The avocado (Persea americana) is native to the Americas but grows well in Kenya. It is very nutritious with a high protein and oil content and is now becoming increasingly important as an export crop.
The fruit is nearly sodium free, rich in potassium dietary fibre and vitamin B6, C, D and E. It is eaten as fresh fruit, in salads, soups, ice cream, also used to make avocado oil, perfume and avocado paste.
The avocado tree may be erect, usually to 9metres tall but sometimes to 18metres or more, with a trunk 30-60cm in diameter, (greater in very old trees) or it may be short and spreading with branches beginning close to the ground.
The leaves are alternate, dark-green and glossy on the upper surface, whitish on the underside, variable in shape (lanceolate, elliptic, oval, ovate or obovate) and 7-40 cm long.
Small, pale-green or yellow-green flowers are borne profusely in racemes near the branch tips. They lack petals but have 2 whorls of 3 perianth lobes, more or less pubescent, and 9 stamens with 2 basal orange nectar glands.
The fruit, pear-shaped, often more or less necked, oval, or nearly round, may be 7-20 cm long and up to about 15cm wide.
The skin may be yellow-green, deep-green or very dark-green, reddish-purple, or so dark a purple as to appear almost black, and is sometimes speckled with tiny yellow dots, it may be smooth or pebbled, glossy or dull, thin or leathery and up to 6mm thick, pliable or granular and brittle.
In some fruits, immediately beneath the skin there is a thin layer of soft, bright-green flesh, but generally the flesh is entirely pale to rich-yellow, buttery and bland or nutlike in flavor.
The single seed is oblate, round, conical or ovoid, 5-6.5 cm long, hard and heavy, ivory in color but enclosed in two brown, thin, papery seed coats often adhering to the flesh cavity, while the seed slips out readily.
Some fruits may be seedless due of lack of pollination or other factors.
In Eastern Kenya, avocadoes do well in Kangundo, Mua Hills, Kathiani (Iveti hills), parts of Mbooni, all in Machakos County, and the high potential areas of the larger Embu and Meru counties.
VARIETIES
They include the following;
- Fuerte – bears thin skinned green-pebbled fruit of very good flavour. It has many lines with different shapes. The pear shaped fruit is preferred for export.
- Haas- bears medium-sized, rounded, rough-skinned, black fruits.
- Nabal – its fruits are green with a good flavour.
- Puebla- bears deep purple to maroon round fruits and is normally used as a rootstock.
- Others– Reed, Simmonds, Booth 7&8, Pinkerton, Bacon, Lula and Taylor
Fuerte‘ is the main variety grown in Kenya although ‘Haas‘ is developing strongly, especially among small growers. These two varieties are grown majorly for the export market. Other varieties like ‘Reed’, ‘Booth 8’, ‘Puebla’, etc. are not exported.
Ecological Requirements
Altitude
Depending on the variety, avocado grows well from 0-2500 m above sea level. Some varieties like ‘Simmonds’, ‘Booth 7&8’ are suited to lowland areas between 90-800 m asl., ‘Hass’ and ‘Nabal’ are suited to altitudes 800-2100 m asl. ,’Fuerte’ and ‘Puebla’ are suited to altitudes 1500-2100 m asl.
Soil
Avocado grows successfully on many types of soil provided they are deep, with good water holding capacity and free draining. Water logged or saline soils are unsuitable because avocado plants are sensitive to excessive soil moisture and high salinity. The optimum pH is 5.5-6.5.
Temperature
Temperatures between 16-24 degrees centigrade are good for growing avocados and the maximum temperature for avocado is 33 degrees centigrade. Above this temperature the fruits and trees can be damaged.
High temperatures and direct sunshine can cause sunburn damage to exposed fruits.
Avocado trees are not tolerant to frost especially those adapted to humid tropics. The sensitivity of trees to low temperature is influenced by a wide range of factors like the age of the tree, its vigour, growth stage and its health, among others.
Rainfall & Humidity
Avocados are highly adapted to different rainfall conditions, however, a well distributed annual rainfall of up to 1600 mm is optimal for good production. Climatic conditions with alternating dry and rainy seasons are best for avocados.
