Population Density and Spatial Pattern of Heterodera glycines in Relation to Soybean Phenology

Population dynamics of Heterodera glycines (SCN) were influenced by initial nematode population density in soil, soybean root growth pattern, soil type, and environmental conditions in two field experiments. Low initial populations (Pi) of SCN increased more rapidly during the growing season than high Pi and resulted in greater numbers of nematodes at harvest. Egg and juvenile (J2) populations increased within 2-6 weeks after planting when early-season soil temperatures were 20 C and above and were delayed by soil temperatures of 17 C or below in May and early June. Frequencies of occurrence and number of nematodes decreased with increasing depth and distance from center of the soybean row. Spatial pattern of SCN paralleled that of soybean roots. Higher clay content in the subsoil 30-45 cm deep in one field restricted soil penetration by roots, indirectly influencing vertical distribution of SCN. Shoot dry weight was a good indicator of the effect of SCN on seed yield. Root dry weight was poorly correlated with soybean growth and yield. The relationship of yield (seed weight) to Pi was best described by a quadratic equation at one site, but did not fit any regression model tested at the second site.     

Effect of Soybean Tip Removal on Penetration and Development of Heterodera glycines

On a few occasions, soybeans with broken root tips were included in tests to evaluate resistance to Heterodera glycines. Although females developed on these plants, the numbers tended to be lower than on similarly treated intact roots. To test the possibility that removal of the root meristem affected nematode development, a culture system using pruned soybeans was devised that permitted access to the roots without disturbing the plants. Treatments included removal of 2 mm of root tip at various times ranging from 24 hours before to 10 days after inoculation, or roots left intact. In each experiment, all roots were inoculated at the same time with equal numbers of freshly hatched second-stage juveniles of Heterodera glycines. No differences in nematode development were detected in plants with root tips removed after inoculation compared to the control. When tips were removed at or before inoculation, fewer juveniles entered roots and relatively fewer nematodes developed. Penetration levels and development correlated with root tip removal such that progressively fewer nematodes entered roots and relatively greater numbers of nematodes remained undeveloped as the time interval between root tip removal and inoculation was increased.     

Influence of Soybean Cultivar on Reproduction of Heterodera glycines in Monoxenic Culture

Nematodes produced in monoxenic culture are used for many research purposes. To maximize the number of Heterodera glycines produced in culture, 24 soybean cultivars (maturity groups 0-8) were evaluated for host suitability. A strain of H. glycines race 3, maintained in monoxenic culture on excised soybean root tips of cv. Kent, was inoculated into 20 petri dishes of each cultivar. The highest numbers of first-generation females per petri dish were produced on cultivars Bass, Williams 82, Kent, Proto, and Chapman, and the lowest on cultivars Lambert and Chesapeake. A diapause-like period with decreased nematode production was recorded on some cultivars but not others. Six generations of cultivation on CX 366 did not affect the number of females produced. The results indicated that soybean maturity group could not be used as a parameter for selecting the optimum cultivars for nematode production, and that only J2 petri dishes needed to be counted to determine a 60-female difference per petri dish among cultivars. This study demonstrated that H. glycines populations in monoxenic culture can be more than quadrupled by selection of an appropriate soybean cultivar.     

Influence of Selected Plant Species on Hatching of Eggs and Development of Juveniles of Heterodera glycines

The influence of selected plant species on egg hatch and subsequent development of Heterodera glycines race 3 was investigated. Plants tested included four soybean cultivars, red clover, alfalfa, hairy vetch, field corn, sweet corn, cabbage, tobacco, cotton, and wheat. Soybean stimulated egg hatching more than any of the other plant species, with H. glycines-resistant cultivars being more stimulating than susceptible ones. Hairy vetch also increased hatch. Roots of cabbage, red clover, alfalfa, and hairy vetch were readily penetrated by juveniles of H. glycines. Maturation to adult occurred only on soybean and hairy vetch.     

