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.
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
• The size of the containers in which the plants are grown, is important.
• The smaller the containers, the greater care will be needed after planting out the trees in the field to ensure their survival.
• The larger the container, the better the root system will have developed and the greater the chances of successful establishment.
• A clay soil mix tends to clog the drainage holes in plastic containers.
• Lighter soil mixes improve drainage and stimulate the development of the root system.
• Where mixes are too light, the soil is washed out easily through the drainage holes, resulting in exposure of the roots.
• The foliage of a tree indicates its health status. A healthy tree has a glossy, dark-green colour.
• Deformed or discoloured leaves are an indication that the tree has not developed normally during the nursery period. It may also be a sign of disease.
• The leaves must be inspected for the presence of insect pests and any deficiency symptoms.
• Any graft union that exhibits excessive thickening at the union, or large differences between rootstock and scion thickness, indicates incompatibility, a poor rootstock or a diseased scion.
• A first-grade tree should exhibit a smooth union, with few or no irregularities on the stem, either above or below the graft union.
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.
• The Fuerte cultivar, which is the most popularly grown cultivar in South Africa, is probably a natural hybrid between the Mexican and Guatemalan races and has a wider climatic tolerance (especially to cold) than the pure Guatemalan types.
• The minimum survival temperature is about - 4 °C, but no frost is tolerated during flowering.
• The optimum growth temperature is 20 to 24 °C, and high temperatures, especially during flowering, are not tolerated well.
• It is more sensitive than others to unfavourable weather conditions during flowering. Hot, dry conditions could result in low yields because of fruit and flower drop.
• Cool subtropical conditions with a mean daily temperature of 20 to 24 °C.
• Light frost can be tolerated, except during flowering and fruit set (August and September).
• For Fuerte, the daily mean temperature during flowering should preferably be above 18,5 °C, but definitely above 13 °C.
• A high humidity is desirable, because it decreases stress conditions (particularly high temperature), that play an important role during flowering and fruit set.
• The mist-belt areas of South Africa are especially suitable in this regard. The humidity should exceed 50 % at 14:00.
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.
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.
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.
• Only reddish-brown, red and dark-brown soils, particularly in the subsoil, are suitable.
• Temporary to permanent waterlogged conditions with concomitant root rot usually occur in yellow, grey, light-brown and white soils.
• Very dark and black soils usually have either a high clay content that could lead to poor root development, or a large percentage of organic matter that could result in excessively acid conditions and aluminium toxicity.
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.
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.
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
• Information on aspects such as pH (water), exchangeable quantities of sodium (Na) and the quantity of free lime present are obtained from chemical analyses of sampled soil from profile pits.
• The pH value (in water) of avocado soils should be between 5,0 and 7,0. Only at great cost and over a long period will it be possible to change pH values below 3,5 to make the soils suitable for avocado production.