Irrigation is essential where rainfall is not adequate.
Too much rain during flowering causes flower abortion resulting in significant reduction in production. Fungal diseases are also problematic in very wet weather.
A short period of dry weather of up to two months usually triggers flowering especially in tropical climates not subject to marked falls in temperature.
The avocado tree requires high relative humidity at flowering, about 70-80 per cent, then moderate levels during the fruit swelling stage. Too much humidity encourages the proliferation of pests and diseases such as scales, scab and anthracnose among others.
FIELD OPERATIONS
Propagation
Propagation is majorly by grafting.
Grafting should be carried out when the seedlings reach pencil thickness. Wedge grafting method is most successful.
It should be done at the point where rootstock is soft, and the scion should be dormant at the time of grafting and should match the size of the stock.
The grafting point should be wrapped thoroughly to exclude water from the union and to prevent it from drying out.
Planting Procedure
- Land preparation. This is made easier and effective by use of CLAMPDOWN 480SL 200ml/20l, a non-selective herbicide which kills all types of weeds.
- Prepare planting holes about 60cm x 60 x 60cm. The general spacing for pure stands of avocadoes is 9m x9m.
- Fill the holes with topsoil mixed with manure and DAP. In order to improve on nutrients uptake by the young plants as well as stimulating growth, it is advisable to incorporate manure and DAP with HUMIPOWER at the rate of 1ton manure and 50kg fertilizer in 1kg Humipower each.
- Water the holes unless the soil is wet enough.
- Plant the grafts in the holes, to the same depth as they were in the nursery. The bud union should be about 300mm above the ground.
- Water the seedlings immediately after planting if it is not in the rainy season.
- Shade the young plants, and if planted in a windy area, a windbreak is also necessary to protect the plants from leaning to one side and to help prevent leaf shedding and bruising.
Cultural practices
Spacing
This is determined by the habit of the cultivar and the kind of the soil. In light soil, 25 x 25 ft. may be sufficient while in deep, rich soil, the tree makes its maximum growth and a spacing of 30-35 ft. may be necessary.
If trees are planted so close that they ultimately touch each other, the branches die back.
Pruning
All suckers and dead branches are removed from main trunk branches.
The canopy is pruned to keep the tree to a height of 5-8 m and for ease of picking.
Pruning should however be minimized because the tree is very susceptible to sunburn.
Weeding
The area around the tree should be kept clean by weeding and removal of all fallen fruits.
Weeds compete for growth factors like nutrients and water and harbour pests and diseases.
CLAMPDOWN 480SL 200ml/20l is a non-selective herbicide which controls all types of weeds in the garden.
PESTS & DISEASE MANAGEMENT
Pests
Fruit flies
Various species of fruit flies attack avocados. Some lay eggs under the skin of the fruit that is just beginning to ripen, but others attack young and old fruit. When the fruit reaches about the size of a golf ball a sting lesion appears as a slight puncture mark surrounded by a white exudate. As the fruit develops the lesion becomes dry and turns into distinct star-shaped crack on the skin surface.
Spray PENTAGON 50EC 10ml/20l or LEXUS 247SC 8ml/20l or PRESENTO 200SP 5g/20l
False coddling moth
After emerging from the egg, the young caterpillar tunnels into the fruit and a discoloration appears at the point of entrance. While inside they feed on the pulp, causing premature ripening and fruit drop.
Spray KINGCODE ELITE 50EC 10ml/20l or LEGACY 50EC 15ml/20l or SINOPHATE 750SP 20g/20l
Thrips
These are small, slender insects with two pairs of fringed wings which feed by sucking the plant sap.
They cause damage to the leaves and fruit and the affected parts become whitish or silvery and are usually covered by dark-coloured droppings.
Attacked fruits develop a leathery brown skin and feeding is most common on young fruits.
Spray ALONZE 50EC 5ml/20l or PROFILE 440EC 30ml/20l or DEFENDER 25EC 40ml/20l
Scales
These are small, stationary brown greenish insects commonly found sucking sap from avocado leaves.
