Plant-Induced Hatching of Eggs of the Soybean Cyst Nematode Heterodera glycines

Root diffusate from soybean plants caused greater hatching of Heterodera glycines eggs during vegetative growth of the host, but the activity declined with plant senescence. Chelation of the root diffusate with ethylenediamine tetraacetic acid (EDTA) significantly increased hatching activity for H. glycines eggs. Diffusate from leafless plants caused little hatching, whereas treatment of intact plants with the growth regulators gibberellin and kinetin had no effect on the hatching activity of root diffusate. Treating H. glycines eggs with zinc chloride and root diffusate reduced egg hatching from zinc chloride alone. Levels of zinc in the root diffusate were insufficient to induce egg hatch, based on analysis by atomic absorption spectrophotometry. The enzymatic activity of leucine aminopeptidase in H. glycines eggs was not altered by treatment with chelated or nonchelated root diffusate.     

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.     

Comparisons of Mitochondrial DNA from the Sibling Species Heterodera glycines and H. schachtii

Restriction fragment patterns of mitochondrial DNA from sibling species of cyst nematodes Heterodera glycines and H. schachtii were examined. Fourteen restriction endonucleases recognizing four, five, and six base-pair sequences yielded a total of 90 scorable fragments of which 10% were shared by both species. Mitochondrial genome sizes for H. glycines and H. schachtii were estimated to be 22.5-23.5 kb and 23.0 kb, respectively. A single wild type mitochondrial genome was identified in all populations of H. glycines examined, although other mitochondrial genomes were present in some populations. The H. schachtii genome exhibited 57 scorable fragments, compared with 33 identified in the H. glycines wild type genome. The estimated nucleotide sequence divergence between the two species was p = 0.145. This estimate suggests these species diverged from a common ancestor 7.3-14.8 million years ago.     

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.     

Scanning Electron Microscopy of Second-Stage Juvenile Cephalic Morphology in Heterodera glycines Races

Scanning electron microscopy (SEM) was used to compare juvenile cephalic morphology of the five described races of Heterodera glycines. Races 1, 2, 3, and 4 were obtained in the United States and race 5 was obtained from Japan. Differences in the gross morphology o f labial discs; ventral, dorsal and lateral lips; amphidial apertures; and fissures on the labial disc o f some specimens were observed. There was considerable interracial and intraracial variation which precluded separation o f juveniles of H. glycines races by SEM.     

Separation and Characterization of Heterodera glycines Acetylcholinesterase Molecular Forms

The composition and biochemical properties of acetylcholinesterases isolated from Heterodera glycines were determined. Heterodera glycines contains three separable AChE molecular forms that can be grouped into two classes corresponding to classes A and C found in some other nematode species. The apparent lack of class B AChE is unusual and may have significant behavioral ramifications. The class C enzyme isolated from H. glycines is similar to that from Meloidogyne arenaria and M. incognita but is somewhat more sensitive to AChE inhibitors such as eserine. Heterodera glycines possesses a larger percentage of its total acetylcholinesterase as class C than other nematodes thus far examined.     

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.     

Response of Soybean to Heterodera glycines Races 1 and 2 in Different Soil Types

Experiments were conducted for 3 years at four locations and 1 year with six soil types at a common location in North Carolina to determine damage and control-cost functions for Heterodera glycines races 1 and 2 on soybean. In the experiments on native loamy sand and sandy soils, tolerance limits for initial population densities were 0 or very low, whereas in a muck, the tolerance limit was 315 eggs/500 cm³ soil. The aggressive race 2 was more damaging than race 1 in Lakeland sand and Norfolk loamy sand. The crop response was not different between races in the Appling sandy clay loam and Belhaven muck. Soybean yield responses to H. glycines were linear in six soil types in microplots at a common site. The amount of damage varied among these soil types, with lowest yields in the muck because of severe drought stress in this soil. An exponential function adequately described soybean yield response relative to nematode control with increasing rates of aldicarb in Norfolk loamy sand. Treatment with aldicarb in the Lakeland sand decreased the effective egg population of H. glycines but had only a minor effect in the muck.     

