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CHAPTER 3 Ecological Relationships Knowledge of the ecological relationships between plant-parasitic nematodes and their environment is important for understanding some of the principles of nematode control. Agricultural land represents a specialized environment, ranging from a dry, barren waste to a moist, lush jungle of plant growth. Plant-parasitic nematodes are mostly those soil-inhabiting species that are capable of withstanding the frequent changes caused by man's agricultural practices. Some of these nematodes, such as species of spiral (Helicotylenchus spp.), stunt (Tylenchorhynchus spp.), and sheath (Hemicycliophora spp.) nematodes, can live in a wide variety of habitats. Some, such as the rice nematode (Hirschmanniella oryzae) in aquatic habitats, are widely distributed but are limited to particular combinations of environmental conditions. Still others, such as sting nematodes (Belonolaimus spp.) in the sandy soils of the southeastern United States, are found only in very special situations. There- fore, it is difficult to recommend control practices for such diverse kinds of nematodes without first knowing how they live and survive in the soil and in host plants. VERTICAL DISTRIBUTION OF NEMATODES The vertical distribution of nematodes in cultivated soil is usually irregular but is generally closely related to the distribution of plant roots and the area adjacent to roots, which is called the rhizosphere. Since the movement of nematodes in the soil by their own activities is limited at most to a few feet per year, it is obvious that the number of plant-parasitic nematodes is greater 17

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18 FACTORS INFLUENCING NEMATODE CONTROL in soils containing plant roots than in soils without plant roots and is corre- lated with the distribution of roots of present and previous hosts. Nematodes are mainly concentrated in the top foot of soil, and as many as six billion have been estimated in the top inch of an acre of soil. Little information exists on the distribution of nematodes deeper than 1 foot in soil; however, a root-knot nematode (Meloidogyne incognita) has been found at a maximum depth of 17 feet in grape vineyards. Consequently, in a chemical treatment the soil must be treated to a greater depth for deep-rooted than for shallow- rooted crops, thus requiring more chemical and more expensive equipment. The fallowing of soil for an adequate period will generally reduce the number of nematodes present. Plant-parasitic nematodes survive longer in the absence of food sources than most nonparasitic nematodes. The cyst nematodes (Heterodera spp.), the most persistent of the plant-parasitic nematodes, de- cline in soil at a steady rate of 35 to 60 percent per year, depending on soil type, moisture, and temperature, regardless of the density of the initial popu- lation per unit of soil. Many questions remain unanswered concerning the in- fluence of soil type, moisture, aeration, and other factors on distribution and the response of nematodes to them. NEMATODE SURVIVAL Plant-parasitic nematodes are able to survive despite unfavorable conditions such as cold and dry periods between host crops. Except in the tropics and heated areas, such as greenhouses, they do not grow and reproduce through- out the year. Nematodes survive unfavorable environments in a dormant con- dition, which is a quiescent or inactive state that is often associated with a lowered metabolic rate. The length of the quiescent period is usually limited by the amount of food reserves in the nematode and the environmental con- ditions. Quiescence may serve to extend a comparatively short life cycle of 20 to 40 days to periods varying from a year for many plant parasites to 20 to 30 years for such nematodes as the stem nematode (Ditylenchus dipsaci) and the wheat nematode (Anguina tritici). NEMATODE POPULATIONS In agricultural soil, the upper population limit for any plant-parasitic nema- tode species depends on the nematode's reproductive potential, the host-plant species, and the length of time the nematode remains in an environment favorable for reproduction. Generally, the reproductive potential of the specialized endoparasites and aboveground parasites is greater than that of