The soft scales excrete large amounts of honeydew, which lead to the development of sooty mold on leaves, branches and fruit. The honeydew also attracts ants. Armored scales may encrust young twigs, leaves and fruit and do not produce honeydew.
Damage can be serious on young tress and small twigs may be killed. Although the presence of scales on the skin of fruit does not cause internal damage, it may lead to rejection of fruit, especially if grown for export.
Spray LOYALTY 700WDG 5g/20l or EMERALD 200SL 10ml/20l or LEXUS 247SC 8ml/20l
Use JAMBO CLEAN 100ml/20l to clean the sooty mold.
Spider mites
Attack by spider mites produces circular necrotic spots covered by dense webbing. As mite populations increase, feeding causes leaf distortion and eventual drop.
The growth of young trees is seriously affected and yields can be reduced significantly.
Spray ALONZE 50EC 5ml/20l or BAZOOKA 18EC 10ml/20l
Bugs
The adults and nymphs of the coconut bug feed on young and mature avocado fruit. Bug feeding causes necrotic bruise-like depressions. A hard lump develops, which can be easily removed when the fruit is peeled.
Helopeltis bugs prefer to feed on young plant tissue piercing the shoots, stems, leaves peduncles, petioles and fruits. Their feeding causes brown necrotic patches. Attacked leaves present angular lesion, which often drop out leaving holes as it attacked by biting insects. Feeding on young shoots causes dieback of the shoots, while on fruits, it causes a dark water-soaked mark around the feeding puncture, turning into a lesion with a light brown centre and black edge. The fruit may exude sap that forms a whitish deposit as it dries.
Stink bugs emit a characteristic unpleasant odour when disturbed and they usually feed on the developing fruit. The feeding punctures cause local necrosis resulting in fruit spotting, and deformation.
Spray LOYALTY 700WDG 5g/20l or EMERALD 200SL 10ml/20l or LEXUS 247SC 8ml/20l
Swarming leaf beetles
Swarms of this insect can cause severe damage to the new terminal growth. Damaged terminals have a burnt look and development in young trees can be severely retarded. Occasionally developing fruits are attacked.
Spray KINGCODE ELITE 50EC 10ml/20l or SINOPHATE 750SP 20g/20l or LEXUS 247SC 8ml/20l
Diseases
Avocado root rot (Phytophthora cinnamomi)
This disease can attack trees of any size and age. Leaves of infected trees are small, usually pale or yellow green, often wilted and fall prematurely giving the tree sparse appearance.
In advanced stages of the disease, branches die-back and fruit remains small and crop yield is drastically reduced. Feeder roots get blackened, decayed and die. The infected tree dies prematurely.
Drench soil with GEARLOCK TURBO 250WP 50g/20l or PYRAMID 700WP 100g/20l or CHANCETYL ELITE 800WDG 100g/20l
Anthracnose (Colletotrichum gloeosporioides)
This is a major post-harvest problem when fruit is at maturity stage. Infection takes place when fruit is still very young and the fungus stays dormant till the fruit ripens. The disease appears as sunken spots on the fruit and the spots are manifested as a rot, which can penetrate deep into the flesh. In wet weather, the spots may be covered with mass of slimy, salmon pink fungal spore mass.
The disease may develop very rapidly in storage if conditions are humid and warm.
Spray RANSOM 600WP 15g/20l or DUCASSE 250EC20ml/20l or EXEMPO CURVE 250SC 15ml/20l
Scab (Sphaceloma perseae)
The fungus readily infects young, succulent tissues of leaves, twigs and fruits.
Lesions appear as small dark spots, slightly raised, oval to elongate. These spots coalesce, giving a corky appearance to the surface of the fruits, impairing the appearance but not the internal quality of the fruit.
Fruits are only susceptible when young until about half size development.
Spray DUCASSE 250EC 2oml/20l or RANSOM 600WP 15g/20l or ABSOLUTE 375SC 10ml/20l
Cercospora leaf & fruit spot (Pseudocercospora purpurea)
This disease is primarily a problem to quality of fruits and the severity of infection varies from season to season and can cause significant losses.
