Tobacco (Nicotina tabacum L.) is a high value crop that is in wide demand throughout the world for the production of ciggaret, cigars and other tobacco product, and is the most widely grown commercial non-food crop in the world (Akerhust,1981). The sale of cured leaf and manufactured product is the major source of income for many countries and many governments rely heavily on taxes levied on sales to consumers. Tobacco originated as a natural hybrid in Central America and has been under cultivation for many countries for many centuries. By the time explorer from Europe come to Americas, tobacco cultivation was widespread in North, Central and South America and since then has widespread all over the world.
In Malaysia, Tobacco industry continues to play an important role in uplifting the socio- economic status of the rural population especially in Kelantan, Terengganu, Kedah and Perlis. Presently, there are 20,524 farm families, 355 tobacco curers, 1,300 grower curers and 25,384 station workers as well as 4,000 personnel employed in the four major cigarette manufacturers. Tobacco industry generates around RM150 million in income a year of which 38% of the income share goes to the farmers, 18% to the curers and the rest to suppliers of fertilizers, plastic materials, farm machinery, firewood for fuels and others.
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Around 70% of the annual local tobacco demand is met by local production and the balance is imported, especially those of higher flavored quality which is not produced in Malaysia, to meet the manufacturers= specification to produce premium brand cigarettes. Depend on the origin and the quality of the tobacco, the price of imported tobacco leaves range from as low as us$ 2.50(RM 9.50) to us$ 8(RM 30.40) perkg in 2002 excluding some related cost and in Malaysia usually the price range is between RM 11 to RM 13 perkg.
Tobacco is cultivated in marginal areas especially on the coastal brisk soil in the east coast of Peninsular Malaysia. Its production is to cater for the local manufacturing of cigarettes. Under the National Agricultural Policy (1992-2010), tobacco industry would still be continued in view of the fact that there are no other alternative crops that could generate competitive returns. Tobacco industry will be geared towards improving its competitive position in order to compete with the other ASEAN tobacco producing countries which produce at a substantially lower cost. The production system will be restructured with a view to reduce its cost of production, increase its quality and yield as well as to maximize income to farmers. This will be done through intensifying the grower curer system and the gradual phasing out of the curer system which recently dominates the production system.
Tobacco does not need a highly fertile soil, but it should be deep and well drained. Much of the flue-cured crop is grown in sandy soils; where nematode problems may develop rapidly whereas air and fire-cured tobaccos are often grow in heavier soil.
Nematodes are the only plant parasites belonging to the animal kingdom that are studied in plant pathology. Nematodes are characterized as that group of unsegmented, worm-shaped animals that arte rounded and generally bilateral symmetrical. Most of the several thousand species of nematodes live in great numbers freely in fresh or salt water or in the soil feeding on the microscopic plant and animals. Numerous species of nematodes attack and parasitize man and animals on which they cause various diseases. Several hundred species however are known to feed on living plants as parasites and to cause a variety of plant disease.
Nematode morphology varies widely, or as stated by Thorne "The infinite form among the nematodes are so variable in their respective characteristics that are so probably no one statement can be made concerning them that does not have exception". This observes variability is probably due to the fact that only a scant of 2% of the estimated number of species has actually been described. Generally, plant parasitic nematodes are small, 300-1000 Î¼ with some up to 4mm long by 15-35 Î¼ wide. Their small diameter makes them invisible to the naked eye, but they can easily be observed under the microscope. Nematodes are, in general, eel-shaped, round in cross section, with smooth unsegmented bodies without legs or other appendages. The female of some species, however become swollen at maturity and have pear shaped or spheroid bodies.
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Nematodes body is more or less transparent. It is covered by a colorless cuticle which is usually marked by striations or may show bristles, punctuation, warts, or other markings. The cuticle molts when nematodes go through their successive larval stages. The cuticle is produce by the hypodermis, which consists of living cells and extends into the body cavity as four chords separating four bands of longitudinal muscles. These muscles enable nematode to move. Additional specialized muscle exists at the mouth and along the digestive tract and the reproductive's structure.
The body cavity is rudimentary in its development and contains a fluid through which circulation and respiration takes place. The digestive system is hollow tube extending from the mouth through the buccal cavity, esophagus, intestine, rectum, and anus. Lips usually a protrusible, hollow, stylet or spear which originates in the buccal cavity and is used to puncture plant cells. The excretory system is not well developed in nematodes. On the contrary, the nervous system is well developed and consists of many nerves, ganglia, and sensory structures.
The reproductive system is well developed. Female nematodes have one or two ovaries followed by an oviduct and uterus terminating in a slit like vulgate male's reproductive structure is similar to the females but has a testis, seminal vesicle, and ejaculatory duct and terminates in a common cloaca with the intestine. A pair of prostrusible, copulatory spicules is also present in the male. Reproduction in nematodes is through eggs and may be sexual, hermaphroditic, or parthenogenetic. Many species lack males.
The life history of most plant parasitic nematodes is in general quite similar. Eggs hatch into larvae, whose appearance and structure are usually similar to those of the adult nematodes. Larvae grow in size and each larval stage is terminated by a molt. All nematodes have four larval stages, with the first molt usually occurring in the egg. After the final molt the nematodes differentiate into adult males and females. The females then produce fertile eggs either after mating with a male or, in the absence of males, parthenogenetically, or it can produce sperm herself.
