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Rubber (Hevea brasilensis)

There are nine species of the genus, Hevea and of these species, Hevea brasilensis tends to be the most superior type because it yields the highest percentage of good quality latex.

Botany of rubber
Hevea brasilensis is an evergreen latex-containing tree rarely exceeding 25m in height. However, some wild types can be as tall as 30-40m. It has a straight and smooth grey bark. The bark is made up of three distinct layers. The innermost layer is made up of soft bast fibre formed from cambium, the middle is composed of intermediate hard bast fibres consisting of a mixture of soft tissues and clusters of stones. The outer protective layer is made up of coracles. The latex vessels are found mainly in the two inner layers. The leaves are spirally arranged, they are trifoliate (three leaves in a pedicel) and glabrous (i.e. smooth) with deciduous stipules. The leaf has a long petiole of about 2-1 cm.

The flowers are small and greenish white and are borne in many flowered auxiliary panicles. Its pollination is caused by bees, moths and flies. Latex is a whitish milky fluid obtained from rubber tree. Its composition is as follows:

Water             65%
Rubber           31%
Resin            1.5%
Protein           1.75%
Ash               0.5%

Also included in the latex are sugars, glucosides, tannins and alkaloids. The Kernel contains 40-50% semi-dry oil, which may be used for soap making.

(i)   Environmental requirement: Rubber is a tropical tree grown in the area lying between latitudes 15 N and 10 S of equator. This is located in the lowland tropical forest. Hevea brasilensis requires a range of rainfall between 2000-3000mm evenly distributed throughout the year for optimum growth and latex yields. Rubber thrives best at optimum temperature range from 25-38 degrees C provided adequate moisture is available. Latex flow is at maximum where there is high relative humidity, which keeps the cells turgid. Rubber thrives best at an altitude of 300m above sea level. In Nigeria, optimum growth and yields of latex is attained where there is deep, porous red clay top soil with sandy clay sub-soil. The soil must be well drained. It can tolerate a pH range from 4-6.5 for optimum growth and yields. It can tolerate a pH range from 3.8- 8.0. Young seedlings are more sensitive to pH. Adverse pH leads to poor growth. Rubber can also be grown on coastal soil that have poor drainage problem.

(ii) Nursery practices: Rubber plants can be raised from seeds. Viable seeds from good clones are selected after clone trials. Selection is based on yield and resistance to pests and diseases. In many estates, the seeds are collected from healthy trees, and also from fallen seeds between August-September. The main nursery must be sited on a well-drained area, near a source of adequate water supply and in an easily accessible area. Seeds are first pre-sprouted in the germination bed.

This bed is normally constructed in a shad area to prevent the penetration of ultra-violet radiation of the sun in the process. Sterilised soil or fine sand or sawdust is used for the germination. The seeds are pressed into the soil at a closest distance and to such a depth that the upper surface can be clearly seen. The micropylar ends should face the same direction to facilitate inspection. Viable seeds germinate within 3-25 days. Newly germinated seeds are removed and planted in either a well-prepared nursery bed at a spacing of 30 x 30cm, or in polybags filled with top soil.

Adequate watering of these seedlings is done and effective pest and disease control is practiced, coupled with the use of wire netting to scare rodents. Shade is necessary and should be provided for the nursery when seedlings are planted in the beds and then removed during the first half of their life in the nursery. Thinning is done to remove weak seedlings. Hand weeding is the most ideal method of weeding in the nursery. Use of herbicides should be avoided. The seedlings are normally transplanted into the permanent site or field when they have grown in the nursery for 12-15 months.

(iii) Lining out: The field is normally blocked. The block areas are lined out using the appropriate spacing method. Planting holes of 60 x 60 x 60cm are normally dug.

(iv) Transplanting: The transplanting of seedlings is usually carried out after topping, and the seedlings are allowed for 7 days in the nursery for the buds to germinate, followed by the uprooting of the seedlings with the whole tap root. The seedlings are finally transplanted into the prepared holes. The side roots should be placed in their natural position with the soil mark on the seedling at the same level with the soil level. Thereafter fill the holes with fertile top soil.

(v) Field maintenance

(a) Weed Control:
Weed control is necessary in rubber plantation, however, where a good ground cover has been established, weeding is confined to 50cm circular patch around each plant to prevent the vines of legumes from climbing rubber trees. Dalapon is used to control Imperata Cylindrical and other graminaceae. Paraquat, MSMA (Monosodium Methyl Arsenate) and DSMA (Disodium Methyl Arsenite) are also effective herbicides used for weed control in Rubber. Atrazine should be avoided, because it causes damage to rubber seedlings.

