^2nd
^mol t
3rd \mol t
growth of the female before death occurs. T h e cyst consists of the female cuticle transformed through the secretions of the nematode into a tough, brown sac that persists in the soil for many years and pro-tects the eggs which have b e e n formed within the body. T h e surface of the cyst shows markings appearing as short, zigzag lines without order.
Development of Disease
T h e infective second stage larvae enter young primary roots or api-cal meristems of secondary roots by direct penetration of the epider-mis (Fig. 121). T h e advance into the cortex is mostly intracellular and results in distortion and death of invaded cells. T h e larvae orient themselves in various directions in relation to the axis of the roots. In many cases they pass through the cortex and pierce their stylets into cells of the endodermis or the pericycle. Cells adjacent to invaded ones are frequently stimulated by n e m a t o de secretions, elongate toward the penetrated region, and fill the s p a ce m a de by the invading larva. Within 2 days from penetration larvae come to rest and fee d on cells of the cortex and stele tissues causing the enlargement of these cells. Nuclei of cells surrounding those penetrated by nematode sty-lets migrate through dissolved walls of the enlarged cells and clump.
Such groups of enlarged cells, called syncytia, are surrounded by a single layer of small, hyperplastic cells which, however, disappear gradually as the cells associated with the feeding site of the nematode enlarge and their cell walls undergo further dissolution and allow en-largement of the syncytia. E n l a r g ed cortical cells are connected through perforated walls to tissues of the endodermis, pericycle, and to vascular elements. During the d e v e l o p m e nt of the third-larval stage cortical cells surrounding the nematode are crushed by the expanding nematode body. Greater distortion of cortical tissue is c a u s ed by de-veloping females than by comparable males. Whe n the nematode feeds on the pericycle adjacent to protophloem tissue, syncytial devel-opment is restricted largely to pericyclic tissue. However, when the nematode feeds near or opposite protoxylem poles, syncytial develop-men t is relatively unrestricted and occurs in tissues of the d e e p er peri-cycle, phloem, and secondary cambium. It has b e e n observed that larvae feeding on cortex, endodermis or pericycle tissues which are opposite protophloem or crushed protophloem d e v e l op into males, while larvae feeding on syncytia which extend into the stele at or near the protoxylem poles and incorporated pericyclic, phloem, and sec-ondary cambial tissue, d e v e l op into females. Syncytia in contact with
Soybean Cyst Nematode 527
d e v e l o p i ng third or fourth stage males b e g in to show signs of degener-ation indicating cessdegener-ation of feeding. Syncytia in contact with females remain healthy u p to and b e y o nd the stage of e g g deposition. D e g e n-eration of syncytia is accompanied by reduction of syncytial volume and results in the receding and collapse of the syncytial wall. T h e re-sulting space is only partly occupied by surrounding parenchymatous tissue.
Whe n soybean varieties resistant to the soybean cyst nematode are attacked, there is no apparent inhibition of penetration of the orga-nism into the host tissues. Syncytia are formed within 2-3 days from inoculation, but by day 5 many of them degenerate and most second-stage larvae associated with them are dead. A few nematodes advance to the third stage, but no adult males or females are produced. Devel-opment of syncytia and s u b s e q u e nt degeneration and necrosis is re-stricted to the periphery of the stele and to regions in the cortex that are invaded and stimulated by infective larvae. T h e root regions va-cated by degenerate syncytia are quickly filled by adjacent rejuve-nated parenchyma cells.
Syncytial d e v e l o p m e nt into the secondary cambial region of suscep-tible varieties results in inhibition of secondary growth of both p h l o em and xylem. Since a short portion of a root may b e attacked by many larvae, the large number of syncytia that d e v e l op may cause w i d e s p r e ad reduction of the conductive elements, resulting in the re-stricted growth and yield of soybean plants, especially under stresses of moisture.
Control
Soil fumigation of soybean cyst nematode-infested fields with a va-riety of nematocides temporarily increases plant growth and soybean yield. N e m a t o de cysts and larvae, however, are almost never eradi-cated from a field completely by fumigation and a small nematode population left over after fumigation can b u i ld up rapidly on the vigor-ous soybean grown in newly fumigated soil. In addition, soybeans are a relatively low-value crop and the cost of fumigation per acre makes its u se impractical.
T h e most practical method of control of the soybean cyst nematode is through crop rotation. Since s o me l e g u m es are the only other culti-vated crops that are hosts of this nematode, a two- to three-year or longer rotation of soybeans with nonsusceptible crop plants is possi-ble and practical. T h e effectiveness of crop rotation is increased by
planting the more resistant soybean varieties which do not allow a quick and excessive build-up of nematode populations. Several soy-bean varieties exhibiting varying degrees of resistance to the soysoy-bean cyst nematode are presently available, and they should always b e pre-ferred for fields infested with this nematode.
Quarantine regulations are presently enforced to prevent the para-site from spreading into nematode-free areas by means of contami-nated soil, products, machinery, or other articles.
