The cashew tree, Anacardium occidentale L., belongs to the Anacardiaceae family of plants, which also includes the mango, the pistachio and the poison ivy. The tree is native to Brazil, but has spread to other parts of tropical South and Central America, Mexico and the West Indies. In the 1600s, Portuguese traders introduced the cashew tree into India and Africa to prevent soil erosion. It is now widely cultivated for its nuts and other products in the coastal regions of South Africa, Madagascar and Tanzania, and in South Asia, from Sri Lanka to the Philippines.
The cashew tree is a tropical evergreen, resistant to drought, unexacting as to soil (although it prefers deep, sandy soil), which grows up to 12 metres high and has a symmetrical spread of up to approximately 25 metres. It has leathery oval leaves. Reddish flowers grow in clusters, and the pear-shaped fruits, referred to as cashew apples, are red or yellowish in colour. At the end of each fruit is a kidney-shaped ovary, the nut, with a hard double shell (Figure 1). Between the shell and nut is a black caustic oil which is difficult to remove and can be used in varnishes and plastics.
The cashew tree grows with a minimum of attention and is easily cultivated. It is usually found from sea level to an altitude of 1000 metres (3000 feet), in regions with annual rainfall as low as 500 mm (20 inches) and as high as 3750 mm (150 inches). For maximum productivity, good soil and adequate moisture are essential. Optimum conditions include an annual rainfall of at least 889 mm (35 inches) and not more than 3048 mm (120 inches). The tree has an extensive root system, which helps it to tolerate a wide range of moisture levels and soil types, but commercial production is only advisable in well-drained, sandy loam or red soils. The cashew tree can flourish in the sand of open beaches, but it grows poorly in heavy clay or limestone.
Most cashew trees start bearing fruit in the third or fourth year, and are likely to reach their mature yield by the seventh year if conditions are favourable. The average yield of nuts of a mature tree is in the range of 7-11 kg per annum. Although the cashew tree is capable of living for 50-60 years, most trees produce nuts for about 15-20 years.
Cashew seedlings present great variation in growth habit, quality of crop and yield. The tree cross-pollinates freely and this has further contributed to high variability although there are as yet very few named varieties of cashew trees. Generally a distinction is made only between those with yellow or red cashew apples. Tests have indicated that very large nuts usually have inferior kernels, low density and slow germination (Caribbean Technological Consultancy Services Network, 1993).
The cashew tree is usually grown from seeds placed directly in the field, since seedlings do not transplant well due to their delicate root system. Seed nuts should be thoroughly dry, clean and free from insect or fungal attack. Seeds should be stored until the next rainy season before they are planted in the field, unless irrigation facilities are available, or seedlings are raised in polythene bags in a nursery where water is available. After a few months, stored nuts gradually lose their germination capacity.
Seeds should be water tested prior to planting - those that sink should be chosen as they have a high success rate and tend to germinate quickly. Seeds should be planted at a depth of about 5 cm. The maximum depth at which a seed should be sown is about 10 cm, depending on the soil conditions. Two or three seeds should be planted together, stem end up, at a slight incline and covered with 5-8 cm of soil. Germination usually takes place in 15-20 days, although seeds of low density (i.e. those that float in water) may require as long as eight weeks to germinate. Using seeds of high density, from selected trees, considerably increases the chance that some of the seeds at one site will perform well. Two months after sprouting, the two weakest seedlings should be removed from the site, leaving only the strongest one to grow. By planting more than one seed, the occurrence of gaps in a plantation is reduced (Ohler, 1979).
In orchard practice, pits measuring from 30 cm ? 30 cm ? 30 cm to 60 cm ? 60 cm ? 60 cm are dug and left to weather for a month or two. They are then filled with soil which has been mixed with rotting manure about two weeks before seeding. In loose soils of sufficient fertility, the root system of the seedlings develops so quickly that, when growing naturally, root lengths would exceed the size of the planting hole within a few months and therefore in optimum growing conditions planting holes are not essential.