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ECOLOGICAL RELATIONSHIPS 19 many of the ectoparasites. Some nematodes have only one or two generations a year, while others have several generations during the growing season. The latter include such important nematode pests as root-knot and cyst nema- todes, lesion nematodes (Pratylenchus spp.), and the citrus nematode (Tylenchulus semipenetrans). The population level of each of these nema- todes is dependent on the nematode's ability to live successfully in soil. The importance of a nematode as a plant parasite depends largely on whether or not the population limit exceeds the level at which economic damage occurs to a crop plant. This concept of population threshold (Figure 2) at which yield loss begins is often used in connection with crop pests to determine the tolerance level of a host crop. For any set of environmental conditions, each host crop has its own tolerance level for a nematode species. Thus, a nematode causes economic damage only if its population density ex- ceeds the tolerance level of the plant grown in the field. For example, the population density of root-knot nematodes is generally much higher than the tolerance level of many host plants, thus accounting for their importance as a nematode pest. In crop rotation, the cultivation of good host crops is alter- nated with poor or nonhost crops. Estimates of thresholds vary with seasons and from field to field. A clear understanding of nematode populations is im- portant to nematode control. 100 13 75 3 50 25 tolerance level of crop and plant threshold of nematode numbers above which yield loss becomes significant yield decreases as nematode numbers increase yield loss reaches maximum Logarithmic scale of nematode numbers per plant FIGURE 2 A diagrammatic relationship between plant-parasitic nematode populations and crop loss. (After Jones, 1965.)

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20 FACTORS INFLUENCING NEMATODE CONTROL THE SOIL ENVIRONMENT All plant-parasitic nematodes inhabit soil for varying lengths of time during their life cycles (Figure 3). For example, the ectoparasitic nematodes spend their entire lives in the soil, usually in the rhizosphere of the plant. The more specialized endoparasites enter plant tissue and thereby spend less of their lives in the soil and rhizosphere. The aboveground parasites are mostly inside plant tissues and spend very little of their lives in the soil. Due to the nema- tode's life habitat in the soil, it is easier to control ectoparasitic than endo- parasitic nematodes. The principal factors in the nematode's soil environment are temperature, moisture, texture, aeration, and the chemistry of soil solution. Only in the Adult Larva 4 Larva 1 Larva 2 ECTOPARASITIC NEMATOOES 1. Hemicydiophora arenaria 2. Trichodorus christiei ENOOPARASITIC NEMATOOES 1. Heterodera schachtii 2. Meloidogyne javanica ABOVEGROUNO PARASITIC NEMATOOES 1. Oitylenchus dipsaci 2. Anguina tritici L2 ANGUINA FIGURE 3 The relationship between parasitic habits of some nematodes and the portion of their life cycle spent in the soil, plant, and rhizosphere.

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ECOLOGICAL RELATIONSHIPS 21 laboratory is it possible to investigate this complex, constantly changing en- vironment; yet, from laboratory data it is difficult to relate varied factors such as nematode distribution, population levels, and pathogenicity to any one factor. Results of field-population studies are necessary to determine the influence of the interdependent and interacting environmental factors. TEMPERATURE Temperature affects nematode activities such as hatching, reproduction, movement, development, and survival and also affects the host plant. Most plant-parasitic nematodes become inactive at a low temperature range of 5 to 15°C, have an optimum range of 15 to 30°C, and become inactive at a high temperature range of 30 to 40°C. Temperatures outside these extremes may be lethal. Little information exists on the effect of constant or alternating temperatures on specific activities of individual nematode species. The Javanese root-knot nematode (M. javanica) is of little concern in the northern states, where it does not overwinter out-of-doors in deeply frozen soils, but the northern root-knot nematode (M. hapla) overwinters and may be a serious pest in these areas. Temperatures, however, do not limit the establishment of some nematodes: the sugar-beet nematode (Heterodera schachtii) is a serious pest in the north as well as in the south, where the soil temperature may ex- ceed 35°C. The less adaptable stem nematode is restricted to cool climates or to warm climates where the host is winter-grown. Determining the influence of temperature on nematode reproduction in plants is complicated, because temperature influences the growth of the plant itself. Changes in plant growth produce corresponding changes in root mor- phology and physiology. Temperature partially determines the choice of crop plantings and rotations. In areas of the United Kingdom, Europe, and the United States, some varieties of potatoes and sugar beets are grown in early spring, when the soils are too cold for reproduction of the potato-cyst and sugar-beet nematodes but are not too cold for the growth of the plants. Pro- tection from nematodes during the early part of the growing season reduces nematode damage at harvest. MOISTURE Fluctuating soil moisture due to rainfall or irrigation is one of the chief factors influencing nematode-population increases. Dry soil conditions may depress populations of a ring nematode (Criconomoides xenoplax), a dagger nematode (Xiphinema americanum), and root-knot and cyst nematodes. Although dry

