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CHAPTER Future Research Needs Plant diseases caused by nematodes have been found in all areas of the world, and many new nematode diseases are being discovered annually. Although the role of more than 150 nematode species in plant diseases is being studied, knowledge about nematodes and their control is far less than that available for other organisms, such as insects and fungi, that attack plants. Nematodes are now recognized as pests of global concern, threatening world food supplies. This has prompted authorities in many countries and subdivisions of countries to enact regulatory and quarantine measures. Such action is particularly necessary when plant materials and soil are transported and infestations are heavy. Although careful inspections and surveys by regulatory and quarantine workers are of great value, all nematodes are not detected by present sampling methods; thus, populations below the discovery level must be considered in any program to control nematodes by regulation or quarantine. Basic informa- tion in such areas as taxonomy, host ranges, plant pathogenicity, and soil and plant sampling is needed to improve methods of preventing nematode spread. Correct species identification is basic to nematode control; thus, nematode taxonomy at all levels must be emphasized. In particular, taxonomists must be urged to study thoroughly the taxonomic relations among nematodes of relatively large groupings and the influence of environmental factors on mor- phological variation, and to search for means of differentiating nematodes, other than by morphology. Additional collections of permanently mounted nematodes must be established throughout the world. Investigators must be encouraged to make at least a correctly preserved mass collection of a nema- tode used in an experiment or series of experiments. Unless these steps are 167
168 RESEARCH NEEDS taken, research data may become useless when taxonomic changes necessi- tate reidentification. Most aspects of the life and survival of nematodes in soil and plants and many of the basic premises of cultural control of nematodes involve ecology encompassing the complex interrelationships among the organism, the soil, and the plant. There are, however, very few detailed research publications or long-range research programs devoted entirely to nematode ecology, and the published data, much of which are contradictory, are based mainly on obser- vations and preliminary experiments. Although nematologists are aware that plant nematodes greatly influence and modify the productivity of soil, a lack of awareness of nematode populations on the part of the horticulturist, soil scientist, and plant pathologist has led in the past to misinterpretations not only of factors limiting crop response but also of experimental results. Future research on nematode ecology should be conducted by research teams, in- cluding investigators in related areas as well as nematologists. In addition to those conducted under natural field conditions, pathogenicity experiments should be conducted in the glasshouse and the laboratory or growth chamber under aseptic conditions involving only nematode and plant. Conclusions should be reached only after evaluating comparative results from at least two types of these experiments. Progress in this and most other nematological research could be facilitated by improved methods of inoculat- ing plants and extracting nematodes from soil and plant parts. The development of versatile, economical, and easy-to-handle nematocides is greatly needed for more effective control of nematode diseases. For ex- ample, nematocides that are effective in cold, wet soils are needed. Also needed are nematocides that are nontoxic to a wide variety of plants, allow- ing application at planting time and in the vicinity of roots, thus resulting in better timing of applications and reduction in the amount of chemical needed. More rapid and effective methods of applying nematocides around roots, es- pecially roots of perennials, need to be developed. An "ideal" nematocide for root pathogens would be a systemic chemical that, when sprayed on tops of plants, is translocated to the roots and kills or repels nematodes feeding on these roots, without imparting harmful residues to parts of the plant harvested for food or fiber. Using such a systemic chemical in place of a soil fumigant would require only a fraction of the quantity of nematocide, because the plant rather than the soil is treated. Interactions among soil, nematocides, and nematodes are not well under- stood. Little is known about the intriguing problem of how and in what quan- tities nematocides enter nematodes and, after entering, how the nematodes are actually killed. In many agricultural areas of the world, it is practical to apply nematocides in irrigation water. To refine this method of application, data are needed on
FUTURE RESEARCH NEEDS 169 the effects of such factors as temperature, humidity, wind velocity, sunlight, and soil penetration of chemicals. Despite these limitations resulting from the lack of basic information, the use of chemicals has increased rapidly and should increase even more rapidly in the future. Control need not await complete understanding. Basic research to gain an understanding of the nematode and the disease and attempts to control by methods such as soil fumigation often proceed together profitably. The high cost of nematode control by methods such as soil nematocide treatments points to a need for development of a wide range of nematode- resistant plant varieties. Additional plant varieties with improved nematode resistance will result in untold benefits, especially to growers of perennial and low-acre-value crops and to crop growers in developing countries, where the cost of nematocide treatments is often prohibitive. However, before major progress in this field is possible, there must be a marked increase in numbers of research teams, consisting of both plant breeders and nematologists, ac- tively engaged in such programs. Future research must be directed toward improving methods for testing the resistance of plants to nematodes. The importance of nematode culture and correct identification must not be overlooked if progress in developing resistant plant varieties is not to be hindered. The biochemical basis of resistance of plants to nematodes has hardly been studied. Research in this area might aid not only in developing nematode- resistant plant varieties, but also in the discovery of new nematocides. Crop rotation is the oldest and still the most widely used field-control measure for nematodes. Rotations, selected on the basis of crop-yield results alone, without considering nematodes, often owe the resulting increased yields to the unwitting control of nematodes. Although, in recent years, data from nematode host-range and population studies have been used in planning rotations, there is a critical need for additional information in these important areas. The need for research in areas such as population dynamics and host ranges should be emphasized. Differences in host specificity of geographical isolates of a nematode species must be considered in any breeding program. Research to determine exactly how resistance-breaking nematodes arise on previously resistant varieties must also receive a high priority. Many plant diseases, especially root diseases, are caused by nematodes in combination with other soil organisms. The fungus-nematode and the bacteria-nematode relationships are so numerous and varied that they present a wide-open field for profitable research. For example, relatively weak fungal and bacterial pathogens, once they gain entry into plant roots in the presence of feeding nematodes, can cause considerable damage. The possible biochemi- cal or physiological role of the nematode in these disease complexes is not known. In fact, in most instances it is not known whether the presence of two
