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contained in the saliva secreted by the nematode in the giant cells during feeding. After male nematodes c e a se to fee d and when female nematodes die or are killed experimentally, the giant cells degener-ate. Whe n giant cells form in the stele, irregular xylem elements may develop around them or d e v e l o p m e nt of xylem vessels may b e inter-rupted. X y l em elements already present may b e crushed or disrupted by the mechanical pressure exerted by the enlarging cells. In the early stages of gall development the cortical cells enlarge in size but, during the later stages, they also divide rapidly. S w e l l i ng of the root results also from hypertrophy and hyperplasia of the vascular parenchyma, pericycle, and endodermis cells surrounding the giant cells and from enlargement of the nematode. As the females enlarge and e g g sacs are formed, they p u sh outward, may split the cortex and may b e c o me ex-p o s ed on the surface of the root or may remain comex-pletely covered, d e p e n d i ng on the position of the nematode in relation to the root sur-face.
In addition to the disturbance c a u s ed to plants by the nematode galls themselves, frequently d a m a ge to infected plants is increased by certain parasitic fungi, which can easily attack the w e a k e n ed root tis-sues and the hypertrophied, undifferentiated cells of the galls. More-over some of these fungi grow and reproduce much faster in the galls than in other areas of the root, thus inducing an earlier breakdown of the root tissues than would have b e e n possible otherwise.
Control
Root knot can b e effectively controlled in the greenhouse with steam sterilization of the soil or soil fumigation with nematocides. In the field the best control of root knot is obtained by fumigating the soil with chemicals such as D D at 25 gallons per acre or E D B at 15 to 20 gallons per acre. E a ch treatment usually gives satisfactory control of root knot for one season. For certain crops and for certain geographical areas and locations, good control of root knot has b e e n obtained by flooding the soil for several weeks or months, by dry summer fallow and dry tillage of the soil, or by changing the time of planting so that crops grow during the period of least activity of the nematodes. Con-trol of root knot by crop rotation although possible is usually impracti-cal b e c a u se of the large number of crop plants that can b e attacked by root-knot nematodes. Varieties resistant to root-knot nematodes are available in relatively few crops, but resistance to root knot has b e e n found in many crops, and increasing emphasis is b e i ng placed on
ef-Soybean Cyst Nematode
S e l e c t ed References
Bird, A. F. 1961. T h e ultrastructure a nd histochemistry of a n e m a t o d e - i n d u c ed giant c e l l . /. Biophys. Biochem. Cytol. 1 1 : 7 0 1 - 7 1 5 .
Brodie, Â. B., a nd W. E . Cooper. 1964. Relation of parasitic n e m a t o d es to post-emer-g e n c e dampinpost-emer-g-off of cotton. Phytopatholopost-emer-gy 54: 1 0 2 3 - 1 0 2 7 .
Christie, J. R. 1936. T h e d e v e l o p m e nt of root-knot n e m a t o de galls. Phytopathology 26:
1-22.
Christie, J. R. 1946. Host-parasite relationships of the root-knot n e m a t o d e, Heterodera marioni. II. S o me effects of the host on the parasite. Phytopathology 36: 3 4 0 - 3 5 2 . D r o p k i n, V. H., a nd P. E. N e l s o n. 1960. T h e histopathology of root-knot n e m a t o de
infections in s o y b e a n s. Phytopathology 50: 4 4 2 - 4 4 7 .
H o d g e s, C. F., a nd D. P. Taylor. 1966. Host-parasite interactions of a root-knot n e m a-tode a nd c r e e p i ng b e n t g r a s s, Agrostis palustris. Phytopathology 56: 8 8 - 9 1 .
Linford, Ì . B. 1937. T h e f e e d i ng of the root-knot n e m a t o de in root-tissue a nd nutrient solution. Phytopathology 27: 8 2 4 - 8 3 5 .
Linford, Ì . B. 1941. Parasitism of the root-knot n e m a t o de in leaves a nd stems. Phyto-pathology 3 1 : 6 3 4 - 6 4 8 .
Sasser, J. N. 1954. Identification a nd host-parasite relationships of certain root-knot n e m a t o d es (Meloidogyne sp.). Maryland Agr. Expt. Sta. Bull. A-77: 30 p p.
Smith, J. J., a nd W. F. Mai. 1965. Host-parasite relationships of Allium cepa a nd Meloi-dogyne hapla. Phytopathology 5 5 : 6 9 3 - 6 9 7 .
Tyler, J o c e l y n. 1933. T h e root-knot n e m a t o d e. Calif. Agr. Expt. Sta. Circ. 330: 34 p p.