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22 FACTORS INFLUENCING NEMATODE CONTROL conditions may depress nematode activity and resulting populations, all nematodes may not be killed. Eggs of most nematodes as well as certain other nematode stages, such as preadult stages of pin nematodes (Paratylenchus spp.), survive drying. Dry fallowing of field soils may not be a practical con- trol measure except in some hot, dry regions, where it reduces the numbers of nematodes so that a profitable crop can be obtained. Saturated soils are not generally favorable for nematode pests of agricul- tural crops. In tropical rain belts and in flooded fields, populations of some species of root-knot, cyst, stunt, and pin nematodes have been reduced by excess water, lack of oxygen, and toxins of anaerobic organisms. However, high populations of some nematodes, such as species of Dolichodorus, Radopholus and Hirschmanniella, are found chiefly in wet locations. It is thought that nematodes are constantly active in soils that have a moisture content of between 40 and 60 percent of field capacity. In dry and wet soils they are quiescent for varying periods. Nematodes need free water films in the soil for hatching and movement, but the influence of moisture on the nematode is little known. Since the interrelationship of soil moisture and soil structure is responsible for the aeration properties of the soil, the oxygen level may be the fundamental factor influencing some activities of nematodes. As soil moisture increases, soil aeration decreases, so that soils become low in oxygen immediately after heavy rains, flooding, or irrigation. From studies of a few plant-parasitic nematodes, it appears that individual nematodes are capable of fermentative as well as oxidative metabolism, which enables them to survive for varying periods of time without oxygen. Low levels of oxygen may induce quiescence and enable nematodes to survive. Growth and develop- ment of nematodes, which are important in determining population levels, are oxygen-dependent; therefore, high populations are usually found in moist, well-aerated soils. SOIL TEXTURE AND STRUCTURE Soil texture describes the sizes of soil particles. A coarse-textured soil usually contains a high percent of sand and has large pores that drain more quickly than the small pores of fine-textured soil, which has a high proportion of clay and silt. Because of the wide variation of the biotic, physical, and chemical environments within textural categories, it is difficult to generalize among soil type, nematode activity, and distribution. Many of the cyst, root-knot, lesion, and stubby-root (Trichodorus spp.) nematodes are found in large numbers in coarse-textured sandy soils. However, the stem, sugar-beet cyst, and some species of lesion and stunt nematodes are numerous in

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ECOLOGICAL RELATIONSHIPS 23 clay soils. Still others, such as the citrus nematode, occur frequently in both sandy and clay soils. The speed of nematode movement through soil is related to soil pore diameter, soil particle size, diameter of the nematode and its relative activity, and the thickness of the soil-water films. A nematode cannot move between soil particles when the pore diameters are less than the nematode width. As mentioned previously, soil structure, moisture, and aeration are interrelated. When the soil pores are full of water, a nematode moves inefficiently, and, when aeration becomes limiting, the nematode becomes inactive. In very dry soils, there is good aeration but not enough water to form films, so that the nematodes do not move. Only a soil of intermediate moisture has suffi- cient aeration and water films for efficient nematode movement. SOIL SOLUTION The chemical constitution of the soil solution, a major constituent of the soil environment, includes soil salinity, pH, organic matter, fertilizers, insecticides, and nematocides. Plant-parasitic nematodes probably derive few nutrients from the soil solution. The hatching of eggs and the survival of larvae may be influenced by various salts and ions. During dry and wet periods, soil nema- todes are subjected to variable salt concentrations in the soil solution. How- ever, nematodes can tolerate osmotic pressures up to about 10 atm, at least for short periods. This is considerably higher than the maximum 2 atm oc- curring in most agricultural soils. A soil pH ranging between 5.0 and 7.0 has little effect on nematodes. Lime, often used to neutralize soil acidity, causes no decrease in population. Fertilizers and organic matter may influence nema- tode populations indirectly by increasing host-plant growth. Occasionally, the use of nematocides and insecticides in soil may eliminate some nematode enemies, thus leading to an increase in population of a plant parasite. CLIMATE Rainfall and temperature above soil level are extremely important to the growth and development of both nematodes and plants. These factors are usually responsible for seasonal fluctuations in nematode populations and may even determine the success of a species in becoming established in a new habitat or region. Climatic factors affecting humidity are particularly im- portant to aboveground parasitic nematodes, which are able to invade seed- lings and move upward on plant surfaces covered by water films or droplets.

