Origin and distribution

It was suggested that avocados may have originated in southern Mexico but were cultivated from the Rio Grande to central Peru long before the arrival of Europeans. They were then carried to the West Indies and to nearly all parts of the tropical and subtropical regions with suitable environmental conditions. The distribution of avocado continued to the Philippines, to the Dutch East Indies by 1750 and Mauritius in 1780. Avocados were planted in Hawaii in 1825 and were common throughout the islands by 1910. It was first brought to Singapore between 1830 and 1840. It was introduced into Florida from Mexico by Dr Henry Perrine in 1833 and into California, also from Mexico, in 1871. Vegetative propagation began in 1890 and stimulated the importation of budwood of various types, primarily to extend the fruiting season. It reached India in 1892 and was grown especially around Madras but Bangalore has never become very popular because of the preference for sweet fruit. Currently avocados are commercially grown not only in the United States but throughout tropical America and the larger islands of the Caribbean. Other countries involved in cultivation of avocados are Polynesia, the Philip pines, Australia, New Zealand, Madagascar, Mauritius, Madeira, the Canary Islands, Algeria, tropical Africa, South Africa, southern Spain and southern France, Sicily, Crete, Israel and Egypt. 2 Early in the 17th century the Spaniards took the avocado to Chile, it was planted from the Peruvian border southward for over 1,600 km. It was reported that actual commercial plantings were not established until California cultivars were introduced in 1930 into two areas within 160 km of Santiago where the industry is now centred. The first avocad’s were planted in Israel in 1908, but named cultivars (Fuerte’and Dickinson) were not introduced until 1924. These aroused interest in the feasibility of the crop for the southern coastal plain and the interior valleys, and development of the industry has steadily gone forward.

Description of the plant

Growth habit The avocado is a dense, evergreen tree, shedding many leaves in early spring. It is fast growing and can reach 24 m, although usually less, and generally branches to form a broad tree. Growth is in frequent flushes during warm weather in southern regions with only one long flush a year in cooler areas. Foliage Avocado leaves are alternate, glossy, elliptic and dark green with paler veins. They normally remain on the tree for 2 to 3 years. The leaves of West Indian varieties are scentless, while Guatemalan types are rarely anise-scented and have medicinal uses. The leaves of Mexican types have a pronounced anise scent when crushed. The leaves are high in oils and slow to compost and may collect in mounds beneath the trees. Flowers Avocado flowers appear in January to March before the first seasonal growth, in terminal panicles of 200 to 300 small yellow-green blooms. Each panicle will produce only one to three fruit. The flowers are perfect, but are either receptive to pollen in the morning or shed pollen the following afternoon (type A), or are receptive to pollen in the afternoon, and shed pollen the following morning (type B). About 5% of flowers are defective in form and sterile. Production is best with cross-pollination between types A and B. The flowers attract bees and hoverflies and pollination is usually good, except during cool weather. Off-season blooms may appear during the year and often set fruit. Some cultivars bloom and set fruit in alternate years. Fruit Guatemalan types produce medium, ovoid or pear-shaped pebbled green fruit that turn blackish green when ripe. The fruit of Mexican varieties is small (170 g) with paper-thin skins that turn glossy green or black when ripe. The flesh of avocados is deep green near the skin, becoming yellowish nearer the single large, inedible ovoid seed. The flesh is hard when harvested

Soil requirements

Soil analysis

Before making a decision to plant avocados on a particular soil, an analysis should be carried out to determine the chemical suitability of the soil, and indicate what amendments need to be made prior to planting.

Soil colour

Only reddish-brown, red and dark-brown soils, particularly in the subsoil, are suitable. Temporary to permanent waterlogging with concomitant root rotting 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 can lead to excessively acid conditions and aluminium toxicity. The aluminium toxicity can be rectified through liming.