Advantages of sowing
Disadvantages of sowing
Cashew seeds should be sown or planted during the rainy season, in areas that lack irrigation facilities. The best time for planting is during periods of regular rainfall, so that the soil does not dry out again. If the soil dries out before the germinated seeds have become securely rooted, they may die. The advantage of early planting is that it gives the seedlings more time to develop before the onset of the dry season.
In climates with irregular rainfall and short rainy seasons, the risk of germinated seedlings drying out can be reduced by pre-soaking the seeds before planting, deep sowing (5-10 cm) and covering the site with mulching material to reduce evaporation. The mulching material should be carefully selected, and any weeds with mature seeds should be removed.
Where it is necessary to raise seedlings in a nursery, the seeds should be sown in containers of a type that can be set in the ground and readily disintegrate. In Cuba, baskets of uva-grass or cona brava are used, and cut away before setting the plant into the ground. In Jamaica, it has been found that nursery seedlings can be raised in the ground and transplanted with 90 percent success, providing the plant is taken up with a good ball of soil and the top is cut back by one third, when it is put into the field. It is recommended that the seedlings are transplanted within a week of emerging, to ensure that the transplantation is successful (Caribbean Technological Consultancy Services Network, 1993).
Seedlings can also be planted using plastic bags as containers. The seedlings should be lifted into their planting holes in the plastic bags, which are then carefully slit with a sharp knife and removed.
The lowest branches of the cashew tree tend to trail on the ground at a distance of several metres from the trunk. Where branches touch the ground, spontaneous rooting may occur. Covering such branches with soil and keeping the area moist encourages rooting, a method which has been used in India for a long time. However, such layers cannot be easily transplanted to other places, and the shape of the material tends to produce low trees of spreading habit. The number of layers that can be obtained in this way is also rather low.
Figure 1: Air-layering
A strip of bark about 0.5 cm wide is removed from either a year-old branch or a pencil-thick shoot (about 1 cm diameter) of the current season, at about 20-30 cm from the growing point. The exposed wood is wrapped with twine to prevent the bark from growing over it during the healing process and covered thickly with moist moss, wood shavings or sand. It is then wrapped securely in a sheet of 100-150-gauge plastic and the ends are tied tightly to the branch with twine (Figure 1).
After 20-30 days, callus is formed at the foot of the layer and 40-50 days later, small roots emerge from the callus tissue. After approximately 75 days, there should be adequate root formation (five or more well formed roots measuring 1.0-1.5 cm long) to separate the twig from the tree. The part of the twig below the layer is cut about halfway through. One week later, the cut is deepened and a few days later the layer can be removed and transplanted into plastic bags or other containers and hardened off before planting. If the layer is separated from the tree in one cut, the shock will be too great for it to survive.
The whole process takes about two and a half months, although the time varies depending upon the period of the year when the layer is being prepared. Layers that are produced early in the rainy season have time to establish themselves and develop a large enough root system to survive the dry season.
Air-layering has been one of the most successful methods of vegetative propagation in cashew. The method is rather laborious and the cost is relatively high, but the advantages of obtaining a plantation from high yielding, uniform material make these costs extremely worthwhile. A large disadvantage of this method, as with all methods of layering, is the relatively small number of layers that can be produced by one tree per year. It is estimated that from one tree, 80 to 120 successful layers can be obtained. In order to obtain sufficient material for a fairly large plantation, the first generation of air layers should be grown to maturity, and only then, from many trees of the same quality, could sufficient material be obtained for later plantings.
The technique of approach-grafting is relatively easy but labour intensive, and like air-layering has to be done in the field. Seedlings to be grown as rootstocks are raised in containers. Once they are 8-9 months old, they are cut back to half their height, and kept in grass baskets for a month in the shade until new shoots appear. The basket is replaced by a sheet of 100-gauge plastic to retain moisture and the seedling is joined to a year old branch of the same diameter on a selected tree as described below.