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24 FACTORS INFLUENCING NEMATODE CONTROL These nematodes may be subjected to severe desiccation and great extremes of temperature owing to the more violent changes in aerial climate compared with soil climate. Perhaps, as an adaptation to this, certain stages of these nematodes, such as second-stage larvae in wheat nematodes and fourth-stage larvae in stem nematodes, are capable of withstanding long periods of desicca- tion. Specific information on the influence of the microclimate of the plant surface on nematode activities is lacking. THE PLANT ENVIRONMENT The plant-host environment, consisting of either root or stem and leaf tissue, greatly influences the endoparasitic nematodes. The plant tissues that are usually attacked are apical meristems that contain cells with thin walls and offer a chemically rich environment. The epidermis and cell wall offer mechanical barriers to nematode entrance and movement. The plant tissue protects endoparasitic nematodes from the soil environment and is their sole source of food, and the quality and quantity influence nematode growth and reproduction. Thus, host susceptibility, tolerance, and resistance to nematodes are dependent on properties of individual plant cells and tissues. Much remains unknown about the nature of these factors and their effect on the nematodes. The periderm and necrotic areas, which are formed in some plants in response to nematode feeding, may affect nematode growth and reproduction, because the quality and quantity of nutrients are deficient in these areas, or the nema- todes may be excluded from suitable plant cells by these areas. In plant para- sites such as root-knot, cyst, and citrus nematodes the host cells are modified to provide specialized feeding sites, and their physiological and nutritional dependence on the host become delicately balanced. Recently, this intricately balanced system has been studied to try to find ways of controlling nema- todes by the use of chemotherapeutic agents or antimetabolites to modify the host-plant environment to one unsuitable to nematodes. This area of re- search needs emphasis to gain an understanding of nematode nutrition and host-parasite relations and to aid in developing methods for systemic control of nematodes. THE RHIZOSPHERE In addition to serving as a source of food for nematodes, plant roots may also modify the soil environment by lowering the concentration of mineral nutri- ents, depleting moisture, increasing carbon dioxide, reducing oxygen, and contributing a variety of organic substances by exudation and sloughing off