Development of Heterodera glycines Life Stages as Influenced by Temperature

The effects of temperature on rates of development of Heterodera glycines egg and juvenile stages were examined as a basis for predicting generation times of the nematode on soybean. The relationship of temperature to H. glycines embryonic development between 15 and 30 C was described by a linear model, The calculated basal temperature threshold was 5 C. Thermal optimum for embryogenesis and hatch with low mortality was 24 C. Development proceeded to first-stage juvenile at 10 C and to second-stage juvenile at 15-30 C. Hatch occurred at 20-30 C. At 36 C, development proceeded to the four-cell stage, then the eggs died. The range of diurnal soil temperature fluctuation and accumulated degree-days between 5 and 30 C (DD5/30) had an impact on rate of development of juveniles in soybean roots. From early June to early July, H. glycines required 534 + 24 DD5/30 (4 weeks) to complete a life cycle in the field. During the midseason (July and August), life cycles were completed in 3 weeks and 429 ± 24 DD5/30 were accumulated. Late in the season (September to November), declining soil temperatures were associated with generation times of 4 weeks and slower rates of development.     

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.     

Selection Against Heterodera glycines Males by Soybean Lines with Genes for Resistance

Soybeans with genes for resistance select against Heterodera glycines with the corresponding genes for avirulence. There may be a differential effect of sex with some specific gene interactions, which would influence the magnitude of gene frequency changes. No effect on H. glycines males was detected with one selected nematode population and the resistant soybean line PI88788. The selective effect of PI89772 against male nematodes was greater with two inbred nematode populations than with one selected (on PI88788) population, presumably due to differences in H. glycines gene frequencies. ''Peking'' also had few males with the one inbred nematode population, whereas Forrest and ''Pickett 71'' had intermediate numbers. Apparently Forrest and Pickett 71 did not get all the Peking genes for resistance that affect male as well as female nematode development. Other H. glycines-soybean genes stop only females, since there were few or no cysts, except on the susceptible soybean Williams. The number of males'' phenotype will help identify specific genes in both organisms.     

Morphometric and Serologic Comparisons of a Number of Populations of Cyst Nematodes

Thirty-five populations of Heterodera glycines and populations of 15 other Heterodera, Globodera, and Punctodera species were studied morphometrically and some were compared serologically. There was a wide range of each measurement within each nematode population. Except for one soybean cyst nematode population from Indiana, which was a tetraploid and considerably larger than the others, morphometric measurements overlapped. In a discriminant function comparison most of the populations were closely grouped but at least three were rather distinctly separated. Morphometrically H. fici, H. cruciferae, H. schachtii, and H. trifolii were closely associated with H. glycines. Serology indicated a close relationship between H. glycines, H. lespedezae, H. trifolii, H. schachtii, and the Heterodera sp. from Rumex, while H. betulae appeared to be more distantly related.     

Repeated Sampling to Determine the Precision of Estimating Nematode Population Densities

The first phase of this study involved repeated samplings of five fields using composite samples of 10, 20, 40, and 80 soil cores, to determine the precision of nematode assays. The second phase focused on randomly selecting two and four 2-ha subunits (data on Meloidogyne spp.) of 24 fields ranging from 6 to 40 ha and computing the precision of estimated means for these numbers ofsubunits versus the general field mean (based on all 2-ha subunits). Average numbers of nematodes from most samples containing Meloidogyne spp., Heterodera glycines, Helicotylenchus dihystera, Scutellonema brachyurum, and (or) Hoplolaimus galeatus were within 50% of the overall means. Coefficient of variation (CV) values were generally lower for 40 cores than for 10, 20, and 80 cores per sample. When data for all nematodes and fields were combined, this value was lowest for 40 and 80 cores. The CV values were higher for Meloidogyne spp. than for H. glycines. Means of two samplings increased the probability of obtaining numbers nearer the mean for that field than numbers from a single composite sample. For the second phase, population estimates of Meloidogyne spp. based on four 2-ha subunits generally were closer to field means than were those for two subunits. Sampling precision with these subunits diminished greatly in large fields with variable soils and (or) mixed cropping histories. Either two or four subunits gave population estimates within 3-20% of the field mean in most instances. The mean man hours required for sampling ca. 2-ha parcels of 4-20-ha fields was 0.54 hours.     