From the stem of the seedling, and from the shoot with which it will be united, strips of bark and inner wood, measuring about 5-8 cm in length, are removed (Figure 2). Both cut surfaces, which should be of the same size, are bound together with twine and the join is firmly tied with string, binding the seedling stem and the shoot together. In 90 days the union should be complete, and the grafted plant is gradually separated from the parent (Figure 3). A "v" cut is made half way through the branch 2 cm below the graft, and a similar cut is made in the rootstock 2 cm above the graft. Seven or eight days later the cuts are deepened and after a further period of four days the severing is completed. About 60 percent take is expected.
Figure 2: Inarching
Figure 3: Inarching
Cashew seedlings are very sensitive to competition with weeds, but in many areas, especially on sloping land, the vegetation should not be removed completely before planting because of the danger of water and wind erosion. In tropical countries with a marked dry season, and where the rainy season tends to start with torrential rains, the danger of erosion increases if large areas of land have been cleared.
If the soil is very sandy and subject to strong winds, clearing the land may result in severe wind erosion, which is virtually impossible to stop. In these areas, the land should be cleared in strips, which are perpendicular to the direction of the prevailing winds. Once the cashew trees have developed to such an extent that their rows act as wind breaks, the remaining land can be cleared. Another important reason for leaving strips of natural vegetation is to ensure that the insect population required for pollinating the cashew flowers is maintained.
The removal of tree stumps is an expensive but essential part of the clearing process. The sprouting stumps need to be slashed each year as they compete with the cashew trees and prevent a good view between the rows.
To promote maximum development and reduce competition for available moisture, it is recommended that the seedlings are planted 10-15 metres apart. This is considered to be the most productive spacing for mature trees. The average yield per hectare will be 700-1000 kg but yields outside these limits are encountered. However, cashew trees are normally planted more closely, which results in overcrowding, and they are often intermixed with other trees either in small orchards or in the wild.
Cashew is often grown as a casual crop by smallholder farmers and as a result its fertilizer requirements are overlooked. Also the trees are long standing and are frequently grown in soils that are of poor quality. As each season passes, the soils become more depleted and productivity gradually declines. Yields of trees grown in this way are very much lower than the potential that could be gained if fertilizer was applied.
Experiments have demonstrated that regular application of the major plant nutrients (nitrogen, potassium and phosphorus) is beneficial for healthy trees and increased cashew yields. In addition, application of magnesium to cashew is beneficial (Fernandopulle, 2000). Two separate mixtures of fertilizer, based on the combination of nitrogen (N), phosphorus (P) and potassium (K), have been recommended according to the growth stage of the plant.
Young plant mixture
This mixture is recommended for cashew plants from field planting up to 5 years of age. It contains N:P:K in the ratio of 4:3:2.
For 100 kg of young plant fertilizer, the following is used:
|Muriate of potash||15|
Adult plant mixture
This mixture is recommended for cashew plants from 5 years onwards. It contains N:P:K in the ratio of 4:3:4.
For 100 kg of adult plant fertilizer, the following is used:
|Muriate of potash||25.5|
Each planting hole should be filled with topsoil mixed with young plant mixture and dolomite and if possible with organic matter. In a hole that measures 60 cm ? 60 cm ? 60 cm, 250 kg young plant mixture and 300 kg Dolomite are required, plus 1-2 kg organic manure. The soil is removed from the hole and mixed with the chemical fertilizer. The hole is then re-filled with the mixture of soil and fertilizer. This should take place before the rains to allow the soil and fertilizer time to settle before the seedlings are planted. If the seedlings are planted directly into the fertilizer, the tender roots may be damaged. To avoid damage, the seedlings should be planted at the beginning of the monsoon or four to six days after refilling the hole.