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ECOLOGICAL RELATIONSHIPS 25 of cells. The rhizosphere, the zone immediately around the plant roots, is a dynamic environment, where the relationships among nematode, host, and environment are often of a chemical nature. A root exudate stimulates the hatching of eggs of the cyst nematodes. Hatching is usually stimulated by chemicals from a wide range of plants, some of which are nonhosts. The com- position of hatching chemicals is unknown, despite more than 20 years of re- search by numerous workers. The eggs of root-knot nematodes, as well as most other plant-parasitic nematodes, hatch freely in water; but in soil, plant- root exudates significantly increase the hatching of root-knot nematode eggs as compared with a water hatch. They stimulate the metabolism of larvae after hatching and may account for their directional movement toward plant roots. The exudates also influence the molting of preadult larvae of the pin nematode. Such examples of stimulation by plant roots appear to be a refine- ment of parasitism. Root exudates and other chemicals may also inhibit egg-hatching or may repel nematodes. From a few observations, it appears that some plants, such as marigold, asparagus, and tobacco, produce chemicals that repel or even kill some species of nematodes. Little is known about the identity of these exu- dates and other chemicals, the nature of the reactions on nematodes, or the receptors in the nematodes. There is also evidence that nematodes may be repelled by small quantities of nematocides. Microorganisms in the rhizosphere may significantly influence nematodes in several crops by antagonism, by competition for food and oxygen, or by excretions that may stimulate or inhibit nematodes. Research in this area should not be overlooked. The ecological system that illustrates the complex interrelationships among plant-parasitic nematodes, plant, climate, and soil environment is summarized in Figure 4. Although the information now available is substantial, the inter- relationships for any one nematode-plant combination are not completely understood. Comprehensive coordinated information on nematode activities, such as length and stage of life cycle, and mechanisms for nematode survival in unfavorable environments and in the absence of a host is limited. A critical evaluation of vertical distribution of nematodes in soil, particu- larly at depths below 2 feet, is needed to determine the possible crop loss caused by these nematodes and the need for their control. Other areas need- ing ecological studies include host-parasite relations, mixed populations of plant parasites, the influence of other microorganisms in the rhizosphere, the influence of plant microclimate on aerial plant parasites, the influence of soil factors on population levels of plant parasites, and the application of nematode-hatching chemicals to infested soil. New methods, fresh approaches, and long-range programs aimed at developing integrated ecological concepts of plant-parasitic nematodes are necessary for progress toward more effective and economical control.

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26 FACTORS INFLUENCING NEMATODE CONTROL FIGURE 4 The ecological system, showing the complex interrelationships among plant- parasitic nematodes, plant, climate, and soil environment. BIBLIOGRAPHY Bird, A. F., and H. R. Wallace. The chemical ecology of Acanthocephala and Nematoda, In Chemical zoology. Academic Press Inc., New York. (In press) Hyman, L. H. 1951. The invertebrates. Vol. 3. McGraw-Hill Book Co., Inc., New York. 572 pp. Jones, F. G. W. 1959. Ecological relationships of nematodes. In C. S. Holton, G. W. Fisher, R. W. Fulton, H. Hart, and S. E. A. McCallan (eds.). Plant pathology problems and progress, pp. 395-411. Univ. Wisconsin Press, Madison. 588 pp. Jones, F. G. W. 1966. The population dynamics and population genetics of the potato cyst-nematode Heterodera rostochiensis Woll. on susceptible and resistant potatoes. Rep. Rothamsted Exp. Sta. 1965. pp. 301-316. Van Gundy, S. D. 1965. Factors in survival of nematodes. Ann. Rev. Phytopathol. 3:43-68. Wallace, H. R. 1964. The biology of plant parasitic nematodes. St. Martin's Press Inc., New York. 280 pp. Winslow, R. D. 1960. Some aspects of the ecology of free-living and plant parasitic nematodes. In J. N. Sasser and W. R. Jenkins (eds.). Nematology, fundamentals and recent advances, pp. 341-415. Univ. North Carolina Press, Chapel Hill. 480 pp.

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CHAPTER The Physiology of Nematodes in Relation to Control Investigations of nematode metabolism and biochemistry, mechanisms of action of nematocidal agents, biochemical bases for plant resistance to nema- todes, and nematode nutrition are increasing. Few basic studies on nematode physiology were conducted before 1950, and, even now, relatively few in- vestigators are conducting research on such subjects. One reason for the limited research on many aspects of nematode physiology has been the diffi- culty of obtaining adequate quantities of specific plant-parasitic nematodes. While the information in this section may not bear directly on specific control measures, it may give insight as to why some of the commonly applied meth- ods are successful. CHEMICAL COMPOSITION Nematodes, like other animals, contain carbohydrates, proteins, lipids, nucleic acids, vitamins, hormones, minerals, and numerous other chemicals, but not much is known of the precise kinds or amounts of these substances present in nematodes. Although the composition of animal-parasitic nema- todes has received considerable study, it is doubtful if all these data are also applicable to the plant parasites. Chemical composition of nematodes, which affects longevity, degree of resistance to temperature extremes, desiccation, atmosphere, osmotic conditions, and chemicals, undoubtedly relates closely to the success of the various control measures utilized. Glucose, fructose, and 15 free and protein amino acids were identified from two species of plant-parasitic nematodes. Plant-parasitic nematodes 27

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74 FACTORS INFLUENCING NEMATODE CONTROL FIGURE 29 Equipment for collecting soil and root samples: a, shovel; b, Veihmeyer soil tube; c, hammer to drive Veihmeyer tube into ground; d, wrapped leather mallet to tap soil from bucket auger; e, 3-inch bucket-type auger; f, Dutch soil auger; g, extension used with e and f for deep samples.

