The avocado tree can be found scattered island wide in St. Kitts and Nevis. The single solid stand of the crop is at the Wingfeild Estate. The trees are mainly seedlings and or the Pollock Simmonds and Lula varieties. In general the bearing season is between July and October. In this period (July-October) the crop has to be marketed locally.
1:1. The avocado pear is a member of the family Lauraceae. The genus is Persea and species Americana. It has been reported that there may be about fifty species in the genus Persea a considerable number of which like the avocado, are native to Mexico and Central America. Only the species Americana is commercially planted.
1:2. The fruit is used fresh and in salads, and is well known and favoured in the Caribbean. The fine oils which the fruit provides are used in the cosmetic industry where it is made into facial creams, shampoos and hair conditioners. The finished products attract some of the higher prices. Technology has been developed in some countries to process and can pure avocado and whole pieces.
1:3. There has been within recent times an increasing demand for fresh fruit in international markets. For the O.E.C.S. countries the normal season is between July to September. Outside of these production months March to June is considered early, while October to February is consider late. Guidelines for exporters of avocadoes, mangoes, pineapples, papayas and passion fruit to the UK market- the brochure prepared by the Commonwealth Secretariat Export Market Development Division on page 22 under the heading when to sell, provides the information under Timing “The market is supplied throughout the year by the major suppliers”.
Israel September- April
South Africa April-October
1.3 In the Caribbean many selected cultivars have been introduced from other avocado growing areas around the world; but there are literally several thousand seedlings scattered and growing. Only very limited selections have been carried out on this vast resource. A research effort in this activity suggests itself.
- There are three classified races, the Mexican which some authorities consider a distinct botanical variety- Persea Americana var drymifolia (Mez) = P drymifolia Cham x Schelecht, The Guatemalan and the West Indies also called Antilles.
- The Mexican race, is a native of the highlands of Mexico. It is the only race with anise scented leaves. The fruits are small approximately 8 oz or less ripening 6 – 8 months after flowering, with thin smooth skin, comparatively large seeds, (which are often loose in the cavity), the oil content of the pulp can be as high as 30 percent. It is the hardiest of all the races being resistant to cold; little commercial value; it however has good value as germ plasm for hybridization with the other races.
- The Guatemalan is a native of the highlands of Central America. It is less resistant to cold than the Mexican. The fruits weigh usually 1 – 2 pounds and ripen in 9 – 12 months after flowering. The fruit is borne on a long stalk, it has a thick skin unlike the Mexican, the seed is held tightly in the cavity.
- West Indian (Antillean) is a native of the lowland of Central America. The fruits are larger and ripen 6-9 months after flowering. They are borne on short stalks; the skin is smooth and leathery but not as thick as the Guatemalan. The seeds are large and often loose in the cavity. It is best suited to the hot low tropics because of its low cold tolerance.
- All the known avocado cultivars can be classified within one of these races or have been produced by inter racial crossings which may have occurred naturally or artificially. The color and scent of the flush gives a good indication of the race as follows, – applicable to those cultivars which are non inter-racial hybrids
|Leaves||Colour of Flush||Race|
|Leaves with anise scent||Green coloured flush||Mexican|
|Anise scent absent||Flush green coloured||West Indian|
|Anise scent absent||Flush reddish/violet||Guatemalan|
Table 1: provides the main differences between the three avocado races (Ref. Morin C and Bakula M 1967).
- THE TREE
- Once soil and climatic conditions are favourable, the tree grows vigorously. The wood tends to be very brittle and can sometimes snap with the weight of its fruit, the tree should be protected from strong winds. Trees can be long lived and some have been recorded as being over a 100 years old and in production. The average height tends to be 30 – 50 feet. Some grafted cultivars are much shorter and tend to spread with a trunk diameter of between 20 and 30 inches. The average commercial productive life is between 40 and 50 years. As the tree ages the trunk can become hollow and snap off. The avocado is generally classified as an evergreen but some cultivars shed their leaves just prior to blossoming but regain them in a short time or almost immediately, when its sends out new flush.
- The tree is shallow rooted and there are no visible root hairs, the root system ramifies into the soil more or less following the leaf canopy. The seedlings are normally tap rooted unless there is some mechanical damage to the tap root, in which case it develops a lateral root system. Grafted trees tend to develop a lateral root system.
- The complete and hermaphrodite flower are borne in pannicles and are crowded at the end of the branches. The flower is small and light green and yellow in colour. The calyx is composed of three sepals and the corolla three petals, the stamens are nine in number and in 3 whorls, with those in the inner whorls been longer, each having two orange coloured nectaries at the base, the ovary is single celled with a slender style and simple stigma. Only a small fraction of the large number of flower produced will set fruit.
The avocado flower is entomophilous, bees been the principles pollinating agent as evidenced by their large numbers and droning sounds once the tree blossoms. The avocado flower exhibits a synchronous dichogamy with two distinct cycles.