Soybean Cyst N e m a t o de Occurrence and Importance
T h e soybean cyst nematode has b e e n found in northeastern Asia, Japan, and the states North Carolina, Virginia, T e n n e s s e e, Missouri, Illinois, Arkansas, Kentucky, Mississippi, Florida, Georgia, and Ala-bama. It continues to spread slowly to n e w areas in spite of the strict quarantine measures i m p o s ed on the presently infested areas. T h e most severely affected host is soybean, but several other l e g u m e s, such as common bean, vetch, l e s p e d e z a, lupine, and a few nonlegumi-nous plants are also attacked by this nematode. D e p e n d i ng on the d e g r ee of infestation, it can cause losses varying from slight to com-forts to incorporate this resistance into horticulturally acceptable vari-eties.
521
plete destruction of the crop. Usually, however, in heavily infested fields yield is r e d u c ed from 30 to 75 %.
Symptoms
Aboveground parts of infected soybean plants appear stunted and have an unthrifty appearance. T h e foliage turns yellow prematurely and falls off early. T h e plants bear only a few flowers and a few small seeds. If infected plants are growing on sandy soil they may die. In-fected plants growing on fertile soils with plenty of moisture may show only slight chlorosis of the older leaves, little or no stunting, and may produce a nearly normal yield for a year or two. In s u b s e q u e nt years, however, d ue to the tremendous build-up of nematodes in the soil, plants in these areas also b e c o me severely chlorotic and dwarfed.
T h e root system of infected plants appears smaller than that of healthy plants, but no macroscopic lesions, galls, or other type of ab-normalities are evident on infected roots. Roots of infected plants usually have considerably fewer bacterial nodules than those of healthy plants (Fig. 119). T h e most characteristic symptom of this dis-ease is the presence of female nematodes in various stages of develop-men t and of cysts attached on the soybean roots (Fig. 120). Youn g fe-males are small, white and partly buried in the root with only part of them protruding on the surface. Older females are larger, almost completely on the surface of the root, and appear yellowish or brown d e p e n d i ng on maturity. D e a d, brown cysts are also present on the roots.
The Pathogen: Heterodera glycines
T h e soybean cyst nematode overwinters as a brown cyst in the up-per 90-100 cm of soil. T h e cysts are the carcasses of the females and are filled with eggs. T h e eggs contain fully d e v e l o p ed second-stage larvae about 440 μ long (Fig. 121). Whe n temperature and moisture b e c o me favorable in the spring, the larvae e m e r ge from the cysts and infect roots of host plants.
At 4-6 days after penetrating the roots, the larvae molt and produce the third-stage larvae. T h e male third-stage larva is slightly more ro-bust than the female, and both are much stouter than the second-stage larvae. About 9-12 days after infection, fourth-stage larvae b e g in to appear. T h e male larva resembles in size and shape the third-stage male larva. T h e female fourth-stage larva, however, loses its some-what slender appearance and develops the typical flask shape,
mea-Soybean Cyst Nematode 523
F i g. 119. (A) S o y b e an roots infected with the s o y b e an cyst n e m a t o d e. N o te a b s e n ce of nitrogen-fixing nodules. (B) H e a l t hy s o y b e an roots b e a r i ng n u m e r o us beneficial n o d u l e s. (Photo by courtesy of U . S. D e p t. Agr.)
suring approximately 0.40 m m in length by 0.12-0.17 m m in width. By day 12 to 15, adult males and females appear.
T h e adult male is slender and wormlike, measuring about 1.3 m m long by 30-40 μ in diameter. T h e males remain in the root for a few days, during which they may or may not fertilize the females, then m o ve into the soil and soon die.
T h e adult females w h en fully d e v e l o p ed are lemon-shaped, mea-suring 0.6-0.8 m m in length and 0.3-0.5 m m in diameter. T h e y are white to pale-yellow at first, b e c o m i ng yellowish brown as they ma-ture. T h e body cavity of the female is almost completely filled by the ovaries, and as the ova gradually d e v e l op into fully formed eggs the
F i g. 120. L e m o n - s h a p ed e n c y s t ed f e m a le n e m a t o d es attached to s o y b e an roots. (Photo by courtesy of U.S. D e p t. Agr.)
body cavity of the female b e c o m es completely filled with eggs. As the female body distends during e g g production it crushes cortical cells, splits the root surface, and protrudes until it is almost entirely e x p o s ed through the root surface. A gelatinous mass, usually mixed with dirt and debris, surrounds the posterior e n d of the females and the nema-todes deposit some of their eggs in it. E g gs deposited in this matrix contain already fully d e v e l o p ed second-stage larvae, the first molt tak-ing place inside the e g g and while the e g g is still inside the female.
E a ch female produces 300-600 eggs, most of which remain inside her body when the female dies. E g gs in the gelatinous matrix may hatch immediately and the emerging second-stage larvae may cause ne w infections. E g gs remaining in the body of the female pass through the embryonic stages and reach the second larval stage, which either emerge s or lies quiescent in the e g g inside the d e ad female. Finally the carcass, darkening to brown, b e c o m es the cyst. Approximately 21-24 days are required for the completion of a life cycle of this nema-tode. T h e size of the brown cyst is determined by the extent of the
Fig. 121. Disease cycle of the soybean cyst nematode Heterodera glycines.