AVOCADO (Persea americana)

Avocado is an evergreen tree belonging to the Lauraceae family that is native to Central Mexico. The fruit is highly nutritious with a high protein content and cholesterol free, highly digestible and nearly sodium-free oil desirable for a healthy diet. The main mineral components in avocado are magnesium, phosphorus, potassium and calcium. It is also rich in vitamins B1, B2, B6, C, D, A. It is a source of essential fatty acids that help to absorb vitamins A, D, E and K.

Varieties

The commonly grown varieties in Kenya are:

  1. Fuerte

It is a hybrid of Guatemalan and Mexican races with green and thin slightly rough skin. The flesh is pale yellow and almost fibre free with an outstanding flavor and fat content of 16-25%. The seed separates cleanly from the flesh. It has spreading growth habit and the recommended spacing should not be less than 8x10m. It matures in 6-8 months after flowering. Due to its long period of bloom, there may be more than one set of fruits on the tree simultaneously.

Fuerte

Hass

It grows vigorously with moderately spreading habit and bears early, regularly and heavily. The recommended spacing is 7 x 8m.The fruits are green at first and change to dark blue when almost ripe. The fruits are ovoid to pear-shaped with a tough pebbly skin which peels easily from the flesh. The fruit has a fat content of 18-23% resulting in an excellent nutty flavour. The fruits store well and have a good shelf life. It matures in 8-9 months after flowering.

Hass
  1. Nabal

This is a typical Guatemalan race. The tree grows vigorously and upright, bears heavily but has a tendency towards alternate bearing. It has green/yellowish flesh of high quality, nutty and melting. The oil content is about 9-16%. Fruits mature in 8-9 months after flowering. The recommended spacing is 8 x8m

  1. Puebla

This is considered a pure Mexican race. The tree is a rapid grower, erect, with drooping branches but does not set fruits regularly. In Kenya, Puebla is grown mainly as a pollinator for Fuerte. The flesh is light green in colour, juicy and of good flavour and an oil content of nearly 20%. It is mainly used as a root stock and matures 5-7 months after flowering.

Close-up of avocados, Cuetzalan, Puebla State, Mexico

Puebla
  1. Pinkerton

The tree is of medium size, but more spreading than Hass, bears early, regularly and heavily. The recommended spacing is 6 x 6m. The fruits are long pear shaped with dark green coloured skin, medium thick, leathery and pebbled. The flesh is pale green with high oil content (8-20%) and a smooth, creamy texture.

Pinkerton

 6) Other varieties

These include Simmonds, Booth 7&8, Becon, Lila, Taylor.

Ecological requirements

 

Altitude

 Depending on variety/race, avocado grows well from 0-3000m asl however altitudes of 1200-2100 m are ideal. Some varieties are adapted to warm low altitudes while others perform well in midland and high land areas. There are three distinct horticultural races of avocado which have specific climatic requirements and have hybrids between them. These avocado trees differ in length of time from flowering to fruit maturity.

 

  1. West Indian race

The West Indian race has large fruits that are pale green when ripe and sometimes with reddish blush. The trees are adapted to warmer lower altitudes of 0-1000m above sea level. They are early maturing e.g. Hardy and Simmonds.

 

  1. Mexican race

Fruits of Mexican race are relatively small, thin skinned and do not store well. They are adapted to higher elevations from 1500-3000m above sea level. They mature early to mid-season e.g. Puebla and Teague

  • Guatemalan race

The fruits of the Guatemalan race have thick, hard skins that are often pebbled or rough. They do well in 1000-2000 m asl especially Hass and Reed. They mature late in the season.

  1. Others

Tonnage, Simmonds, Booth 7 & 8 are suited to the lowland areas between 90-800m above sea level. Hass and Nabal are suited to altitudes between 800-2100 m. Fuerte hybrid and Puebla are suited to altitudes between 1500-2100m. Commercial varieties are Hass, Pikaton and Fuete;

 

Rainfall

Avocado is moderately drought tolerant but requires adequate and well distributed rainfall preferably not less than 1,000 mm per annum. If supplementary water is required, moderate irrigation of not more than 50 mm at a time may be applied. Critical stages of moisture requirement occur during flowering, fruit-set and fruit development.