On infection, lesions appear as small light-yellow spots on fruits and leaves, and later become reddish brown and eventually become hard and crack.
On leaves, the infected tissue becomes thin and brittle, and often drops out, leaving a ragged hole.
Spray EXEMPO CURVE 250SC 15ml/20l or RANSOM 600WP 5g/20l or DUCASSE 250EC 20ml/20l
Bacterial soft rot (Erwinia carotovora)
Infected fruit has a darkened metallic sheen externally. Internally, the flesh is grey to black and soft with a putrid smell.
Spray Copper-based fungicides like GREENCOP 500WP 50g/20l or TRINITY GOLD 425WP 50g/20l or COLONIZER 440wp 50g/20l
Stem end rot (Dothiorella dominicana, Phomopsis spp., Botryodiplodia theobromae & Lasiodiplodia theobromae)
A dark brown to black rot begins at the stem end as a dark brown ring and the rot proceeds towards the other end. This rot produces dark streaking of the water-conducting tissues, and this symptom distinguishes stem end rot from anthracnose.
Spray RANSOM 600WP 15g/20l or EXEMPO CURVE 250SC 15ml/20l or ABSOLUTE 375SC 10ml/20l
Nutrition
To obtain good growth and high fruit yields, it is important to supply the plants with necessary nutrients.
Basal and foliar fertilizers should be applied.
Basal fertilizers are absorbed by the plants through the roots and include DAP, CAN, NPK, UREA, among others. Farmyard manure could also be added, depending on the organic matter of the soil.
Foliar fertilizers are absorbed by the plants through the foliage and they supply both macro and micro nutrient elements. They include OPTIMIZER, DIMIPHITE, ZINC GOLD, LAVENDER, GOLDCHANCE SERIES, VITABOR GOLD, among others.
Application of these fertilizers prevents nutritional deficiencies.
NOTE
- Whenever doing foliar sprays, it is advisable to mix the product (insecticide, fungicide, foliar fertilizer or herbicide) with INTEGRA 3ml/20l. This is a sticker, spreader, wetter and penetrant, which improves the efficacy of the respective product.
- Alternation of various chemicals (especially fungicides and insecticides) throughout a crop’s season help in preventing resistance build-up by the pest, which could happen if only a single chemical was used.
- Timely application of products (fertilizers, insecticides, fungicides & herbicides) is very crucial.
- All basal fertilizers and manure should be mixed with HUMIPOWER, which adds organic matter, improves nutrient uptake, stimulates beneficial microbial activities and promotes electrochemical balance, among other benefits.
Maturity, Harvesting & Postharvest Handling
Avocados are ready for harvesting at 5-10 months after flowering. This depends on the variety and the ecological conditions of the region.
It may not be easy to tell when the fruits are ready for harvesting unless they are of the varieties that change colour at maturity. For instance; dark-colored varieties are usually mature when they start to turn from green to dark color, while green-colored varieties become smoother, may develop corky spots, and a yellow tint to skin and stem.
Clippers are used for low hanging fruits and for those higher up, a long handled picking pole with a sharp “V” on the metal rim is used to cut the stem and a strong cloth bag to catch the fruit.
Avocados do not ripen while they are still attached to the tree.
If allowed to remain too long on the tree, the fruits may be blown down by wind and they will be bruised or broken by the fall.
Immature fruits do not ripen but become rubbery, shriveled and discolored.
If picked when fully grown and firm, avocados ripen in 4-5 weeks at room temperature.
A ripe avocado fruit yields to a light pressure when gently squeezed and can be stored in the refrigerator for two to three days.
Avocados are easily bruised or scratched and must be handled with care and are packed and padded in single or double-layer boxes or cartons for shipment.
Avocados ship well and are exported under refrigeration in surface vessels. The fruits are subject to chilling injury (dark-brown or gray discoloration of the mesocarp) in refrigerated storage and degree of susceptibility varies with the variety and stage at harvesting and length of time in storage. Most commercial varieties can be held safely at temperatures between 4-13ºC for at least two weeks. The best ripening temperature after removal from storage is 15ºC.