Susceptibility of Soybean Introductions to Races 1, 2, 3, and 4 of Heterodera glycines

Thirteen soybean plant introduction (PI) lines, selected for their apparent susceptibility to Heterodera glycines, were compared with cultivar Lee 74 as hosts of H. glycines races 1, 2, 3, and 4. Race 3 produced the highest average number of females of the four races. Compared to Lee 74, more (P = 0.05) females of H. glycines race 1 were extracted from eI 274420, PI 274423, and PI 317333; PI 86457 had more females of H. glycines race 2; and PI 86443, PI 86457, PI 261467, PI 274420, PI 274421, and PI 274423 had more females of H. glycines race 3. Similar numbers of females of H. glycines race 4 developed on all of the soybean lines and Lee 74. PI 274421, PI 274420, or PI 196159 could provide a more or equally susceptible host for H. glycines races 1, 2, 3, and 4 than Lee 74. One of these three lines could be substituted for Lee as the standard susceptible cultivar in the race determination test.     

Relationship Between Time of Infection with Heterodera glycines and Soybean Yield

Experiments were conducted to determine the relationship between time of infection by Heterodera glycines and soybean growth in the greenhouse and yield of plants grown in the field. Soybean cultivar Essex seedlings growing in the greenhouse were inoculated with H. glycines at 2, 4, or 6 weeks after planting. Seedling growth was inhibited by H. glycines infection at 2 or 4 weeks after planting but not at 6 weeks. Infection of Essex by H. glycines in the field was delayed 2-6 weeks by nematicides. Yields were significantly increased when H. glycines infection was delayed 2 weeks by nematicide treatment. Essex yields were highest when infection was delayed 6 weeks, equalling the yield of the H. glycines-resistant cultivar Asgrow 5474. The effect of H. glycines on soybean growth in the greenhouse and yields in the field declined when infection was delayed 6 weeks. Thus, soybean sensitivity to H. glycines seemed to diminish with age of the soybean plants.     

Effect of Soil Temperature and pH on Resistance of Soybean to Heterodera glycines

Soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is a major pest of soybean, Glycine max L. Merr. Soybean cultivars resistant to SCN are commonly grown in nematode-infested fields. The objective of this study was to examine the stability of SCN resistance in soybean genotypes at different soil temperatures and pH levels. Reactions of five SCN-resistant genotypes, Peking, Plant Introduction (PI) 88788, Custer, Bedford, and Forrest, to SCN races 3, 5, and 14 were studied at 20, 26, and 32 C, and at soil pH''s 5.5, 6.5, and 7.5. Soybean cultivar Essex was included as a susceptible check. Temperature, SCN race, soybean genotype, and their interactions significantly affected SCN reproduction. The effect of temperature on reproduction was quadratic with the three races producing significantly greater numbers of cysts at 26 C; however, reproduction on resistant genotypes remained at a low level. Higher numbers of females matured at the soil pH levels of 6.5 and 7.5 than at pH 5.5. Across the ranges of temperature and soil pH studied, resistance to SCN in the soybean genotypes remained stable.     

Dormancy of Heterodera glycines in Missouri

A 2-year study was conducted in field microplots to determine the relative importance of soybean phenology and soil temperature on induction of dormancy in Heterodera glycines in Missouri. Four near-isogenic soybean lines differing for maturity date were planted in microplots infested with a race 5 isolate of H. glycines. Soil temperature was monitored at a depth of 15 cm. Eggs of H. glycines, extracted from cysts collected monthly from each microplot, were used in hatching tests and bioassays to determine dormancy. Egg hatching and second-stage juvenile (J2) infectivity rates decreased sharply from their highest levels in midsummer (July-August) to a low level by October of each year and remained low (< 10% hatching and < 0.2 J2/cm root) until May or June of the following year. The patterns of numbers of females and eggs in the bioassays were similar. The decreases were not related to soil temperature and did not differ consistently among soybean isolines. The monophasic changes in all nematode responses with peak midsummer rates suggest that H. glycines produces one primary generation per year in central Missouri. Changes in hatching rates and the timing of minimum and maximum rates suggested that H. glycines eggs exhibit more than one type of dormancy.     