 

Temperature

Different varieties require different temperatures from hot and humid coastal lowlands to the cooler areas. The maximum temperature for avocado is 33°C. Prolonged exposure to temperatures above 35oC damages the fruits and leaves thus reducing productivity. Avocado trees do perform not well in areas where frost is likely to occur.

 

Soils

Avocado grows successfully on a wide range of soils but does not tolerate flooding and poorly drained soils. Deep, fertile and well aerated soils with neutral pH are desirable. The optimum pH is 5.5-6.5. The roots are highly susceptible to high soil salinity.

 

Propagation

Avocado trees can be grown from seed but they do not grow true to type and most cultivars are therefore grafted or budded onto rootstocks that can improve tree health, yields and adaptability to a wide range of conditions.

 

Nursery Select healthy, egg sized seeds and plant them in boxes or seed beds. Immediately after germination, transplant the seedlings into 4 litre pots of polythene bags or tins. Water them until they are pencil size thick.

Grafting Grafting should be carried out when the seedling reaches pencil thickness. The wedge grafting method is the most suitable. Grafting should be done at the point where root stock is soft. The scion should be dormant at the time of grafting and should match the size of the stock. Wrap the grafting point thoroughly with grafting tape to exclude water from the joint area. A windbreak may be required to protect the plants from leaning to one side and prevent leaf shedding, fruit drop and bruising

 

Grafted seedling
Wedge grafting method

Orchard establishment

Site selection

When establishing avocado orchards, site selection is very crucial and protecting trees from wind is essential. Wind breaks are ideally placed at right angles to the prevailing winds.

 

Land preparation

The field should be cleared by removing the tree stumps, bushes and perennial weeds. The ground should then be ploughed about 30cm deep.

Planting holes of 60 x 60 x 60cm should be dug few weeks before planting and keep topsoil and subsoil separate.

 

Spacing

  • 9m x 9m for extensive management
  • 7m x7m for intensive management.

 

Planting

  • Compound fertilizers at the rate of 120g/hole and 15kg/hole of well decomposed manure should be thoroughly mixed with the top soil.
  • The seedlings are then removed from their containers and placed into the planting hole with the grafting union 30cm above the ground.
  • The topsoil/fertilizer/manure mixture should be placed in the hole first followed by the subsoil and then firm up. The remaining subsoil can be used to make a basin around the tree.
  • In the dry season, the plants should be watered immediately after planting.
  • Shade the plants with banana leaves or similar material when they are putting up a new flush.
  • Establish micro-water catchments at the end of the rainy season.
  • Watering is necessary during the dry season for 2 – 3 years.

 

Pruning

Pruning starts at planting where roots that are too long are cut back and branches cut back to one-third of their length to reduce water loss in critical times. During the early growing period, pruning is done to encourage lateral growth and multiple framework branching. In later years more pruning is done to remove undesired shoots inside the canopy. These include broken or diseased branches and the shortening of those touching the ground. This is important as incidence of body rots on fruits is more prominent on fruits at the bottom of the trees. There should be no sprouts below the graft union.

 

Weed control

Weeds can be controlled through use of manual weeding, cover crops (preferably with legumes), mulching or herbicides. However, use of heavy machinery is undesirable because avocado roots are usually superficial and can be easily damaged. The area around the tree corresponding to the diameter of the tree canopy should be kept weed-free.

Fertilizer application

Application of manure and fertilizer is desirable on the onset of each long and short rain. To determine the right amount, the soil should be tested annually. The general recommendation is to apply a total 30g of Nitrogen per tree in the first year and double the amount each subsequent year until 480g are attained in the fifth year.

Phosphate fertilizers will not be absorbed if soil is too acidic and this should be addressed by liming. Potassium is essential for yield and fruit quality of mature avocado trees as opposed to young non-bearing trees.