Selected References
A n o n y m o u s. 1961. S o y b e an cyst n e m a t o d e. US. Dept. Agr., Agr. Res. Serv. Spec. Rept.
2 2 - 7 2 : 2 0 p p.
E n d o, Â. V . 1964. Penetration a nd d e v e l o p m e nt of Heterodera glycines in s o y b e an roots and related anatomical c h a n g e s. Phytopathology 54: 79-88.
E n d o, Â. V. 1965. Histological r e s p o n s es of resistant a nd s u s c e p t i b le s o y b e an varieties, a nd backcross p r o g e ny to entry a nd d e v e l o p m e nt of Heterodera glycines. Phyto-pathology 55 : 3 7 5 - 3 8 1 .
E n d o, Â. V., a nd J. N. Sasser. 1958. Soil fumigation e x p e r i m e n ts for the control of the s o y b e an cyst n e m a t o d e, Heterodera glycines. Phytopathology 48: 5 7 1 - 5 7 4.
E p p s, J. M., a nd Á. V. C h a m b e r s. 1965. Population dynamics of Heterodera glycines under various c r o p p i ng s e q u e n c es in field bins. Phytopathology 55: 100-103.
Ichinohe, M. 1959. S t u d i es on the s o y b e an cyst n e m a t o d e. Heterodera glycines, a nd its injury to s o y b e an plants in J a p a n. Plant Disease Reptr. Suppl. 260: 2 3 9 - 2 4 8 .
Skotland, C. B. 1957. Biological studies of the s o y b e an cyst n e m a t o d e. Phytopathology 47: 6 2 3 - 6 2 5 .
Smart, G. C , Jr. 1962. Distribution of cysts of Heterodera glycines in soil at different depths. Phytopathology 52: 1221 (abstr.).
L e s i on Nematodes
Occurrence and Importance
L e s i on or m e a d ow nematodes occur in all parts of the world, where they attack the roots of many different kinds of plants. A m o ng the hosts of economic importance are field crops, such as tobacco, alfalfa, cotton; cereal crops, such as wheat, corn, oats; vegetable crops, such as tomato, potato, carrot; fruit trees, such as apple, peach, cherry; and many ornamentals, both herbaceous and shrubs.
T h e severity of d a m a ge c a u s ed by the lesion nematodes is difficult to estimate. It varies with the crop attacked and is greater in
subtropi-Lesion Nematodes 529
cal than in temperate regions. T h e d a m a ge to plants consists in root reduction or inhibition by formation of local lesions on young roots which may b e followed by root rotting d ue to secondary fungi, bacte-ria, etc. As a result of the root d a m a ge affected plants grow poorly, produce low yields, and may finally die.
Symptoms
Susceptible herbaceous host plants affected by lesion nematodes appear stunted and chlorotic as though they are suffering from mineral deficiencies or lack of water. Usually several plants are affected in one area, producing patches of plants with r e d u c ed growth and yellowish-green color which can b e s e en from a distance. As the season pro-gresses stunting b e c o m es more pronounced, the foliage wilts during hot s u m m er days, and the color of the leaves b e c o m es yellowish brown. S u ch plants can b e easily p u l l ed from the soil b e c a u se of the extensive destruction of the root system. Yields of affected plants are r e d u c ed in varying degrees, and in severe infections the plants are killed.
Whe n shrubs or trees are attacked by lesion nematodes, d a m a ge is usually slow to appear; it is less obvious than that on herbaceous hosts, and it rarely kills the plants. T h e symptoms usually consist of isolated trees or patches of trees gradually b e c o m i ng unthrifty and producing poor crops. T h e leaves are smaller in size, their color b e i ng a dull green or yellow. Terminal branches may lose their leaves pre-maturely and die back. T h e whole appearance of affected trees indi-cates that the trees are w e a k e n ed and are in a condition of decline.
T h e patches of affected trees may slowly increase in size, although this h a p p e ns over a rather long period.
T h e root symptoms of affected plants consist of lesions which first appear as tiny, elongate, water-soaked, or cloudy yellow spots, but which soon turn brown to almost black. T h e lesions appear mainly on the young feeder roots and they are most concentrated in the area of the root hairs, but they may appear anywhere along the roots. T h e le-sions enlarge mostly longitudinally following the root axis and they may coalesce with other lesions, but at the s a me time they slowly ex-p a nd laterally until they finally girdle the entire root, which they kill.
As the lesions enlarge the affected cells in the cortex collapse and the discolored area appears constricted. Secondary fungi and bacteria usually accompany nematode infections in the soil and contribute to further discoloration and rotting of the affected root areas, which may slough off. Moderately affected plants exhibit varying degrees of root
survival, and in some hosts production of adventitious roots may b e stimulated by the infection, but generally the individual roots are dis-colored and stubby, and the whole root system is severely r e d u c ed by the root pruning that results from the formation of lesions (Fig. 122).
The Pathogen: Pratylenchus sp.