A Technique for Evaluating Heterodera glycines Development in Susceptible and Resistant Soybean

A technique was developed to evaluate Heterodera glycines development in susceptible and resistant soybean. Roots of 3-day-old soybean were exposed to infective juveniles of H. glyci.nes in sand for 8 hours followed by washing and transfer to hydroponic culture. The cotyledons and apical meristem were removed and plants were maintained under constant light, which resulted in a dwarfed plant system. After 15 or 20 days at 27 C, nematodes were rated for development. Emerged males were sieved from the culture water and females were counted directly from the roots. Nematodes remaining in the roots were rated for development after staining and clearing the tissues. The proportion of nematodes at each stage of development and the frequency of completed molts for each stage were calculated from these data. This technique showed that resistance to H. glycines was stage related and did not affect males and females equally in all resistant hosts. The resistance of plant introduction PI 209332 primarily affected development of third and fourth-stage juveniles; ''Pickett'' mainly affected second and third-stage juveniles, whereas PI 89772 affected all stages. Male development was markedly affected in PI 89772 and ''Pickett'' but not in PI 209332.     

Heterodera glycines in Indiana: I. Reproduction of Geographical Isolates on Soybean Differentials

Four of five geographical isolates of Heterodera glycines from Indiana classified as Race 3 using standard differentials showed many differences when classified using another group of differentials comprised of five soybean breeding lines and cultivars. Two isolates from northern Indiana produced cysts on more of the differentials tested than did three isolates from southern Indiana, suggesting that potential resistant lines should be tested on a range of H. glycines populations originating from the areas for which cultivars are being developed.     

Structural Changes Associated with Resistance of Soybean to Heterodera glycines

Subcellular responses to infection by Race 3 of Heterodera glycines in susceptible (''Lee'') and resistant (''Forrest'' and ''Bedford'') soybean cultivars were compared. Syncytial formation, initiated in susceptible as well as resistant soybean cultivars, was characterized by wall perforations, dense cytoplasm, and increased endoplasmic reticulum, In susceptible plants, syncytia developed continuously until nematode maturity. This included hypertrophy of nuclei, increase of rough endoplasmic reticulum in early stages of infection, and formation of wall ingrowths at a late stage of infection. In the resistant reaction in Forrest, a necrotic layer surrounded syncytium component cells demarcating them from surrounding normal cells and leading to syncytial necrosis. Wall appositions were prominently formed near the necrotic layer, and the cytoplasm of the syncytium component cells was extremely condensed. The whole syncytium became necrotic at a late stage of infection. Bedford had nuclear degeneration prior to cytoplasmic degradation. Chromatin was often scattered throughout the syncytial cytoplasm. Finally the whole syncytium became degenerated with plasmalemma completely detached from the syncytial cell walls. The differences in resistant responses reflect a difference in genetic composition of the soybean cultivars tested.     

Tolerance of Soybean to Heterodera glycines

Seven soybeans were selected from 200 entries evaluated for tolerance to soybean cyst nematode (SCN), Heterodera glycines. Tolerance to SCN was measured by comparing the seed yield from aldicarb-treated vs. nontreated plots. A yield response index (YRI) was calculated for each entry: YRI = (seed yield from nontreated plot/seed yield from treated plot) × 100. The soybean entries Coker 156, PI 97100, and S79-8059 exhibited high tolerance (YRI) to SCN when compared to Essex even though they became heavily infected with SCN. Tolerance in soybeans to SCN may be useful in pest management programs designed to stabilize soybean yield.     

Variability in Host Preference Among Field Populations of Heterodera glycines

Eighteen soybean fields, six each with race 3, race 4, or Bedford population of Heterodera glycines, were selected for testing of host variability. Each field was divided into three sections, and a bulk soil sample was taken from each section. The 54 bulk soil populations (BSP) and 270 single cyst populations (SCP) were subjected to race determination tests. Tests of the 18 BSP and 90 SCP from the race 3 fields revealed that race 3 was the predominant race; however, 68 of the populations tested were other races. Tests of the 18-BSP and 90 SCP from race 4 fields demonstrated that races 2 and 4 were predominant, with 38 and 39 populations; respectively. Tests of the 18 BSP and 90 SCP from the Bedford population fields revealed tremendous variability. Races 2, 4, and 6 were the predominant races, with 32, 31, and 28 populations, respectively. These results indicate that of the three races studied, the Bedford population is the most variable, race 3 shows considerable variability, and race 4 shows very little.     