In the first flower opening the female organ (stigma) matures (protogynous) and is ready to receive the pollen but the stamen are not ready to shed them. In the second opening the flower performs as male (Protandrous) and pollen is shed from the stamen- the stigma is non receptive at this time. This phenomenon is described as obligatory cross pollination and is a constraint for self-pollination. It is normally not restricted to the same flower but to the entire tree, and in most cases to all trees within the same cultivar. Cultivars are grouped into two distinct types A and B according to the performance of their flower.
Group A – Flowers of the plants in this group open first in the morning as female and close by midday. They open again in the afternoon of the following day but as male. The cycle takes 36 hours – from midday day one to afternoon. It should be noted in the same day and on the same tree many flowers are protogynous, while others will be opening in the afternoon (Protandrous).
“A flowers” are female (receptive to pollen) in the morning and male (shedding pollen) in the afternoon.
Group B – The cycle is that flowers open in the afternoon and close in the night. (Protogynous) the second opening is on the following morning. The total cycle is 24 hours. As in the case of Group A, flowers the same day and for the same tree many flowers will open (second opening) in the morning (male) and many others first open in the afternoon as females.
“B flowers” are male (shedding pollen) in the morning and female (receptive to pollen) in the afternoon.
- To ensure pollination and fertilization it is necessary to interplant cultivars of both groups. The periodicity is most marked in bright clear warm weather, (typical of the Caribbean) but is less marked on cool cloudy days.
Figure 1 Performance of the Avocado Flower
|Group||Flower opens in the Morning as||Flower opens in the Afternoon as||Total cycle|
The fruit is a berry with a single seed. As has already been mentioned it varies in size. There are four fruit shapes (a) Elongated (b) Pear (c) Round (d) Oblong, the shape more or less conforming to seed shape. The fruit is attached to the tree by a long peduncle which depending on the point of attachment is classified as either symmetrical or asymmetrical. The peel (exocarp or skin) presents a wide variation and can on the ripened fruit be light green and purple. The seed is covered by two thin usually papery and light brown layers that contain variable amounts of inhibitors to germination. It is recommended that these layers be peeled off or a slice be removed at the distal end before planting to accelerate the process of germination. When fruits are mature seeds are either close in the cavity or loose. In those cultivars where the seed is loose a light shaking of the fruit is used to determine fruit maturity. For long journeys the loose seed is an undesirable feature as it could lead to poor quality fruit at journeys end.
- The principle commercial cultivars in the Caribbean belong either to the West Indies (Antillean) or are hybrids between West Indian x Guatemalan or West Indian x Mexican. As has already been mentioned there is a vest number of seedling trees with the obvious germ plasm bank potential. This writer had made some attempts at the selection, but it needs a dedicated and determined effort to continue the exercise. When attempting a particular seedling tree or collecting scions from a particular seedling. The parameters which are suggests are as follows:
- THE TREE
Location – where is it growing and the existing conditions and if it is a single tree or one of several.
Type of Growth – Upright or Spreading
Age – the approximate age should be ascertained if possible
Health and Vigour – Does it appear healthy? Is the growth vigorous?
Flower Type – Group A or B
Time from flowering to maturity (Early, Normal, Late)
Race – Hybrid, West Indian, Guatemalan or Mexican (this may need the assistance of a specialists.
Yield – High, Medium, Low- an estimate is necessary
- THE FRUIT
Shape – elongated, pyriform, round or oblong
Skin – smooth, thin, thick, peels easily, brittle
Size – large, medium or small
Weight – in grams (Mean of several)
Mesocarp – width- in millimeters/centimeters
Mescorp – freedom from string
Mescorp – dry, watery, oily
Mescorp – organoleptic
Seed cavity – large, medium, small, tight, and loose
Seed size – large, medium, and small
Pest and disease – susceptibility
AVOCADO INTRODUCTION DIARY
It is advisable that one be kept in which the above should be noted in detail, Pictures should be included to provide visual data.
- MAJOR DISEASES AND PESTS
- The disease of major economic importance worldwide is Phytopthora Root rot. The disease has been reported in all the major avocado growing regions of the world. (Australia, Caribbean, Israel, Kenya and the U.S.A). The disease is caused by a fungus Phythophthora cinnamomi or cinnamon fungus.
- On infected trees the leaves are fewer and smaller than normal and are usually pale and appear thin, yellow green instead of dark green; twigs often die back and fruit drop. The diseased tree will often fruit profusely due perhaps to the girdling effect through root loss; fruits remain small and undeveloped and as the disease progresses fall off. These are the above ground diagnostic symptoms. The roots become black and brittle and soon die. They are unable to absorb water and nutritional elements, with the results been the eventual death of the tree.
- The casual organism (pathogen) is a fungus and a water mould because it thrives in wet places requiring water for completion if its life cycle. It can exist in soil for up to six years in the absence of a host plant by forming resistant thick walled spores. Infection of a healthy plant commences with the attachment of zoo spores (swimming spores) to healthy feeder roots which it penetrates. The life cycle of the pathogen is completed with the production of both sexual and asexual spores.
