Control of Plant-Parasitic Nematodes (1968) / Chapter Skim
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FACTORS INFLUENCING NEMATODE CONTROL
FACTORS INFLUENCING NEMATODE CONTROL
Pages 17-84
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From page 17... ...
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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From page 18... ...
. NEMATODE POPULATIONS In agricultural soil, the upper population limit for any plant-parasitic nematode 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.
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From page 19... ...
The latter include such important nematode pests as root-knot and cyst nematodes, lesion nematodes (Pratylenchus spp.) , and the citrus nematode (Tylenchulus semipenetrans)
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From page 20... ...
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.
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From page 21... ...
Protection 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.
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From page 22... ...
Growth and development 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.
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From page 23... ...
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.
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From page 24... ...
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.
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From page 25... ...
Such examples of stimulation by plant roots appear to be a refinement of parasitism. Root exudates and other chemicals may also inhibit egg-hatching or may repel nematodes.
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From page 26... ...
1960. Some aspects of the ecology of free-living and plant parasitic nematodes.
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From page 27... ...
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.
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From page 28... ...
Such variations may also represent weak points in metabolism, toward which attempts to develop specific control measures could be directed. Synthesis of amino acids by two species of plant-parasitic nematodes incubated in 14C-labeled glucose and acetate solutions in the absence of plant tissues has been shown, illustrating that it is not imperative for nematodes to feed on cells in order to take in chemicals.
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From page 29... ...
Oxygen consumption by intact specimens of several species of plantparasitic nematodes has been measured by microrespirometry. Rates of oxygen uptake varied according to nematode species, condition of the nematodes, and carbon dioxide and osmotic concentrations of the incubation solution.
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From page 30... ...
MOISTURE The natural environment of nematodes is aquatic. The soil solution that covers soil particles and is in soil pores is the medium in which nematodes live and through which they move to contact plant tissues.
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From page 31... ...
are available only for the short time of flower embryo development, after which weeks or months may elapse before plant tissues are again available. Larvae of the wheat nematode have been revived from galls after 28 years of storage, stem nematodes (Ditylenchus dipsaci)
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From page 32... ...
HATCHING, MOLTING, GROWTH, AND SEX DETERMINATION Eggs of most nematode species hatch freely in water, soil solution, or other aqueous solutions of low osmotic concentration. However, hatch of eggs of certain cyst nematodes is greatly increased by incubation in solutions leached from the host-plant root system.
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From page 33... ...
The free-living nematode Caenorhabditis briggsae is known to require six B-vitamins and ten amino acids. Although all plant-parasitic nematodes require living plant tissues for reproduction, there is great variation in the host ranges of different nematodes.
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From page 34... ...
Laboratory culturing of plant-parasitic nematodes on a scale adequate for biochemical studies has been successful with about 12 nematode species. Sterile root cultures, grown on nutrient agar media, were first utilized for culturing nematodes.
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From page 35... ...
1959. Ecological relationships of nematodes, pp.
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From page 36... ...
The stem nematode (Ditylenchus dipsaci) causes swelling and distortion of stems and leaves (Figure 7)
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From page 37... ...
can be seen on the roots of host plants (Figure 12) if the soil is carefully removed from the roots.
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From page 38... ...
by the wheat-gall nematode, Anguina tritici (Courtesy of Shell Development Company.) EXPERIMENTAL DETERMINATION OF PATHOGENICITY In general, because nematodes are unobtrusive plant parasites, their pathogenicity must be established experimentally.
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From page 39... ...
Differences in nematode population levels can be created by soil fumigation, by previous cropping, or by adding nematodes to uninfested soil. Subsequent plant growth in soil thus treated can be correlated with nematode population level.
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From page 40... ...
40 FACTORS INFLUENCING NEMATODE CONTROL FIGURE 8 Angular discoloration produced by the chrysanthemum foliar nematode, Aphelenchoides ritzemabosi, in chrysanthemum leaves. (Courtesy of Nematology Investigations, USDA.)