During the early stages of growth, it is better if the amount is split and applied in two separate doses at the end of each season, to avoid the heavy rains.
|Time after transplant||Young plant mixture|
After 5 years of age, the rate of application of adult mixture varies with the plant yield.
|Average yield/plant||Adult plant mixture|
Fertilizer should only be applied after weeding and cleaning the base of the individual trees within a 1-2 metre radius, to avoid the competition for nutrients from weeds. In the early years (up to 1.5 year), fertilizer should be broadcast close to the plant, covering an entire full circle up to a distance of 0.5 m from the base of the plant. The fertilizer should be lightly mixed with the soil. As the plant grows older, the area should be gradually extended to reach 1.0 m. Mulching the fertilized area is encouraged as it is beneficial. With adult plants, the fertilizer should be broadcast in a circular strip (1-1.5 m wide) and about 0.5-1.0 m away from the base of the tree. The fertilizer should be lightly mixed with the soil.
Fertilizer should be applied to young plants twice a year at the end of each monsoon rain. It is applied to adult plants annually at the end of the monsoon rains and before flowering. The fertilizer should be applied when the soil is moist, at the end of the monsoon season so as to avoid the heavy rains. Application of fertilizer during the dry season is not advisable as the nutrients require water to be dissolved and absorbed by the roots.
Cashew seedlings can be affected by a number of different diseases. The fungus, Colletotrichum glocosporioides, is one of the most common pathogens in cashew (Ohler, 1979). Initial symptoms show the development of reddish-brown, shiny, water-soaked lesions, followed by resin oozing out onto the affected parts. As the lesions grow, the affected shoots and inflorescences are killed and the leaves become crumpled. The affected nuts and apples decay and shrivel, and the flowers turn black and fall off. The trees can be sprayed with various fungicides, including Bordeaux mixture, to control the fungus.
Several diseases that result in the terminal twigs dying off are grouped under the name "die-back" or "pink disease" which is produced by Glocosporium. There may be associated pitting of the surface of the nut. This serious disease requires the affected branches to be pruned and sprayed with a 1 percent Bordeaux mixture or other copper-based fungicides.
Characteristic symptoms of other infections which attack cashew include wilting and withering; the yellowing of the lower leaves; the seedlings turning pale and showing water-soaked girdles of darkened tissue around the stems; or the rotting of the underground portion of the stem. Powdery mildew may appear on young leaves and inflorescences during dry weather.
Insect pests are a major source of crop loss in all cashew-growing areas of the world. More than 60 species of insects attack the crop during its different stages of growth. These pests include sap-sucking bugs, leaf-chewing caterpillars, beetles, aphids, scales, thrips and some mites. They can cause considerable damage to the tree and the crop by bringing about the death of the floral-flushing shoots, the early abortion of young nuts and loss of yield.
The adult weevil, or borer, is dark grey in colour and about 2 cm long. The larva has a curled whitish body with wrinkled skin and dark brown head. The eggs are laid singly in small holes made by the female in the bark of the trunk or the main branches of the cashew tree. After hatching, the larvae tunnel down just beneath the bark, eating the sapwood. The grubs should be removed as soon as they are detected because they cause gum leakage and will eventually kill the tree. To control the borer, all heavily-infested trees should be cut down and the bark should be cut away in places that show signs of weevil attack. Once the wood has dried out sufficiently, it should be burned, to kill all the remaining larvae and eggs. Alternatively, the base of the tree can be sprayed with Malathion and the bore holes filled with the spray.
The tea mosquito bug (Helopeltis antonii Sign.) is a common pest found in Sri Lanka, causing up to 30 percent loss of yield. The adults and nymphs of the species feed on tender shoots and floral branches, as well as on the developing nuts and apples, by piercing and sucking the sap (Ranaweera, 2000). Helopeltis populations increase during the rainy season, reaching a maximum at the end of the rainy season and the beginning of the dry season. This coincides with the emergence of a new flush. Insecticide application should be started when the new flush emerges. The damage caused by Helopeltis is of two types. Primary damage, which is seen as brown lesions surrounding the feeding punctures, causes affected leaves and young shoots to turn brown, wither and die. Secondary damage occurs when the puncture holes are infected by secondary pathogens, and the whole stem or branch dies back or is affected by blight
Helopeltis can be controlled by chemical spray - three applications of a 0.1 percent mixture of Carbaryl is recommended at flushing, onset of flowering and the fruiting season. However, Carbaryl is indiscriminate and will kill other beneficial insects. Special attention must be paid to the second spray at the onset of flowering, to minimize the effect of insecticide on pollinators. The optimum time to spray is between 0600 hours and 0800 hours and again between 1600 hours and 1900 hours, when the Helopeltis bug is active.