Effects of Heterodera glycines and Meloidogyne incognita on Early Growth of Soybean

Greenhouse and field microplot studies were conducted to compare soybean shoot and root growth responses to root penetration by Heterodera glycines (Hg) and Meloidogyne incognita (Mi) individually and in combination. Soybean cultivars Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were selected for study. In the greenhouse, pot size and number of plants per pot had no effect on Hg or Mi penetration of Coker 237 roots; root weight was higher in the presence of either nematode species compared with the noninoculated controls. In greenhouse studies using a sand or soil medium, and in field microplot studies, each cultivar was grown with increasing initial population densities (Pi) of Hg or Mi. Interactions between Hg and Mi did not affect early plant growth or number of nematodes penetrating roots. Root penetration was the only response related to Pi. Mi penetration was higher in sand than in soil, and higher in the greenhouse than in the field, whereas Hg penetration was similar under all conditions. At 14 days after planting, more second-stage juveniles were present in roots of susceptible than in roots of resistant plants. Roots continued to lengthen in the greenhouse in the presence of either Mi or Hg regardless of host genotype, but only in the presence of Mi in microplots; otherwise, responses in field and greenhouse studies were similar and differed only in magnitude and variability.     

Hatch and Emergence of Heterodera glycines in Root Leachate from Resistant and Susceptible Soybean Cultivars

Egg hatch and emergence of second-stage juveniles (J2) of Heterodera glycines races 3 and 4 from cysts exposed to soybean root leachate of cv. Fayette (resistant to H. glycines) and H. glycines-susceptible cultivars A2575, A3127, and Williams 82 were determined in three sets of experiments. In the first experiment, cysts of both race 3 and race 4 were exposed to leachate of 8-week-old plants for a 2-week period. In the second experiment, cysts from populations of races 3 and 4 were raised on cultivars A2575, A3127, and Williams 82. Cysts then were exposed to leachate from 8-week-old plants for a 2-week period in all possible race-per-cultivar combinations. In the third experiment, cysts of races 3 and 4 were exposed at 4-day intervals to leachate from plants as the plants developed 7 to 59 days after planting. In experiments 1 and 2, leachate from 8-week-old Williams 82 and A3127 stimulated more hatch and emergence of H. glycines than leachate from A2575, Fayette, or the control. In the first experiment, cumulative hatch and emergence were greater for race 3 than for race 4. In experiment 2, no apparent relationship developed between leachate from a cultivar and the population developed on that cultivar in terms of stimulation of hatch and emergence. In the third experiment, A2575 stimulated more hatch and emergence of both race 3 and race 4 than A3127, Fayette, and Williams 82. Leachate from Fayette stimulated less hatch and emergence of both race 3 and race 4. Hatch and emergence were greatest during the initial 12 days of the experiment.     

Coating Soybean Seed with Oxamyl for Control of Heterodera glycines

Oxamyl coated on soybean (Glycine max (L.) Merr. cv. Elgin) seeds in solutions of 20, 40, 80, and 160 mg/ml had no serious deleterious effects on seedling emergence and growth when planted in sterile soil. Seedling emergence on day 3 was less than that of the uncoated control, but by day 7 emergence was equal to, or greater than, the control. Shoot and root growth from seed coated with oxamyl in 40 and 80 mg/ml solutions was greater than that of the control. In soil infested with soybean cyst nematode, Heterodera glycines, shoot weight of soybean plants from seeds coated with oxamyl in 80 mg/ml solution was 11 and 9% greater at weeks 3 and 7, respectively, than from uncoated seeds. Numbers of juveniles (J3 and J4) and adults of H. glycines observed on the roots of plants from oxamyl-coated seeds were 83, 42, and 49% less at weeks 3, 5, and 7, respectively, than numbers on the roots of the untreated control. Numbers of J2 extracted from the roots of plants from oxamyl-coated seeds were 75% less at weeks 5 and 7 than those extracted from roots of uncoated seeds. The numbers of J2 extracted from the soil planted to oxamyl-coated seeds were 51 and 33% less at weeks 5 and 7, respectively, than from soil planted to uncoated seed.     

