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Plant Diseases

Nematology is a recognized discipline now in almost all the countries of the world. In its infancy or in some instances even now, it has not generated any significant interest amongst zoologists, in spite of the fact that more than 10,000 species have been described-no doubt only a fraction of the actually existing ones. At the same time, botanists as well as plant pathologists have kept away from this area because of the animal nature of the nematodes. However, with more and more information accumulating, regarding their role in plant diseases, either singly or in complex associations with other soil micro-organisms, like fungi and bacteria, as also the role of some of the nematode species in vectoring soil-borne viruses, the science of nematology, to be more precise, agro-nematology, has stimulated interests of the agricultural scientists. The unique nature of the plant parasitic nematodes, their ubiquitousness and persistence, as soil-borne plant pathogens, the unobtrusiveness of their attack, the tremendous losses they have caused to some of the economically important agricultural crops, have further established this group of -pests on par with other allied disciplines.

 

[n spite of the tremendous importance of the science of agro-nematology, several countries of the world still lack in the knowledge of various types of nematodes present in soil, their role and behavior, frequency, distribution, etc. The extensive as well as intensive works reported from U.S.A., England and The Netherlands, the leaders in this branch of Science, nave geared the thinking of agricultural scientists to this field in several countries of the world, particularly the developing countries. With the vast increase in exchange and distribution of plant material, during modern times, it has brought forth a necessity of technical know-how of looking for plant nematodes so that the noxious types are not introduced in countries where the particular types are not recorded to exist.

The importance of nematodes has been beautifully expressed by Cobb, father of modern nematology, as if all the matter in universe except the nematodes were swept away, our world would still be dimly recognizable... we would find its mountains, hills, valleys, rivers, lakes, and oceans represented by a film of nematodes.

    Plant nematology forms a link between zoology and botany as its Study involves not only the morphology and taxonomy of the animal but also their biology, host-parasite relations, control, etc. To have proficiency in the subject, therefore, one has to have knowledge not only of plant anatomy, plant pathology, physiology but also of zoology and parasitology. The plant diseases caused by nematodes, symptoms, identification of the causal nematodes, etc., therefore, form an important aspect in the overall understanding of the subject.

    Plant diseases have two important aspects, economic and ecologic- One or both of these aspects may cause a tremendous effect on the development of the discipline. Identification of the presence of the golden nematode of potato in Long Islands (USA.), bulb and stem nematodes in Europe, 'molya' disease of wheat and barley (cereal root eelworm) in India are some of the outstanding examples where nematodes have caused economic as well as ecological impact on the scientists as well as the administrators. Several disease problems still remain unrecognized or have not been able to attract attention because of the limited expertise available or uneconomic crop plants attacked by them. The subterranean habit of most nematodes, their microscopic size, lack of any clear cut above ground symptoms, the slow debilitating type of damage, rather than a killing effect on the host, want of any diagnostic service to provide advice on the basis of soil and plant tests, etc., are some of the important reasons which have precluded any sustained attentions

from the farmers and administrators. Furthermore, the plants are rarely subject to association with only one pathogenic organisms. There is a growing amount of evidence that nematodes, in quite a few cases, particularly in the case of soil-borne diseases, may act as initiators, thus facilitating parasitism by weak pathogens. Indications to their role as aggravators, resistance breakers and deterrents of plant diseases are now well recognized as well as understood. In these complex associations, therefore, the disease syndrome is influenced by two or more concurrent pathogenic organisms. Such associations may be more common in nature than is being realized at the present juncture. The total disease picture, therefore, necessitates a thorough understanding of the influencing factors of the various organisms in the symptom expression of the diseases.

A very commendable effort was made for the first time by Christie³ when he complied all the available information in his book 'Plant Nematodes, their Bionomics and Control. More than thirty years have elapsed since its publication. A large volume of literature has been since added and several new disease problems have come to the forefront. However, in spite of intensified researches, the literature is mainly confined to well known nematode problems only. Informa­tion on ectoparasites or nematodes like the lance or spiral nematodes, which are the most common as well as often prepon­derant populations in agricultural soils, ale still meagre. In the present compilation, an attempt has been made to bring together as much up-to-date information as possible for the students of nematology.

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