A copper fungicide or Bordeaux mixture (1 percent) can be used to control inflorescence die-back, which may occur. Helopeltis damage can be limited by careful cultural practices. The plantation should be kept weed-free to remove alternative hosts of the bug. Removal of non-productive and diseased branches from the trees allows for improved air circulation and solar radiation within the crop canopy and can then reduce the humidity of the canopy. This in turn can reduce the damage caused by the pest.
Thrips are known in many countries to attack cashew leaves, causing a symptom called "silvery leaves". They suck the leaf juices and leave a "scorched" area. Heavy attacks will stunt the growth of young plants but can be controlled by spraying with a 0.05 percent solution of Malathion.
The caterpillar of the leaf miner attacks young plants by tunnelling through new leaf tissues. The first signs are winding trails on the leaves. Later the infestation shows as patches of white blisters. The adult is a tiny, silvery-grey moth. The larvae appear red when they are fully-grown and are about 6 mm long. Affected trees should be sprayed with a 0.06 percent solution of "Folidol E.605" (30ml in 50 litres of water).
A sporadic leaf-eating caterpillar, which has poisonous stinging hairs, sometimes occurs in swarms and defoliates the cashew tree. It pupates in silky cocoons and the adult moth is large and reddish-brown in colour.
The mealy bug attacks the cashew inflorescence but can be controlled by spraying with a 0.06 percent solution of "Folidol E.605" (30ml in 50 litres of water). Among the other pests are leaf webbers, flea beetles, spider mites and scales.
Rodents, such as rats, squirrels and porcupines, may cause serious damage to cashew seedlings, particularly when they emerge above the ground. Cashew apples are sometimes attacked by fruit flies. Monkeys are partial to ripe apples and can cause damage to the cashew trees whilst foraging for the fruit. Bats and parrots also eat cashew apples.
The harvesting and processing of cashew is very labour intensive. After producing clusters of flowers, cashews produce the edible apple, and also a nut encased in a heavy shell, which is the true cashew fruit. The cashew tree flowers for two or three months and fruit mature about two months after the bloom. The cashew nut forms first at the end of the stem. Subsequently, the stem swells to form the "apple" with the nut attached externally.
The cashew nut is 2.5-4.0 cm (1.0-1.5 inches) long and kidney shaped. Its shell is about 5 mm thick, with a soft leathery outer skin and a thin hard inner skin. When fully ripe, it falls to the ground. Harvesting generally involves collecting the nuts once they have dropped to the ground after maturing. Workers scour the area and detach the nut from the fruit. For the nuts to be easily traced, the surface under the tree has to be free from weeds. In some places, the whole area under the tree is swept free of dry leaves. The nuts are generally collected in baskets or sacks. Cashew fruit are generally left to fall to the ground before being collected, as this is an indication that the kernel is mature. If fruit are picked from the trees, the cashew apple will be ripe, but the kernel will still be immature.
The quantity of nuts, which can be harvested, depends upon the yield of the trees. Where many nuts fall together, much less time is required for walking in search of them. On average, each individual can harvest a maximum of 50 kg per day. A very limited number of nuts fall at the beginning of the production season. A peak in the number of nuts falling is gradually attained and production slowly declines. Although activities are labour intensive and time consuming, they are not heavy and women and children can help.
In very dry climates where the topsoil remains dry overnight, nuts can be left under the trees for several weeks without their quality being affected. However, where humidity of the air or soil causes moisture and dew formation, the nuts should be reaped at least twice a week. This is not very economical, unless it is carried out on smallholdings with relatively high labour intensity, as there will not be adequate numbers of nuts to harvest sufficient quantities to achieve collection levels of 50 kg per day (Ohler, 1979).