Impact of Herbicides on Heterodera glycines Susceptible and Resistant Soybean Cultivars

Several abiotic and biotic stresses can affect soybean in a growing season. Heterodera glycines, soybean cyst nematode, reduces yield of soybean more than any other pathogen in the United States. Field and greenhouse studies were conducted to determine whether preemergence and postemergence herbicides modified the reproduction of H. glycines, and to determine the effects of possible interactive stresses caused by herbicides and H. glycines on soybean growth and yield. Heterodera glycines reproduction factor (Rf) generally was less on resistant than susceptible cultivars, resulting in a yield advantage for resistant cultivars. The yield advantage of resistant cultivars was due to more pods per plant on resistant than susceptible cultivars. Pendimethalin reduced H. glycines Rf on the susceptible cultivars in 1998 at Champaign, Illinois, and in greenhouse studies reduced dry root weight of H. glycines-resistant and susceptible cultivars, therefore reducing Rf on the susceptible cultivars. The interactive stresses from acifluorfen or imazethapyr and H. glycines reduced the dry shoot weight of the resistant cultivar Jack in a greenhouse study. Herbicides did not affect resistant cultivars'' ability to suppress H. glycines Rf; therefore, growers planting resistant cultivars should make herbicide decisions based on weeds present and cultivar tolerance to the herbicide.     

Impact of Planting 'Bell', a Soybean Cultivar Resistant to Heterodera glycines, in Wisconsin

Although the soybean cyst nematode (SCN), Heterodera glycines, has been known to exist in Wisconsin for at least 14 years, relatively few growers sample for SCN or use host resistance as a means to manage this nematode. The benefit of planting the SCN-resistant cultivar Bell on a sandy soil in Wisconsin was evaluated in 1992 and 1993. A range of SCN population densities was achieved by planting 11 crops with varying degrees of susceptibility for 1 or 2 years before the evaluation. Averaged over nematode population densities, yield of ''Bell'' was 30 to 43% greater than that of the susceptible cultivars, ''Corsoy 79'' and ''BSR 101''. Counts of cysts collected the fall preceding soybean were more predictive of yield than counts taken at planting. Yields of all three cultivars were negatively related (P < 0.001) to cyst populations. Fewer (P < 0.01) eggs were produced on ''Bell'' than on the susceptible cultivars. The annual (fall to fall) change in cyst population densities was dependent on initial nematode density for all cultivars in 1992 and for the susceptible cultivars in 1993. Yield reductions induced by the SCN under the conditions of this study indicate that planting a SCN-resistant cultivar in Wisconsin can be beneficial if any cysts are detected.     

Management of Heterodera glycines by Cropping and Cultural Practices

Heterodera glycines was identified in North Carolina in 1954, although symptoms of the disease were noted in the state at least 8 years earlier. Crop rotation experiments designed to develop management systems were initiated in 1956. Two or more years in production of a nonhost crop resulted in decreases of the nematode to low or undetectable levels with acceptable subsequent yields of soybean (Glycine max). Because of almost complete dependence on resistant cultivars and (or) nematicides for nematode control, crop rotation experiments were not conducted from 1962 to 1980. Research on control of H. glycines, beginning in 1981, emphasized biological and ecological aspects of the nematode in order to determine cropping systems that restrict the nematode to nondamaging levels. Mortality during embryogenesis was high at temperatures above 30 C. Hatching of eggs occurs readily in May and June. Postinfection development takes 2-3 weeks at weekly mean temperatures of 22-29 C and is slow above and below those temperatures. Egg production is high during the late growing season. Some cultural practices such as planting early maturing cultivars in mid-to-late June and rotation with a nonhost effectively keeps populations at low levels.