The following table can act as a guide on fertilizer application in the field:

Age of tree CAN (g/tree) TSP (g/tree) Manure (kg/tree)
1-3yrs 120 120 15
4-5yrs 220 450 15
6-7yrs 450 650 30
Over 8yrs 650 650 30

 

Trace Elements

Deficiencies in one or more minor elements may occur in avocado orchards. Deficiency symptoms are usually seen as varying degrees of leaf discoloration. Accurate analysis can be made at the National Agricultural Laboratories of the Kenya Agricultural Research Institute or other analytical laboratories to which leaf samples should be sent. Application of foliar feeds corrects these deficiencies as indicated in the table below:

Element Deficiency symptoms Correction
Zinc Mottled leaves with light yellow areas between the veins and abnormal development of the growing shoots Application of 250g of zinc sulphate for each year of growth to a maximum of 4.5kg. The application should be done in a 60cm strip round the drip area.
Iron Progressive yellowing of the margins and interveinal parts of the leaves while the veins remain green. Apply 360gms iron chelate per tree in acid soils. In normal soils 250gms of Iron sulphate in 10 litres of water corrects iron deficiency
Manganese Leaf chlorosis Apply a foliar spray of manganese sulphate to the young leaves

Pests

Chemical control of pests may not be necessary in avocado orchards except when infestations of economic importance have been determined. This is also important as it encourages build up of natural enemies like ladybirds, hover flies and parasitoid wasps in the orchard, which helps in biological control of the insect pests.

Pest Damage/Symptoms Control measures
Red spider mite (Tetranychus spp.) Clusters of mites on underside of leaves. Immature falling of leaves and fruits. Plant disease free material. Avoid excess irrigation.

Remove affected part of the tree.

Thrips (Scirtothrips perseae) Feeding is most common on young fruit; economic damage generally occurs on fruit up to 2cm in length (2-3 weeks after fruit set).

Older fruit with thicker skin is less susceptible to attack.  Silvering of the infested plant tissue which will then turn brown and dry up.

Other indications of attack are small black/shinning spots of excreta on the infested parts of the plant.

The naturally occurring predaceous thrips, Franklinothrips orizabensis, is the only biological control agent observed to respond in large numbers to the presence of avocado thrips populations.

 

Other general predators, such as lacewings and six spotted thrips, also feed on avocado thrips.

 

Chemical sprays are rarely necessary

Fruit flies (Ceratitis spp.) Eggs are deposited under the

skin of fruit which is just beginning to ripen, often in an

area where some break in the skin already has occurred.

Remove old fruit from under the trees and destroy them.

If the problem persists, spray the soil under the canopy with Neemroc EC, or Neemros or a solution of neem oil.

White flies (Aleurodicus spp)

 

Whiteflies are small (3 mm or less), flying insects that derive their name from the white wax covering their wings and body.

Whiteflies feed on plant sap. Honeydew excreted by nymphs collects dust and supports growth of sooty mould fungus and can attract ants, which interfere with the biological control of whiteflies and other pests

Several natural enemies attack the immature stages of whiteflies and provide partial to complete biological control when undisturbed by ants, dust, or insecticide treatment.

 

Diseases

Poor orchard management as well as poor post-harvest handling practices predisposes the fruits to diseases due to bruising and cracking. Maintenance of a clean orchard through removal and destruction of fallen fruits, leaves and twigs is recommended to remove debri that acts as habitat for diseases.

Disease Symptoms / Description Control
Anthracnose (Colletotrichum gloeosporioides) This is considered the most important postharvest disease in avocado fruits.

The infected ripening fruits exhibit circular brown to black spots on the skin.

The lesions are later sunken and in moist environment, they support growth of pink spore masses while the flesh beneath decays rapidly.

Orchard sanitation and pruning of canopies

Pre-harvest spraying with copper based fungicides and other suitable fungicides is also recommended. Such as Equation pro, Delan ,Tata shujaa, Daconil 750 SC, Bravo 500 SC, Captan, Ranko 75WP, Rankonil 500SC, Merpan 83WP, Rodazim SC, Bavistin, Kocide DF, Cobox, copper nodox, Delan etc

Cercospora fruit spot (Pseudocercospora purpurea) The lesions on fruits appear as small, scattered, brown, slightly sunken spots that have definite outline but irregular shape.  These fruit spots develop cracks or fissures, which permit entry of other fungi that, cause decay especially anthracnose Removal and destruction of  infected fruits

Scab(Sphaceloma perceae)

Symptoms appear as corky, raised, brownish, oval-shaped spots on fruit.

