Dr Guy Witney, project manager at Great Brak Avos in Mossel Bay, Western Cape, says avocados likely originated from the mountainous rainforests straddling Mexico and Guatemala. Today, wild avocados are found in the coastal tropical lowlands and mountainous highlands of Central and South America.

Avocados were probably introduced to South Africa by Dutch settlers coming from the West Indies between 1652 and 1700.

“Interestingly, in some parts of South Africa, large, low-oil fruit from West Indian seedling trees, often referred to as ‘butter avocados’, are still common backyard trees and are preferred over higher-quality Mexican-Guatemalan hybrid avocado varieties introduced later from California,” he explains.

Dr Guy Witney (pictured) says avocado tree grafting involves the connection of a branch of an avocado cultivar with the rootstock of a different tree. Grafting provides strong support for the tree as well as disease resistance, specifically against Phytophthora root rot.

Hass and Fuerte

Between 1932 and 1938, the first experimental avocado cultivar evaluation block was planted by the then department of agriculture near Mbombela, then known as Nelspruit.

Once it was confirmed that imported California varieties performed well in the region, the first commercial plantings took place in the same area in 1938. These first commercial plantings were mainly of the Fuerte variety and were established in the mountainous areas of the Lowveld near Tzaneen and Mbombela.

“Although there are hundreds of varieties of avocado in the world, around 90% of cross-border international trade in avocados is of the Hass variety. This variety is also the main cultivated variety in South Africa, followed closely by Fuerte. Other important varieties grown in South Africa are GEM, Lamb Hass, Maluma, Carmen, Pinkerton, Ryan, and Reed.

“Mexican-Guatemalan hybrid varieties introduced from California in the 1930s now dominate commercial production in South Africa. The green-skinned Fuerte dominated production until early in the 21st century, and while it remains important, it has been overtaken by Hass and newer dark-skinned Hass-like varieties like GEM and Lamb Hass,” says Witney.

The local avocado industry currently consists of around seven million trees planted on approximately 20 000ha. Production varies between 120 000t and 160 000t per year.

The most important production areas are the humid, subtropical mountain foothills of Limpopo and Mpumalanga, the KwaZulu-Natal Midlands, the Southern Cape coast of the Western Cape, and the interior valleys of the Amatole Mountain foothills in the Eastern Cape.

Moderate temperatures

Witney explains that avocados are adapted to fairly moderate temperatures and do best in areas with daytime highs of between 24°C and 34°C and nighttime lows of 8°C to 20°C.

Most varieties do not fare well at sustained daytime temperatures of above 35°C and are damaged when temperatures drop to -1°C, even for just a few hours. Sustained temperatures below 0°C will kill avocado trees.

“Conventional wisdom at the turn of the century dictated that avocados could only be grown on deep, well-drained, red-coloured Hutton series soils often found in moderate- to high-rainfall areas in Limpopo, Mpumalanga and KwaZulu-Natal, with summer rainfall ranging between 800mm and 1 400mm.

“Today, however, new soil preparation technologies and new clonal rootstock varieties have allowed for successful orchard development in areas with soils previously thought unsuitable for avocado growing. These areas, such as those in the Western Cape, may have shallower soils and receive only 400mm to 800mm of rainfall per year,” he continues.

Planning an orchard

Witney warns prospective producers that before they even consider establishing an avocado orchard, they must ensure the area earmarked to be planted to avocados has adequate high-quality water to support a mature avocado orchard.

Avocados require 3 000m³ to 5 000m³ of water per hectare per year, depending on rainfall. Witney also strongly recommends that a soil survey be done to make sure there is enough well-drained topsoil to make proper ridges on which to plant the trees.

According to him, a properly planned and established avocado orchard should last at least a generation, perhaps longer. In fact, some of the very first Hass groves planted in California more than 70 years ago are still in production.

“For this reason, I often point out to potential growers that they only have one chance to get land preparation right; and while preparing the ground properly is expensive, the payoff way exceeds the initial input. A professional contour survey may be required first to determine the best orientation and spacing of tree rows so that water drains away during periods of heavy rainfall,” he says.