170 RESEARCH NEEDS or more pathogens on a single host has only an additive or a synergistic effect on disease severity. Nematode attacks sometimes lower the resistance of plants to diseases caused by other organisms, such as resistance to vascular- wilt diseases caused by fungi and bacteria; however, the basis for this break- down of resistance is not understood. Future challenging and interesting re- search areas include determining why plants with nematode-damaged roots are more susceptible to cold injury and why fungal infection of roots some- times increases nematode buildup in these roots. Although nematodes are known to transmit many soilborne viruses, the nature of virus transmission by nematodes is essentially unknown, and the im- portance of nematodes in the damage caused by these virus diseases is difficult to ascertain. To better understand the whole gamut of host-nematode- bacteria-fungus-virus relationships, nematologists need to know more about generally overlooked peripheral areas. These areas include changes induced in plant constituents by parasitism, immunology, and enzyme action, as well as the physiology, biochemistry, and genetics of nematodes. New approaches to nematode control will undoubtedly be found. Perhaps through research, viruses and enemies such as fungi, bacteria, and invertebrate animals (including predacious nematodes), which greatly reduce nematode populations under certain naturally occurring conditions, can be used effec- tively to control specific diseases. Perhaps results of future research will show how attractants can be used to influence nematodes to move toward a nema- tocide, how male sterility can be used to reduce nematode populations, and how repellents rather than nematocidal chemicals can be used in control pro- grams. Additional basic research in nematode biology is needed to further the development of such control measures. Progress in nematode control would be accelerated by the cooperative efforts of teams composed of such special- ists as a nematologist, an organic chemist, a biochemist, soils chemist, plant pathologist, an agronomist, and a horticulturist. Basic research on nematode physiology, biochemistry, ecological relation- ships, and host-parasite interactions, so important in developing control methods for nematode diseases, has gained impetus only recently. Even today only a few investigators are working in this area. Despite some research on plant-parasitic nematodes, the physiology and biochemistry of nematode parasites of man and animals have received considerably more attention. Be- cause of the great differences in these types of nematodes and in their en- vironments, it is dangerous to assume that data obtained with parasites of man and animals apply equally to plant parasites. However, it is probable that many of the concepts and techniques developed by helminthologists deserve more consideration by nematologists, and vice versa. Nematodes, like other animals, contain carbohydrates, proteins, lipids, nucleic acids, vitamins, hormones, minerals, and numerous other substances,
FUTURE RESEARCH NEEDS 171 but most of the existing information on types or amounts present concerns animal parasites. Too little is known of intermediary metabolisms in plant- parasitic nematodes to make comparisons with those of animal parasites. Re- search on fundamental metabolic cycles, such as sequences of sugar breakdown and terminal oxidation, is needed to understand the nematode's use of plant tissues for food. Although water and certain ions pass directly through the nematode cuti- cle, the excretory system appears to be responsible for osmotic regulation of the body fluid, as well as for excretion. Information on osmotic regulation in nematodes is important to nematode control, because chemicals used to kill nematodes are transported in the water phase. The use of radioactive tracers may help to elucidate how nematodes control their osmotic concentration and to understand nematode metabolism. A primary reason for the limited research on many important aspects of nematode physiology, biochemistry, and host-parasite relationships is the dif- ficulty of obtaining specific plant-parasitic nematodes in large enough quanti- ties for study. Although several plant-parasitic species can be grown on excised roots, on callus tissue, or in association with a fungus or a bacterium, the ma- jority of species cannot be grown even using these methods. Ability to grow plant-parasitic nematodes on chemically defined media under sterile conditions would aid many types of nematological research. Despite some progress in recent years, relatively little is known about mechanisms involved in changes induced in plant cells and tissues by plant- parasitic nematodes. For example, although it is known that the mechanism of nematode-induced galling of plant tissues involves growth regulators, little research effort has been directed toward specifically identifying the com- pounds involved. Furthermore, the majority of the microscopic studies of tissue and cellular alterations (histopathological studies) associated with nema- tode parasitism have dealt with plant tissues in which parasitic relationships were already established at the time of sampling. Especially needed are studies of tissue changes during the critical period of the establishment of the host- parasite relationship and comparisons of reactions in nematode-resistant and nematode-susceptible plant tissues. Ideally, histopathological, cytological, and histochemical studies of plant tissue should be conducted simultaneously, and, where feasible, electron microscopy should be used. Such comparative studies, combined with physiological and biochemical studies of infected and noninfected tissues, will provide the information needed to understand the fundamental basis of plant parasitism by nematodes. Plant-pathogenic nematodes will become more important to agriculture within the next few decades because of their spread by man and because of more intensive plant culture, particularly monoculture, on the better agricul- tural soils of the world. Despite the widespread parasitism of every crop plant,
172 RESEARCH NEEDS less than 100 nematode diseases are considered of serious proportions. Many other nematodes causing less obvious or less severe damage are generally unrecognized. Well-trained nematologists are absolutely essential to fruitful research programs. Fortunately, many colleges and universities recognize the wisdom of training nematologists who will eventually teach and conduct research in this area. Some colleges in the United States and India have established de- partments of nematology. In most universities, instruction and research in nematology is centered in departments of plant pathology, entomology, and economic zoology; however, as the science of nematology gains stature and status, the establishment of separate departments of nematology will increase. Nematologists should be trained with full recognition of the urgent needs of nematology as a rapidly growing area of biology as well as of the needs directly related to agriculture. With recognition of the importance of nematology, the better training of nematologists, and more and higher caliber research in nematology, man's control of nematode infestations on important food crops may become one of the most dramatic developments in world food production in the latter half of the twentieth century.
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