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From page 41... ...
- - / . • FIGURE 9 Galling of carrots by a root-knot nematode, Meloidogyne incognita, (Courtesy of the Department of Nematology, University of California, Riverside.)
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From page 42... ...
Courtesy of Plant Disease Reporter.)
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From page 43... ...
FIGURE 12 Mature females of the golden nematode, Heterodera rostochiensis, on potato roots. (Courtesy of the Department of Plant Pathology, Cornell University.)
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From page 44... ...
Involvement of a nematode species in a plant disease may be judged by comparing plants inoculated with a water suspension of nematodes isolated from roots or soil with plants inoculated with an otherwise similar suspension but freed of nematodes by a technique such as sieving. Freeing the suspension of nematodes by sieving provides a valid check on associated microorganisms if these microorganisms pass through the sieves that are employed.
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From page 45... ...
By correlating findings from these studies, it may be possible to determine the fundamental basis for planttissue alterations that occur during nematode parasitism. For convenience, the gross symptoms of nematode parasitism of plant tissues may be separated into galling; necrosis, or death, of cells or tissues; distortion; and inhibition of growth.
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From page 46... ...
Galls enlarge by hyperplasia and hypertrophy of cortical parenchyma cells, while nematodes are simultaneously destroying cortical cells and thus forming cavities. Galls may continue to enlarge while the plant is actively growing.
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From page 47... ...
PATHOGENIC RELATIONSHIPS 47 FIGURE 14 Cross section of an alfalfa stem galled by Ditylenchus dipsaci, 20 days after nematode inoculation (After L
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From page 48... ...
. Under the continued influence of nematode feeding, walls between enlarging cells and adjacent cells dissolve, the protoplasts combine, and, as additional surrounding cells are incorporated, so-called giant cells are formed (Figure 18)
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From page 49... ...
Simultaneous with giant-cell formation, other changes occur in plant tissues surrounding the feeding root-knot nematode. Hypertrophy and hyperplasia occur in the pericycle, cortex, and epidermis adjacent to the enlarging nematode, so that it is continually enveloped by plant tissues (Figure 18)
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From page 50... ...
Cyst nematodes also induce giant cells in plant tissues, but galls are not formed, as there is little or no hypertrophy or hyperplasia in cells surrounding the nematode. Instead, the nematode breaks out of the root as it grows, so that only the head and neck are embedded in the root when the nematode reaches maturity (Figure 19)
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From page 51... ...
(N) , giant cells (GC)
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From page 52... ...
in root tissues. (After B
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From page 53... ...
Except in a few instances, little is known about progressive histopathological changes occurring in host tissues during penetration of nematodes into plants and establishment of the parasitic relationship. Lesion nematodes apparently penetrate almost anywhere along the roots.
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From page 54... ...
. Localized cortical necrosis of plant roots is caused by spiral (Helicotylenchus spp.
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From page 55... ...
PATHOGENIC RELATIONSHIPS 55 FIGURE 21 A cross section of a grapefruit root infected by the burrowing nematode, Radopholus similis, showing cavities in cortical parenchyma containing nematodes (N)
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From page 56... ...
Inhibition of Apical Growth Inhibition of apical growth of plant tissues may accompany galling or necrotic responses to nematode parasitism. However, growth inhibition may be the only visible response to nematode feeding.
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From page 57... ...
No histopathological evidence of feeding was found in sections of alfalfa roots on which specimens of tobacco stunt nematode were microscopically observed feeding on epidermal cells. Certain nematodes feeding ectoparasitically near meristematic tissues of aboveground plant parts may also inhibit apical growth.
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From page 58... ...
Such enzymes may be involved in inhibition of apical growth in plants affected by nematodes. Recent studies have elucidated some of the biochemical changes resulting from nematode infection of plant tissues.
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From page 59... ...
