crawlers, eventually become permanently fixed in one spot. In this way the new leaves become infested. There are 2 generations per year. The summer generation lasts 5 months and the winter generation 7 months. Crawlers are produced mainly during November and April.

The scales occur on the back of avocado leaves where they suck the sap from the leaves.

Fruit is never attacked, but the scales secrete considerable quantities of honeydew, landing on the leaves, branches and fruit on which sooty mould grows, causing a black discoloration of the plant and fruit and interfering with photosynthesis.

Dust blown from dirt roads alongside avocado orchards settles on the trees, creating conditions that are unfavourable to the natural enemies of the scale. Therefore greater numbers of scales occur in such areas.

The heaviest infestations occur on trees of the Hass cultivar.

Wasps, ladybirds, larvae of a lacewing and a fly species (Cecidomyidae) play an important role in the biological control of this scale.

As a result of the many natural enemies, chemical control is usually not necessary.

Avocado diseases

Phytophthora root rot

Root rot, caused by Phytophthora cinnamomi occurs worldwide. Locally it is one of the most important avocado diseases and is found in all production areas, as well as home gardens. The severity of infection varies, but the potential loss is very big if no control measures are taken.

Symptoms

The pathogen

Dispersal of the pathogen requires free-moving water. It is therefore important to avoid planting avocados on soils that are prone to waterlogging. The fungus has a very wide host range, which includes papaya, granadilla, macadamia and various ornamental species. If possible, planting of avocados on lands previously used for alternate hosts should be avoided.

Control

Avocados should not be established in areas where waterlogging is a problem.

Planting material should be disease free.

Irrigation should not leave water standing on the soil surface, particularly around the trunks. Sprinklers and microjets are therefore preferred to flood irrigation. Avoid overirrigation.

Stem canker

This is another form of Phytophthora infection, but it may be caused by P. citricola, P. cactorum and P. cinnamomi.

Symptoms

Control

Do not cause injuries to the stems near ground level and avoid constant wetting of the stem.

Where lesions occur, remove dead tissue and cover with a bituminous sealant.

Anthracnose

This is a pre as well as a post-harvest problem and can cause serious losses. It is present in most, if not all, production areas of the world.

Symptoms

Control

A registered post-harvest dip is available specifically for anthracnose control. It should be applied in the packhouse. Control of Cercospora spot should also give good control of anthracnose.

Cercospora spot

This is a fruit-spotting disease, caused by the fungus Pseudocercospora purpurea.

Symptoms

Control

Several chemicals are registered for use against this disease. These programmes will also help to control sooty mould.

Always consult product labels before any chemicals are applied.

Consult the latest issue of A Guide for the Control of Plant Diseases for information on chemical disease and pest control. It is obtainable from the Resource Centre, Private Bag X144, Pretoria 0001

Handling avocados

Picking

Picking maturity

The maturity of the fruit is closely related to moisture content. The fruit is normally ready to be picked when it has a moisture content of about 80 % or less.

The following procedure may be applied to determine maturity:

Harvesting

Always handle fruit carefully during harvesting and packing because avocados are bruised and scratched easily.

Precautions

Handling

Grading and packing

Cold storage

To delay ripening, fruit must be stored at a low temperature as soon as possible. The lower the temperature, the longer the fruit will take to ripen. However, storage temperatures that are too low will cause cold damage of fruit. A temperature of 5,5 °C is generally best. Early-maturing avocados may be kept at a slightly higher temperature while late-season fruit may be kept at a slightly lower temperature.  

For further information contact the
ARC-Institute for Tropical and Subtropical Crops
Private Bag X11208, Nelspruit 1200
Tel (013) 753 2071
Fax (013) 752 3854



This publication is also available on the website of the

National Department of Agriculture at:

www.nda.agric.za/publications

ISBN 1-86871-078-5

2000

Compiled by Directorate Communication,
National Department of Agriculture in cooperation with
ARC-Institute for Tropical and Subtropical Crops

Printed and published by National Department of Agriculture
and obtainable from Resource Centre, Directorate Communication
Private Bag X144, Pretoria 0001, South Africa