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CONSIDERATIONS BASIC TO NEMATODE CONTROL 75 FIGURE 30 Food blender for macerating plant parts.

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76 FACTORS INFLUENCING NEMATODE CONTROL FIGURE 31 Soil sieves, pans, and beakers for wet-sieving soil. FIGURE 32 Dissecting microscope with clear-glass stage and sub- stage mirror. Lamp with cover partly removed to show fluorescent bulbs.

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CONSIDERATIONS BASIC TO NEMATODE CONTROL 77 FIGURE 33 Seinhorst soil elutriators. from soil than other nematode forms, and special apparatus have been devised for their recovery (e.g., Figure 34). Increasing the specific gravity of the soil solution with sugar and subsequent centrifugation is another commonly used method. SEPARATION BY NEMATODE MOVEMENT The equipment most widely used for separation by nematode movement is the Baermann funnel. A wire screen is fitted across a glass funnel about half

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78 FACTORS INFLUENCING NEMATODE CONTROL FIGURE 34 Fenwick can to separate Heterodera cysts from soil. an inch below the top rim. A sample of soil, wet-sieve residue, or plant parts is placed on a paper tissue supported on the screen. The tissue used is pref- erably of the silicone-treated type, such as Kimwipes (Kimberly-Clark Corp., Neenah, Wisconsin), which do not disintegrate when wet. The bottom of the funnel may be extended by rubber or plastic tubing and closed with a pinch- cock or a small vial inserted in the end of the tubing (Figure 35). The funnel is then filled with water to a level to make contact with the sample on the wire. The nematodes move through the paper and screen into the water in the funnel. They settle by gravity to the bottom of the funnel, where they are collected for identification. A modification of this method is one set up inside a mist chamber (Figure 36). Substitution of a fine mist for the water bath allows better aeration and results in less microbial and chemical interference. In using a mist, the rubber tubing attached to a funnel can be inserted into a large test tube. The funnel is then placed in a mist chamber, where it is exposed to intermittent, regular fine sprays. The water filters through the sample, down the funnel, and into the test tube, where it accumulates and ultimately overflows. The nematodes carried in the water settle by gravity into the bottom of the tube. Both the Baermann-funnel and mist-chamber techniques depend on nematode activity and ability to move through soil or out of plant material and through the tissue paper into the funnel.

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CONSIDERATIONS BASIC TO NEMATODE CONTROL 79 FIGURE 35 a, Large Baermann funnel fitted with several layers of cheesecloth to separate Xiphinema spp. from soil sievings; b, funnel showing wire screen in place; c, funnel with screen and tissues; d, funnel with pinchcock and water added; e and f, funnels with soil and root pieces on tissues; g and a are funnels fitted with small vials instead of pinchcocks. FIGURE 36 Baermann funnels on racks in mist chamber. Funnel tips are inserted directly into large test tubes instead of being closed by pinchcocks or vials.