As fruits mature these spots coalesce and the centre become sunken with the skin becoming rough and cracked. These act as entry points for post-harvest pathogens especially anthracnose.

The disease is only capable of direct penetration of susceptible young developing fruits.  However they are said to become immune after reaching a certain stage.

Orchard sanitation
Root rot (Phytophthora cinnamomi) This is the most serious avocado disease that can destroy the whole orchard.  It attacks trees of any size and age.

Young feeder roots become black, brittle and die.

In advanced stages, only remnants of the root system remain.

Loss of feeder roots affects uptake of water and nutrients.  Initially there is heavy flowering and fruit set, but fruits remain small.

Later, leaves curl, droop and slowly become yellow.

 

Fruit shrivel on the tree and a scabby lesion may develop on the stem as a result of water stress, even though soil moisture is adequate.

The soil beneath affected trees remains wet long after rain, because the tree is unable to absorb the moisture

There is no simple solution to Phytophthora root rot in avocados, but good horticultural management can minimize losses.

Pick a site with excellent drainage and aeration.

Mulching promotes development of beneficial microorganisms antagonistic to Phytophthora cinnamomi and annual application of gypsum supplies calcium.

For best results, apply mulch and gypsum when the orchard is established.

Rotate infested plots with resistant crops such as citrus, vegetables, flowers or macadamia.

External anthracnose symptoms

Internal anthracnose symptoms

 

 

Cercospora spots

Scab symptoms

 

Thrips damage

Avocado thrips

Adult fruit fly

 

White flies

 

Harvesting

Harvesting usually begins in the 3rd year of tree growth and harvesting period varies between varieties. Commercial yields are obtained after the 5th year. An avocado tree can bear 250 to 300kg of fruit per year (8 to 10 tonnes per hectare per season).

  • Fruits are harvested when mature but still hard and then allowed to ripen. Grafted seedlings should be allowed to retain flowers as from the 3rd year, when the tree is able to support fruit weight.
  • Generally oil content is used to determine maturity universally. Avocado fruit at the time of harvest should not contain less than 8% oil in weight, excluding skin and seed. The oil content is determined by weighing 5-10 g of avocado pulp and then extracting the oil with a solvent (e.g. benzene or petroleum ether). This method is best for cultivars naturally high in oil content
  • For varieties that remain green on maturation e.g. fuerte, harvesting indicators include glossiness, colour change and skin texture. Mature fruits have no glossy shine and the skin is smooth at the end.
  • For varieties like Hass, maturity determination is easier as the fruit turns from green to purple and the skin gets rougher.

It is recommended that fruits be clipped from the tree by sharp clippers leaving at least 0.5cm of the stem slightly above the fruit shoulder and should never be dropped on the ground. Dropping fruits on the ground during harvesting exposes them to bruising which acts as entry point to post-harvest pathogens. However, frequent sterilization of the harvesting tools reduces the incidence of these rots.

 

Post harvest handling

It is also recommended that freshly harvested fruits be placed in cool storage within 24 hours to minimize the potential for development of high incidence and severity of ripe rots. It is important to keep the fruits in a cool environment away from direct sunlight to avoid build up of heat and consequent sunburn or loss of moisture.

 

Utilization

The fruit is eaten as a fresh fruit and salad. It is used as a flavour in the preparation of ice cream, a base for milk shakes and soups. Avocado oil is used in the cosmetic industry for the manufacture of creams, lotions and soaps.

 

Marketing

The major export destination for Kenyan avocado is the European Union market mainly France, for which Kenya competes with South Africa, Israel, Mexico and Spain. A smaller volume is also exported to Middle East. The main export varieties are Fuerte and Hass.