A life cycle from egg to egg may be completed within 3 or 4 weeks under optimum environmental, especially temperature, conditions but will take longer in a cooler temperatures. In some species of nematodes the firs or second larval stages cannot infected plants and depend for their metabolic function on the energy stored in the egg. When the infective stages are produce, however, they must feed on a susceptible host or starve to death. Absence of suitable host may result in death of all individuals of certain nematodes species within a few months, but in order species eggs may remain dormant in the soil for years.
Almost all plant pathogenic nematodes live part of their lives in the soil. Many of these live freely in the soil, feeding superficially on roots and underground stems, and although they may cause injury to plants they are not strictly parasitic. Even in the most highly specialized sedentary parasites, the eggs, the preparasitic larval stages, and the males are found in the soil for all part of their lives. Soil temperature, structure (porosity), moisture and aeration affect survival and movement of nematodes in the soil. Nematodes occur in greatest abundance in a layer of soil from 0-15 cm deep, although species vary in this respect and their preference is influence by location and by the host plant. Distribution of nematodes in cultivated soil is irregular and is greatest in or around roots of susceptible plants, which they follow sometimes to considerable depths (30-150 cm or more).The greater concentration of nematodes in the region of host plant roots is due primarily to their more rapid reproduction on the food supply available and also attraction of nematodes by substance released by the rhizophere. To these must be added the so-called hatching factor effect of the substance originating from the root which diffuse or are carried into the surrounding soil and markedly stimulate the hatching of the eggs of certain species, although most nematodes eggs hatched freely in water in the absence of any special stimulus.
Nematodes spread through the soil very slowly under their own power. The overall distance traveled by nematodes probably does not exceed a few feet per season. The speed of movement in the soil seems to be related to pore diameter, particle size, and water content of the soil, and diameter sand relative activity of the nematodes. Nematodes move faster in the soil when the pores are lined with a thin film of water than when the soil is waterlogged. In addition to their own movement, however, nematodes can be easily spread by anything that move and carry particles of soil. A few nematodes that attack aboveground parts of plant spread through the soil and are splashed to the plant by the falling rain, overhead wintering, or ascent wet plant stem or leaf surface on their own power. Further spread takes place upon contact of infected plant parts with adjacent healthy plants.
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In term of habitat, pathogenic nematodes are either ectoparasites (species that do not normally enter root tissue but feed only on the cells near the root surface) or endoparasites (species that enter the host and feed from within) and both of these can either be migratory (species that live freely in the soil and feed on plants without becoming attached), or sedentary (species that once within a root, do not move about). The ectoparasites nematodes include ring nematode (sedentary), and the dagger, stubby root and sting nematode (all migratory).The endoparasites nematode include root knot, cyst and citrus nematodes (all sedentary),and the lesion, stem and bulb , burrowing, leaf stunt, lance , and spiral nematode (all somewhat migratory). Of these, the cyst, lance, and spiral nematodes may be somewhat ectoparasitic, at least during part of their lives.
Nematode infection of plant results in the appearance of symptoms on the root as well as on the aboveground parts of plants. Root symptoms may appear as hypertrophy, necrosis, or abnormal growth which includes root knot or root gall, root lesion, excessive root branching, injured root tip, root rot. All of these symptoms are usually accompanied by no characteristic symptoms in the aboveground parts of plants appearing primarily as reduced growth, symptoms of nutrient deficiencies such as yellowing of foliage, excessive wilting in hot or dry weather, reduced yields, and poor quality of product. Certain species of nematodes invade the above ground portions of plants rather than the roots, and on these they cause gall, necrotic lesions and rots, twisting or distortion of leaves and stems and abnormal development of the floral parts.
Nematodes damage plants only slightly by direct mechanical injury inflicted upon the plants during feeding. On the contrary, most of the damage seems to be cause by secretion injected into the plants while nematodes are feeding. This secretion, calls saliva, is produced in three glands from which it flows forward into the esophagus and is injected through stylet. Some nematode species are rapid feeders. They puncture cell wall, injected saliva into the cell, suck part of the cell contents and move on within a few second. Other feed much more slowly and may remain at the same time puncture for several hours or days. These, as well as the females of species which become permanently established in or on roots, inject saliva intermittently as long as they feeding.
The saliva of plant nematodes seem to aid the parasite in the penetration of cell walls and possibly, in liquefying the cell contents, making them easier to ingest and assimilate. The feeding process cause the affected plant cells to react resulting in dead or devitalized root tips and buds, lesion formation and tissue break down, swellings and galls of various kind, and crinkled and distorted stem and foliage. Some of these manifestation are cause by dissolution of middle lamellae and cell walls of infected tissue by nematodes enzymes, which, with or without help of toxic metabolites, cause tissue disintegration and death of cells. Others are caused by abnormal cell enlargement. Plant disease syndrome cause by nematodes is complex. Root feeding species probably decrease the ability of plant to take up nutrient and water from soil thus cause symptoms of water and nutrient deficiencies in the aboveground parts of plants.