Nematodes of the genus Pratylenchus are approximately 0.4-0.7 mm long and 2 0 - 25 μ in diameter. T h e y appear as stout, cylindroid
F i g. 122. Roots of b e an plants s h o w i ng s y m p t o ms c a u s ed by lesion n e m a t o d e s.
Healthy plant on left. (Photo by courtesy of the D e p a r t m e nt of Plant Pathology, Cornell University.)
Lesion Nematodes
nematodes with a blunt head, strong, stout spear, and bluntly rounded tail (Fig. 123). T h e y are migratory, endoparasitic nematodes affecting the roots of many kinds of plants. D e v e l o p m e nt and reproduction of Pratylenchus nematodes is rather slow, the life cycle of the various species b e i ng completed within 4 5 - 6 5 days. T h e se nematodes over-winter in infected roots or in soil as eggs, larvae, or adults, except for the egg-producing females which s e em to b e unable to survive the winter. Adults and larvae of various ages can enter and leave roots of susceptible hosts. T h e females, with or without fertilization, lay their eggs singly or in small groups inside infected roots. T h e eggs remain in the roots and hatch there, or when the root tissues break down, they are released into the soil. T h e first larval stage and the first molt occur in the egg. T h e emerging second-stage larva moves about in the soil or enters the root, in either case d e v e l o p i ng into the s u b s e q u e nt larval stages and the adults. Whe n in the soil the nematodes are susceptible to drying and during periods of drought they lie quiescent until the moisture increases and the plants r e s u me growth.
Development of Disease
Larvae and adult Pratylenchus nematodes enter roots usually in a radial direction by penetrating cells directly or by forcing their way b e t w e en the cortical cells. Penetration occurs anywhere along roots (Fig. 123). Intracellular penetration is accomplished by a persistent thrusting of the stylet and head which s e e ms to soften and break the cell wall. T h e contents of invaded cells and of some adjacent ones b e c o me disorganized and much of the cytoplasm disappears or re-treats, together with the nucleus, against the cell wall. T h e cell walls and, frequently, the adherent cytoplasm usually turn light brown in color and appear as small, discolored spots within a few hours after inoculation. T h e nematodes m o ve into the cortex and they either fee d and reproduce in the mid-cortical layers or they move quickly to the d e e p er layers of the cortex and align themselves with the longitudinal axis of the root, just outside the endodermis. In s o me hosts, lesion nematodes penetrate the endodermis and invade vascular and paren-chymatous cells of the stele, but usually the endodermis is not at-tacked even when the nematodes completely fill the area b e t w e en the endodermis and the epidermis (Fig. 124). Necrosis of cortical cells fol-lows the path of nematodes, but discoloration of the affected and ad-joining cells varies with the host plant. Sometimes only 1 or 2 cells on
each side of the nematode tunnels are affected, especially when nematodes move parallel to the root axis, but at other times, they move
531
F i g. 123. L e s i on n e m a t o d es (Pratylenchus sp.) penetrating y o u ng carrot root. (Photo by courtesy of R. A. Rohde.)
in a more circular fashion and the lesion involves over half the circum-ference of the root. T h e part of the endodermal layer adjacent to the nematode also b e c o m es discolored, taking on a d e e p and permanent brown color which extends into rather large groups of cells. As the feeding of the nematode on cortical cells continues, cell walls break down, and cavities appear in the cortex with their walls sometimes lined with brown tissue.
Neither hypertrophy nor hyperplasia s e em to b e c a u s ed by the le-sion nematodes, although some cellular divile-sions around points of invasion occur in some host plants. E a ch lesion is usually inhabited by more than one nematode, and sometimes single host cells are si-multaneously trans versed by four or more nematodes. T h e females lay their eggs in the cortex, and frequently eggs, larvae, and a few adults form " n e s t s "; these occur in great numbers in the cortex. U p on hatch-ing of the eggs, the nematodes fee d on the parenchyma cells and move mostly lengthwise within the cortex, thus enlarging the lesion (Fig.
125). S o me of the nematodes leave the lesion, e m e r ge from the root, and travel to other points of the root or other roots where they cause
Lesion Nematodes 533
ne w infections. Necrotic cortical tissues of large lesions are sloughed off or are invaded by secondary fungi and bacteria with resulting rot-ting and breakdown of the root tissues around the point of infection and s u b s e q u e nt death of the distal part of the root b e y o nd the point of infection. T h u s, the number of functioning roots of the plant is drasti-cally reduced, absorption of water and nutrients b e c o m es insufficient, and the aboveground parts of the plant b e c o me stunted and chlorotic, showing symptoms of water and nutrient deficiencies.
F i g. 124. (A) L e s i on n e m a t o de f e e d i ng within a y o u ng carrot root. (B) C r o ss section of a y o u ng carrot root s h o w i ng six lesion n e m a t o d es in the cortex a nd one in the stele. (Photos by courtesy of R. A. Rohde.)
Fig. 125. Disease cycle of the lesion nematode Pratylenchus sp.