Adding ameliorants

Generally, the next step is to deep rip and cross-rip the soil to around 1m deep, which will break impervious layers in the soil profile and help loosen the top layer for further preparation.

“After ripping, soil ameliorants are added to adjust pH and soil fertility, following the recommendations made from the soil survey. Ameliorants are generally spread using GPS-enabled equipment and are then disked in,” adds Witney.

The final step is to build ridges of between 0,7m and 1m high, around 3m wide at the base and 1m wide at the top, and anywhere from 6m to 8m apart.

Ridges are generally made by contractors using either grading equipment, bulldozers, excavators, or tractor-drawn offset discs. The equipment scrapes together all the loosened topsoil and incorporates it homogeneously into the ridges along with the required ameliorants.

Although the preparations for the establishment of an avocado orchard could be considered expensive,
Dr Guy Witney explains that the return on the initial investment far exceeds the investment itself.

Irrigation

The final step in the preparation prior to planting the trees is to install an irrigation system.

Modern micro-irrigation systems used in avocado farming were pioneered by Israeli companies and are extremely efficient in providing the exact water requirements of the trees without waste. This makes avocado farming one of the most efficient of all agricultural enterprises in terms of fruit nutritional value per litre of water used.

Irrigation systems commonly used in South Africa are similar to those in California, but Israeli-designed mini-sprinkler systems dominate the industry.

Grafting

“All commercial avocado trees are grafted trees. This is because there is tremendous genetic variability between seedling trees. Varieties have been selected by plant breeders that have the best fruit quality characteristics, called clonal scion varieties, and clonal rootstocks have been selected that provide strong support for the tree as well as disease resistance, specifically Phytophthora root rot resistance.

Other useful horticultural characteristics of clonal rootstocks include salt tolerance in challenging soils, or inducing precocity to the attached scion variety,” explains Witney.

This means each tree is the sum of two clonal parts. A clonal varietal top that produces fruit of superior quality is grafted onto a clonal varietal rootstock that has specific superior horticultural traits.

Top genetics

According to Witney, the combination of the top and rootstock makes for a highly productive tree but one that is relatively expensive among nursery fruit and nut trees.

Whereas a commercial macadamia tree may cost around R100, for example, a clonal avocado nursery tree could cost as much as R320, including plant breeder royalties.

While in the short term this high price may be a barrier to rapid orchard expansion, in the long term the superior genetics of the tree pays for itself over and over again.

“Compared with most crops, avocados suffer from relatively few pest and disease problems. Only during flowering and early fruit development are the fruit susceptible to attack by small sucking insects like stink bugs, and these are generally controlled by means of integrated pest management biologicals, among others practices,” he says.

Hedgerows

Most orchards are now trained to hedgerows with trees spaced approximately 3m to 4m apart in the row. Rows are spaced 5m to 6m apart. A combination of mechanical hedgerow cutting and plant growth regulators (triazoles) are used to control vegetative growth.

While in many growing areas summer rainfall is abundant, winters are generally dry and irrigation is required to maintain production and optimal tree condition.

Root rot

“Phytophthora root rot remains the most important disease challenge for commercial avocado producers. Agricultural controls, such as building ridges for the trees to grow upon to provide good drainage, providing organic mulches for disease suppression, as well as using root rot-resistant rootstocks, form the backbone of Phytophthora management,” advises Witney.

Avocados are susceptible to one serious viroid disease, namely avocado sunblotch viroid (ASBVd). The disease is spread through contaminated nursery stock and may spread in the field through contamination of field tools, or from tree to tree.

The industry is active in eliminating this scourge from commercial trees and has implemented an ASBVd-free nursery certification programme.

Most commercial avocado orchards use a system of annual leaf and soil analysis, coupled with crop removal information, to determine exactly how much fertiliser to apply each season.

Generally, soluble fertiliser applied via automated fertigation and manually applied granular fertiliser provide the precise nutrition an orchard requires.

Email Dr Guy Witney at guywitney@me.com.