Water logged conditions, poorly drained soils and soils with poor aeration predisposes the avocado roots to infection.
- Use of healthy planting stock
- By the careful selection of soil types – well drained deep and those in which avocado is not known to have succumbed
- Use of a resistant/ tolerate rootstock
- Use of chemotherapeutics – metalaxyl – Ridomil plus Phosetyl – Al (Aliette) at the recommended rates. Ridomil 0.15 g a.i/m2 as a soil drench and Aliette injectable formulation at 0.49 a.i. /m2 where m2 = tree canopy. The above treatment shows promise under Australian conditions.
- The avocado fruit can have severe blemishes which are caused by fungal organisms- these blemishes may result in fruit rot which render the fruit unmarketable. The pathogen Botryodiplodia theobromae , a fungus, usually occurs in the stem causing a stem end rot with greyish water soaked spots in the center, and the whole fruit rotting. The anthracnose fungus – Collectortrichum gloesporoioides infects the immature fruit where it remains inactive until the fruit begins to ripen when black lesions will appear then the fruit softens.
Avocado scab is caused by the fungus Elsino persea. Some varieties appear to be resistant/tolerate to the disease. A reduction of humidity and aeration of the tree by pruning and allowing light into the canopy will assist in controlling the disease. In the cases of stem rot and anthracnose a spray programme with a copper fungicide (where this is feasible) or the use of post-harvest treatments including storage controls of temperature and humidity. Post-harvest dips are also useful. (Benomyl Imazil, Thiabenzaole).
- Pest of the avocado include scale insects, thrips, mealy bugs, aphids white flies and mites. The tree is very susceptible to termite attack.
- PROPAGATION OF THE AVOCADO
5.1 The avocado can be propagated sexually, or asexually by cuttings. By far the most widely used for commercial production is asexually where a chosen seed is grown as a rootstock and a graft or bud is put on as the scion.
5.2 In avocado propagation because the tree is so susceptible to the disease P. cinnamomi special care must be applied at the nursery.
Fruit for stock should come from vigorous healthy trees. Seeds should, where possible receive hot water treatment by the immersion in hot water at 52 degrees C for 10-15 minutes.
The seeds are then sliced, and a piece removed at the distal end with a disinfected knife (Dipped in alcohol).
Potting bags where used should be of the right colour (black preferred) thickness 300 mm gauge and sized 6 by 14 inches.
A shade house is required in which 40 – 55% of the incident light is excluded. Seeds after sowing germinate in 12-15 days. Depending on the nutrient status of the soil etc they grow vigorously and are ready grafting 12 – 14 days later or when the seedling is changing from red to green.
The favoured method of grafting is the use of a terminal wedge graft. The scion should be yellow on top and green on the bottom, and should be collected only from healthy trees of the named variety. Scions should be taken which match approximately the diameter of the stock. The scion once removed from the parent plant should be placed in a clean piece of muslin cloth previously dampened. Scions should be wrapped in this damp muslin when being taken to the nursery. Scion should best be collected early in the morning.
The principal tool is a clean sharp budding knife. Budding tape (polyethylene), of which there are proprietary brands, should be used.
As mentioned earlier, the top of the seedling is cut off at a given height, the scion is cut into a wedge, the stock is cut vertically downward and the wedge shaped scion inserted between the cut surfaces. The interested scion and stock are then bound tightly. The graft is then completely covered with a suitable sized clear polyethylene bag. The bag is tied loosely around the grafted stock. The covering prevents water entering the cut surface, and allows the grafted plant to be watered normally. In a well-managed nursery the grafted plants are carefully labelled as to cultivar.
As the grafted plant is growing several axillary buds emerge, these should be pinched off. Once the graft is caught (as evidenced by new flush on the scion) the grafting tape should be slackened and retied loosely to allow normal growth and to prevent constriction of the new plant at the union stock and scion.
6.1 Special care should be taken when selecting the soils in which the avocado plants are to be transplanted. These have already been discussed. Planting distance ranges from 25’ x 25’ – 28’ x 28’. Planting holes are generally 18’’ x 18’’ x 18’’. It would be useful to incorporate some well-rotted pen manure which was previously steam sterilized, a compete fertilizer (12:12:17:17) about 2 ozs may be used as a basal dressing. The plant should be carefully removed from the bag. The top soil which was placed in the same manner in which it was removed should be back in the same way, and pressed firmly around the plant.
The plant should be sheltered from the wind by the use of a feed bag or palm frond etc. Plants should be watered as a necessary. In the first 2 – 3 years the plant should receive a little compound fertilizer applied often.
- PRUNING AND AFTER CARE
7.1 The tree appears to require little pruning. De-heading to shorten the tree and force more growth (horizontally) in younger branches appears to be necessary in varieties like the Lula. A wound dressing should be put on the cut surface after de-heading.
7.2 The normal agronomic practices like for most crops are necessary- weed control, integrated pest management fertilization etc.