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80 FACTORS INFLUENCING NEMATODE CONTROL Endoparasitic species can be recovered from inside roots or other plant material by washing the plant parts free of soil and storing in a Mason jar or closed plastic bag (Figure 37). The lid of the Mason jar should be reversed and screwed down lightly enough to avoid desiccation but not so tightly as to lead to suffocation. The nematodes emerge from the roots, and after several days storage they are recovered by rinsing the jar with fresh water. The rinse water can be examined directly or cleared through the Baermann funnel. After the nematodes have been recovered by any of these or similar methods, identifications can be made from temporary mounts or specimens prepared in glycerine. The nematodes are picked up by a pipette, by a slender pick made of a bamboo splinter (Figure 38), or by nylon bristle from a tooth brush and transferred to a separate drop of water on a glass slide. Gentle heat from a small flame, sufficient only to stop movement, will kill the nematodes in the water. Before placing a cover glass on the drop of water, small glass rods are placed in the water to prevent the nematodes from flattening. A fingernail-polish or paraffin seal prevents drying and permits examination for several hours. Permanent mounts are made by kilhng the nematodes in water by gentle heat and fixing in 2V£ percent formalin for 24 hours. The nematodes are then transferred to 2V4 percent glycerine in 30 percent alcohol for 24 hours in a FIGURE 37 Moist roots in plastic bag or in Mason jar.

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CONSIDERATIONS BASIC TO NEMATODE CONTROL 81 FIGURE 38 Tools and equipment for killing, fixing, preserving, and mounting nematodes for identification. sealed cavity slide and finally to 5 percent glycerine in 30 percent alcohol in a B.P.I. dish stored in a petri dish. After the mixture dries to a viscous condition (about 7 to 10 days), it is placed in a desiccator for 24 hours, and the speci- mens are mounted in dehydrated glycerine. Glass rod supports are again re- quired to prevent flattening. Zut slide ringing compound (Bennett's, Salt Lake City, Utah) is used to seal the cover slip. It is not always possible to prepare permanent slides for reference and re- study, in which case it is essential that mass collections be made whenever possible. Nematodes recovered in water are concentrated by allowing to settle for an hour or more, decanting the excess liquid, and adding hot 5 percent formalin in equal volume. Again allow to settle, decant, and store in small vials sealed by paraffin. Preserved material of this kind can be made into permanent mounts any time in the future when and if specific identification must be checked. The fact that there are usually mixtures of various plant- parasitic species in the same soil, often two or more species of the same genus that could be confused or overlooked, is further reason for keeping mass collections. Taxonomy of nematodes becomes increasingly complex and technical every year, as many new and closely related species are described. For this reason, increased training of scientists in this field is needed. Whenever practi- cal, there should be at least one trained taxonomist in every laboratory where nematological research is conducted.

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82 FACTORS INFLUENCING NEMATODE CONTROL POPULATION LEVELS AND PREDICTION OF PLANT DAMAGE For most plant-parasitic nematode species, there is insufficient information on the levels of population or density of infestations that are likely to cause damage to particular crop plants. One notable exception is the golden nema- tode (Heterodera rostochiensis). In Great Britain and the Netherlands, advisory services are established for the control of this nematode by crop rotation. Soil samples are examined to calculate nematode populations surviving in the cyst stage. Based on the number of cysts with viable contents per gram of soil, ad- vice is given to growers as to whether to plant potatoes or to continue to plant nonhost crops. Similar advisory services are now available in the Netherlands for other nematodes pathogenic to various agricultural crops. The growers pay a fee for this service, which includes identification of nematode infestations in soil, calculations of nematode population densities, and advice as to types of crops to plant, based on the nematode analyses. Before such services can be developed further, much information is needed on the host-range preferences of many plant-parasitic nematode species, the minimum sampling procedures on which to judge infestations, and how to predict damage from population densities. Unfortunately, examination of soil samples is laborious, time-consuming, and expensive. Because of this high cost it is probable that similar advisory services will be developed only for agricultural crops of high acre value. BIBLIOGRAPHY Goodey, J. B. 1963. Laboratory methods for work with plant and soil nematodes. Ministry of Agriculture, Fisheries and Food. London. Tech. BulL No. 2. 72 pp. Goodey, T. Revised by J. B. Goodey. 1963. Soil and freshwater nematodes. Methuen and Co., Ltd., London. 544 pp. Southey, J. F. 1965. Plant nematology. Ministry of Agriculture, Fisheries and Food. London. Tech. Bull. No. 7. 282 pp. Thorne, G. 1961. Principles of nematology. McGraw-Hill Book Co., Inc., New York. 572pp.

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PART BASIC PRINCIPLES OF CONTROL

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