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Importance of Plant Parasitic Nematodes in Agriculture

Plant parasitic nematodes are unique in their ubiquitous ness and persistence as soil borne pathogens. The unobtrusiveness of their attack allows their presence to often pass unnoticed while crops slowly decline in vigour and yield. Rarely is any crop free from attack of these tiny and microscopic pathogens. They are present every where, in fields, glasshouses, orchards, home gardens, and yet their presence is generally not felt till the concerned individual is baffled by the continuous decline of the crop in spite of best agrono­mic practices. The losses, as a result of nematode attack do not consist of yield reductions only but other aspects like lesser ability of infected roots to utilize fully the available nutrients from soil or the necessity to grow uneconomical rotational crops, in an effort to control the noxious nematodes, are some of the effects which follow as a result of nematode infestation. Considerable information has been generated on the losses caused by the nematodes to staple and other crops, particularly in U.S.A., U.K. and the Netherlands.

    Amongst the soil inhabitants, nematodes constitute one of the most important groups of organisms which quite often play a vital role in the deterioration of the health of the standing crops. With intensification in agriculture and also with continued and uninter­rupted new introductions in countries, it is obvious that there is

every likelihood of hitherto unknown problems which may become serious in the near future. The golden nematode problem in long Islands, U.S.A. and in the southern part of India is one of the most outstanding examples of such type.

    There are still no reliable information on the exact tosses caused by the nematodes to various crops. Moat of the reports are approximations based on indirect evidences, interpreted on the basis of pot trials and chemical trials under field conditions. The validity of the results on yield increases, as a result of chemical applications, is open to question since the total effect on biological flora of the soil is not fully understood and therefore, it might not be justifiable to interpret results of field trials with nematode damage alone. The quantitative aspect of the nematode damage to the crops is not only affected by the size of the nematode population but also on other variables such as the species of the host plant, temperature and other environmental conditions, the biological flora and fauna of the soil, which singly or jointly may contribute in the overall expression of disease syndrome. A range from death of seedlings/plants, in some cases, to an absence of recognizable damage may not be uncommon in nematode infestations. One of the most important factors influenc­ing yield of crops is the size of the nematode population in soil and also availability of a susceptible host. Measurable damage occurs only when the population density exceeds a certain limit. Experi­ments have demonstrates that about 100 nematodes (Tylenchorhynchus claytoni) can reduce the top growth of inoculated tobacco plants. On the other hand, another species of the same genus (T. martini) causes deterioration of sugarcane roots at a population between 600 to 6400. In onions, only 5 individuals of Pratylenchus penetrans per seedling are reported to cause serious damage.

    Amongst the best known plant parasitic nematodes are the root-knot nematodes, belonging to the genus Meloijoyne, which have a wide host range and are widely prevalent all over the world. These produce conspicuous galls on the roots and the infestation can be recognized easily in fields. The wide distribution of these nematodes through tropics, sub-tropics and temperate regions and their occur­rences in cultivated fields and greenhouses as well, make them as one of the most common agricultural pests. It is especially a serious

problem with vegetable growers as also to cotton and tobacco growers. In majority of the cases, soil microorganisms like fungi and bacteria are also associated with root-knot nematode infestations. Plants resistant against fungal and bacterial pathogens may become susceptible in the presence of nematode infestation. In some instan­ce s, root injuries caused by nematode larval penetration or feeding ma y provide an easy entry avenue for fungal and bacterial colonization, which otherwise they might not have been able to do so independently. In such cases the disease expression may be quite severe.

    Involvement of various ecto-parasitic nematodes, that feed ectoparasitically on plant roots, besides the other endo or semi-endoparasites, notably P atylenchus species (lesion nematodes), Tylenchulus semipenetrans (citrus nematode) and Rotylenchulus renifo rmis (reniform nematode) in the plant disease, adds to the dimensions of losses caused by these noxious nematodes to economi­cally important crop plants. As stated earlier, many of these may not cause conspicuous damage immediately but with continued root feeding, the root growth may stop and the affected plants may express unthriftiness and stunted growth. However, not all the soil nematodes are harmful to the plants. Some of dorylaims and mononchs prey on nematodes, rotifers and soil micro-organisms. These and some predacious fungi, which attack nematodes, may well be maintaining a natural balance in the soil fauna in favor of the plant growth.

    Even though a very large number of plant parasitic species have been recorded and documented, only 15 have been identified, so far, as world problems. Besides these major nematode problems, there are several other important nematode diseases which cause heavy losses in certain parts of the world and under specific conditions. For example, the ear-cockle disease of wheat (Anguina tritici) could lead to as high as 80 per cent or more crop loss in certain pockets of India, if contaminated seed is regularly used for sowing. The maize cyst nematode (Heterodera zeae) is another nematode problem which has gained an economic aspect recently in India as well as U.S.A. The 'ufra,' and white tip diseases of rice, respectively caused by Dltylenchas angustus and Aphelenchoides

besseyi, often are responsible for serious crop losses in south-east Asian countries. Similarly, the soybean cyst nematode, Heterodera glycines, is of great economic importance in U.S.A. and has neces­sitated development of intensive nematode management programmes. The lesion nematodes, in general, and particularly PratyienchuS cqffcee on coffee in southern parts of India and P. loosi on tea in Ceylon (Sri Lanka) have attracted great deal of attention from the growers as well as the scientists. The red ring disease of coconut caused by Rhadinaphelenchus cocophilus in Surinam and British Guiana and on oil palm in Venezuela is rated as one of the most devastating disease throughout the world.

    There are several other known examples where nematodes have been established definitely as the causal organisms of the well known maladies. However, many are still unexplored but at the same time, the importance of nematodes, as one of the important limiting factors in crop production is now well established. Nematode control could play a primary role in reducing the existing gap between food and population.