Soil texture

Avocados do best in soil with a clay content between 20 and 40%. If the clay content is below 20%, the soil has a limited water retention ability and unless optimum irrigation is applied, the trees will sometimes suffer from temporary drought. A high clay percentage makes irrigation difficult because over- irrigation and high rainfall lead to oversaturation of the soil. This means water drains away relatively slowly, which promotes root rot. At a higher clay percentage the resistance of the soil to root penetration is great and this adversely affects tree growth. An advantage of a higher clay percentage is that more water is available for the trees, which means that the irrigation cycles can be extended in accordance with tensiometer readings. The clay content can be up to 40%, provided the subsoil is apedal. In such soils the trees should be planted on ridges.

Soil structure

In soils with a moderately to strongly developed block structure, i.e. soils that can be broken into hard clods when dry, root development will be re- 7 stricted. Ideal avocado soils display only small, fine cracks when a dry profile wall is examined—in other words, it has a poorly defined structure or micro structure. Soils with a prismatic or column-like structure are therefore quite unsuitable.

CULTIVATION PRACTICES

Planting

Spacing is determined by the habit of the cultivar and the character of the soil. In light soil, 7,5 x 7,5 m may be sufficient. In deep, rich soil, the tree makes its maximum growth and a spacing of 9,1 or 10,7 m may be necessary. If trees are planted so close that they will ultimately touch one other, the branches will die back. Some growers plant 3 to 4,5 m apart initially and remove every other tree at 7 to 8 years of age. If the surplus trees are not bulldozed but just cut down leaving a stump, application of herbicide may be needed to prevent regrowth. Ammonium sulphamate has been proven to be effective. In modernised plantings, space between rows is necessary for mechanical operations. Holes at least 0,6 m deep and wide are prepared well in advance with enriched soil formed into a mound. After the young plant is put into place mulch is beneficial, weeds should be controlled, and watering is necessary until the roots are well established. Keeping the upper soil moist has been greatly facilitated by drip irrigation, which also may carry 80% of the fertiliser requirement. Because some cultivars tend to grow too tall for practical purposes, commercial growers cut trees back to 4 to 5,4 m, let them grow back to 9,1 m and top them again. However, decapitation is not a perfect remedy because the tendency of the avocado tree is to grow a new top very quickly. Avocado branches frequently need propping to avoid breaking with the weight of the developing fruit. Some growers find it profitable to interplant bananas until the avocado trees reach bearing age. Slope: Cold pockets will occur in low-lying areas such as the bottom of a valley as illustrated on the following page. It is important to remember that cold air is heavier than warm air. The cold air will accumulate in the lowerlying areas of an orchard, which could lead to frost damage. This should be kept in mind when establishing plums in areas that are prone to frost. 8 The steeper the terrain the more expensive orchard management becomes.

Fertilisation

Commence feeding of young trees after one year of growth, using a balanced fertiliser, four times a year. Older trees benefit from feeding with nitrogenous fertiliser applied in late winter and early summer. Generally small quantities of fertiliser are given every 2 months with the quantity gradually increasing until fruiting begins. Bearing trees need, on the average, 1,5 to 2 kg three times a year, beginning when the tree is making vegetal growth. No fertiliser should be given at blooming time; one must wait until the fruit is firmly set. Nitrogen has the greatest influence on tree growth, its resistance to cold temperatures, and on fruit size and yield. Fertiliser mixtures vary greatly with the type of soil. Mineral deficiencies determined by leaf analysis, are usually remedied by foliar spraying. Yellowed leaves (chlorosis) indicate iron deficiency. This can usually be corrected by a chelated foliar spray of trace elements containing iron. Mature trees often also show a zinc deficiency.

Irrigation

Avocado trees may not need irrigation during the winter rainy season, but watch for prolonged midwinter dry spells. Overirrigation can induce root rot which is the most common cause of avocado failure. To test to see if irrigation is necessary, dig a hole 30cm deep and test the soil by squeezing. If it is moist (holds together), do not irrigate; if it crumbles in the hand, it may be watered. Watch soil moisture carefully at the end of the irrigating season. Never enter winter with wet soil. Avocados tolerate some salts, though they will show leaf tip burn and stunting of leaves. Deep irrigation will leach salt accumulation.