Apples to be used for processing into products such as jam or juices, should be picked from the tree before they fall naturally. On falling to the ground, apples may become damaged. Once damaged, the apples may ferment and deteriorate quite rapidly. The riper the apple, the sweeter the taste. It is therefore recommended that the apple is picked as it is about to fall. At this stage the nut is fully-grown for about two weeks and is ripe and ready for harvest.
Apples which are not within reach of the pickerís hands can be harvested using a small basket or sack attached to a ring at the end of a long stick. Fully ripe apples will drop into the sack when the tree is shaken. Apples that have not matured completely, should be cut off with a small knife attached to the stick. The nuts must remain attached to the apple, since some juice may be lost on their removal.
The cashew apple will only keep for 24 hours after it has been picked. Transporting large quantities of apples is difficult for this reason . When stacked in layers, apples may burst and lose their juice because of the weight on top of them.
Cashew nuts are dried in the sun for two reasons:
Cashew nuts should keep for 12 months or more, provided that they are dried a moisture content to eight percent or below, packed in sealed polythene bags and stored under dry conditions. The moisture content of cashew nuts at harvest, is dependent on climatic conditions, moisture content of the soil on which the nuts have fallen, weed growth density under the tree and the time between nut fall and harvest. High moisture content may cause deterioration of the kernel due to mould or bacterial attack, or enzyme action. Drying the nuts immediately after harvesting is essential in preserving their quality, but this process is often neglected.
Sun drying of cashew nuts can be done on specially prepared drying floors, or mats made of bamboo or palm leaves. The drying areas should be smooth and slightly sloping, so as to allow rainwater to run off. The cashew-nut layer on the drying floor should not be thicker than 10 cm, thus allowing for about 60 kg of nuts per square metre. The nuts should be constantly raked in order to ensure that they all receive the same benefit of the sunís rays and therefore they are dried evenly. The nuts should be heaped together and covered in the evenings. If the nuts are heaped while still warm, they will continue to dry under the cover of a tarpaulin. The nuts should be checked the following morning to ascertain the need for further drying.
Dried nuts should make a rattling sound when falling. Drying may take between one and three days depending upon local climatic conditions. As soon as the nuts are dry, they should be stored and protected from rain.
Technical requirements for storage are dependent on weather conditions. As cashew nuts are usually produced in climates with a long dry season, simple buildings with concrete floors and walls, and roofs of corrugated metal, should provide adequate storage.
Certain prerequisites must be satisfied to ensure safe storage:
|1||a waterproof, dry floor;|
|2||a firm and secure roof;|
|3||openings in the wall must be protected in order prevent water from entering the room;|
|4||headroom must be adequate, so as to allow the bags in a stack to be moved around if large quantities are to be stored;|
|5||the store should be easily inspected: there must be sufficient clearance between the wall and the bags, so as to allow individuals to walk around and check the condition of the stack;|
|6||the stack must be placed on a raised wooden platform, in order to prevent moisture from being drawn from the floor to the nuts|
Raw cashew nuts, stored in sacks, sometimes in the open awaiting shipment, and frequently without protection from rain, are subject to infestation through the stem-end, and this may go under detected until damage has progressed to the point of heavy loss. Infestation also occurs in the shelled kernels at various stages of handling.
The nut is encased in a rock hard shell that is virtually impossible to penetrate after harvest. In order to extract the nut, the whole shell is soaked in water, softened by steaming, and carefully air-dried to the final moisture content (9 percent). Each nut is hand massaged and cracked via a manual process that entails putting the nut against one sharp blade and bringing another blade, which is on a foot powered lever, through the outer shell. The blade on the foot lever is raised by an enthusiastic stomp allowing the outer shell to separate from the nut. The nut inside is carefully picked out of the outer shell using a nut pick.