These findings, as well as results from histopathological studies, indicate that root galls induced by root-knot nematodes are very active metabolically. Levels of certain biochemicals in plant tissues affected by other nematodes were investigated.
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From page 60... ...
Most investigations concern host-parasite interactions of root-knot and stem nematodes. Based on paper chromatographic and biological activity assays, galled plant tissues were reported to contain more auxin or indole compounds than healthy tissues.
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From page 61... ...
Such comparative studies, combined with physiological and biochemical studies of nematode-diseased plant tissues, should provide the information needed to understand the fundamental basis of nematode parasitism of plants. These kinds of studies should be encouraged.
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From page 62... ...
1963. The biology of plant parasitic nematodes.
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From page 63... ...
Specific examples are the tobacco varieties developed for resistance to the black shank fungus, Phytophthora parasitica var. nicotianae (Figure 26)
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From page 64... ...
The plants on the right were grown in soil inoculated with the black shank fungus only; the plants on the left were grown in soil inoculated with the black shank fungus and root-knot nematodes. Root-knot nematodes alone, not shown, caused only slight stunting.
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From page 65... ...
It is not unusual to find that certain species are primary in a given disease situation; however, whether or not a given nematode is the predominant species in causing disease in a plant will depend on factors such as the host, the initial population level of that nematode species as well as other nematodes, relative reproductive rates of the species involved, soil type, and other environmental factors. Furthermore, any advantage one nematode may have over another in a given situation is likely to be temporary and may change with the planting of a different crop and as the environment is otherwise modified.
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From page 66... ...
The fanleaf disease of grapes was the first virus disease shown to be transmitted by nematodes. Nematode species in three genera -- Xiphinema, Trichodorus, and Longidorus -- are now known to transmit some 20 viruses, many of economic importance, which together attack a wide range of host plants.
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From page 67... ...
For example, the effects of nematode parasitism of plant roots or other parts on leaf diseases caused by fungi, bacteria, and viruses as well as the physiological and biochemical bases for changes in susceptibility or disease expression need to be investigated.
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From page 68... ...
1963. Role of plant-parasitic nematodes in bacterial diseases.
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From page 69... ...
Nematode eggs, larvae, or adults may be present in the soil and are usually more vulnerable to toxic chemicals or cultural practices than are nematodes within plant tissues. Certain developmental stages of some nematodes, such as the cysts of the sugar-beet nematode (Heterodera schachtii)
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From page 70... ...
Suitable crop rotations, chemical treatments, or use of resistant varieties may reduce infestations to levels below those that cause economic losses in crop yield. But a few nematodes survive even under the most rigorous control programs, and, under favorable conditions and with repeated plantings of suitable host plants, these survivors inevitably build up again to damaging levels.
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From page 71... ...
However, endoparasites, which feed and reproduce inside plant root tissues, such as root-knot nematodes, are not as readily killed in living plant roots; consequently, control has not been satisfactory when measured in growth response and yields. Another example of the importance of differences in life habits is encountered when field soil is turned repeatedly during the summer to control nematodes.
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From page 72... ...
Nematodes in the soil are more difficult to recover for identification than those in plant tissues. Control of nematodes in perennial tree and vine crops is more difficult than in annual or herbaceous crops.
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From page 73... ...
are sometimes used to macerate plant tissues to free the nematodes. The resulting suspension of plant parts and nematodes can be examined directly or can be fractionated with sieves before examination.
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From page 74... ...
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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From page 75... ...
CONSIDERATIONS BASIC TO NEMATODE CONTROL 75 FIGURE 30 Food blender for macerating plant parts.
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From page 76... ...
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 substage mirror.
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From page 77... ...
. Increasing the specific gravity of the soil solution with sugar and subsequent centrifugation is another commonly used method.
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From page 78... ...
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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From page 79... ...
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.
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From page 80... ...
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)
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From page 81... ...
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.
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From page 82... ...
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, advice is given to growers as to whether to plant potatoes or to continue to plant nonhost crops.
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