(vi) Nutrient requirements: Phosphorus is required at the early stage. Each plant is supplied with about 248g of rock phosphate at planting. Phosphate tends to drastically affect the building blocks of latex (i.e. Isoprene), in rubber trees. Nitrogen requirement is not specified especially in the presence of legume ground cover. However, nitrogen is required in the establishment phase. Organic manure tends to promote the rapid growth of rubber plants in a field. Other maintenance operation such as pruning, watering, pest and disease control and mulching should, as a matter of fact, be carefully carried out.

(vii) Harvesting: Harvesting of latex in rubber trees is done when 70% of the trees have attained a height of 100cm and a girth of 50cm. Trees attain the point of tapping age at 5-7 years. But the basic condition, which is emphasised, is usually the technical maturity. This depends on age, clone, type of seedlings (from seed or budded).

Tapping is simply the controlled removal of thin bark at interval from the lower surface of a groove made into the bark to a depth of about 1 mm from the cambium. The tools commonly used for tapping are the Jebong knife and the gauge. Tapping of latex is usually done in the early morning hours because the latex flow is highest at this period owing to the increase in turgor pressure. Therefore, optimum yield of latex should be obtained at the point when humidity is at its maximum and transpiration pull is at it lowest during the early hours of the day precisely, between 6 a.m -7 a.m.

(viii) Tapping system: The three basic tapping systems of rubber include
Brazilian, Herringbone and panel systems.

(1) Brazilian system: This is the earliest method used by the American, Indians and then the Brazilians. It involves extracting latex by daily cut of 2.5 to 5cm long along the stem of rubber for 100-150 days in a year and the trees are then left untampered with in the remaining part of the year. It is not a good system based on the reasons that:
(a) It creates deep wound.
(b) Leads to development of bumpy scars on the stem.
(c) Rapid consumption of trappable area and excessive contamination of latex by bark fragments.

(2) Herringbone system: This was evolved by Riley in 1897. In this system, oblique cuts are made in the bark with all the cut converging at the central vertical. The depth of cut is normally about 4-5mm. This system has been used in tapping Lagos rubber.

(3) Panel system (or method): This is the modern method of tapping and comprises of removal of thin shavings of the bark from the surface of a groove made at intervals into the bark at a depth of 1.0-1.5mm away from the cambium. A pole is used to demarcate the height at which cutting is to be done and a metal panel is placed where the cut is to be made. Cutting is done from high left to low right preferably at angle 30′ for budded plant and 25′ for seedling trees during the tapping operation. Tapping of latex facilitated by opening of the panel is never done in all the three systems during wet season because of the damage of tapping panel disease.

(ix) Processing: There are four general processing methods that can be used and the choice of the method that may be adopted depends on the good quality and quantity of rubber produced, the storage and transportation facilities and market demand.

The types of rubber produced are concentrated latex, crepe, sheet, and comminuted or crumb. Coagulation of latex obtained from rubber is done using coagulants such as formic acid, acetic acid or tannic acid. Anticoagulants such as sodium sulpite, ammonia prevents the coagulation of rubber latex.

Rubber diseases

(a) Mouldy Rot

Causative agent: Fungus.
Species: Ceratostomella timbriata.
Transmission: Transmitted through contaminated panel of tapping equipment.
Symptoms:
(i) Necrotic and chlorotic spots on panel.
(ii) Grayish mould cover and darkened spots in severe infection.
Prevention/control:
(i) By spraying fungicides on infected panels.
(ii) By using hygienic equipment in tapping operation.

(b) Black Thread or Black Stripe Disease

Causative agent: Fungus.
Species: Phytophthora SPP.
Transmission: Transmitted through air-borne spores deposited on tapping area in the trunk.
Symptoms:
Appearance of black stripe in the cortex and wood of the infected trees.
Prevention/Control:
(i) By regular pruning of trees.
(ii) By breeding resistant varieties
(iii) By “praying fungicides on infected area.

(c) Root diseases

White root rot, caused by Fungus: Forme ligneous. Brown root rot caused by Fungus Forme noxius. Red root rot caused by Fungus: Ganoderma Pseudoferreum.
Transmission: The rhizoids develop from the mycelia along the roots and remain attached firmly.
Symptoms:
(i) Poor growth.
(ii) Growth of thin foliage on trees.
(iii) Defoliation (leaves fall off).
Prevention/Control
i) By spraying with fungicides such as captan, Bordeaux.
ii) By regular weed control.

(d) Pest

Termites, cock chafergrubs, caterpillars, mealybugs aphids, snails, slugs, rodents, bats.
Prevention/Control:
(i) By spraying with insecticides.
(ii) By treating with Rigor 40.
(iii) Destruction of pests.

Rubber is a tropical crop grown in tropical forests. It yields optimum amount of latex in a mixture of well-drained red clay top soil and sandy clay sub-soil. It requires high amount of phosphorus for maximum yields. Latex is obtained from rubber.

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