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.     

Influence of Heterodera glycines on Interspecific and Intraspecific Competition Associated with Glycine max and Chenopodium album

The influence of Heterodera glycines (soybean cyst nematode) on the interspecific and intraspecific competition associated with Glycine max (soybean) and Chenopodium album (common lambsquarters) was studied in 1988 and 1989 in three de Wit replacement series experiments in growth chambers and microplots. Glycine max was grown alone (1 plant/experimental unit), in intraspecific competition (2 plants/experimental unit), in interspecific competition with C. album, and in presence or absence of H. glycines. No significant effects of H. glycines and C. album on G. max growth were observed 14 days after planting. By 42 days after planting, both H. glycines and C. album had a negative (P = 0.05) influence on the growth of G. max. Relative crowding coefficients for G. max were lower and deviated (P = 0.05 and P = 0.001) from 1.0 in the presence of H. glycines, compared to that of C. album and early emerged C. album in the absence of the nematode, respectively. Glycine max, therefore, became less competitive than C. album. There was a trend that the presence of H. glycines decreased the competitiveness of G. max on measures of the aggressivity and relative mixture response. Heterodera glycines decreased the aggressivity of G. max (ca. 150-350%) and increased the relative effects of intraspecific interference on G. max (ca. 10-50%) and interspecific interference (ca. 60-350%) after 42 days of plant growth, compared with plants grown in the absence of H. glycines. No H. glycines x C. album interactions were detected. Observations showed that H. glycines and early emerged C. album inhibited the growth of G. max 5-13%, as measured by plant dry weight.     

Multiple Pest Interactions in Soybean: Effects on Heterodera glycines Egg Populations and Crop Yield

Population changes of Heterodera glycines eggs on soybean in small field plots were influenced by the lepidopterous insect pest, Helicoverpa zea; however, few effects on eggs due to the presence of annual weeds were detected. Soybeans defoliated 15-35% by H. zea during August remained green and continued to produce new flowers and pods later into the season than soybeans without H. zea, resulting in higher numbers of H. glycines eggs at harvest on insect-defoliated soybeans. Final H. glycines populations also were influenced by soil population density (Pi) of the nematode at planting. Fecundity of H. glycines was generally greater at the undetected and low Pi than at high Pi levels. Soybean yields were suppressed 12, 22, and 30% by low, moderate, and high H. glycines Pi, respectively. When weed competition and H. zea feeding damage effects were added, yields were suppressed 34, 40, and 57% by the three respective nematode Pi levels. Effects among the three pests on soybean yield were primarily additive.     

Field Interrelationships among Heterodera glycines,Pratylenchus scribneri,and Three Other Nematode Species Associated with Soybean

Field experiments were conducted in 1982 and 1983 to assess interactions between Heterodera glycines and Pratylenchus scribneri on soybean in southern Illinois. Soybean cyst nematode susceptible cultivar Williams 79 and resistant cultivar Fayette were treated or not treated with aldicarb 15G. Initial population densities were 35 H. glycines cysts containing eggs, 100 P. scribneri, 30 Helicotylenchus pseudorobustus, 225 Paratylenchus projectus, and 85 Tylenchorhynchus martini per 250 cm³ soil in 1982, whereas in 1983 populations were 11 H. glycines cysts, 330 P. scribneri, and 620 H. pseudorobustus. In both years H. glycines populations increased on nontreated Williams 79, decreased on both treated and nontreated Fayette, and remained at initial levels on treated Williams 79. Recovery of P. scribneri per gram dry root was different between nontreated cultivars in 1982 but not in 1983. Aldicarb treatment suppressed soil and root populations of P. scribneri on both cultivars in both years. Populations of H. pseudorobustus, P. projectus, and T. martini at harvest indicated little population increase on either nontreated cultivar. In 1982 H. glycines caused yield suppression but P. scribneri did not, as differences in yield occurred between cultivars but not between aldicarb treatments. In 1983, however, there were no yield differences between cultivars, but aldicarb application resulted in yield increase in both cultivars. In 1983 the yield increase resulting from P. scribneri control was approximately 25%. No synergistic effect on yield was observed between H. glycines and P. scribneri.     

Effects of Continuous Cropping of Resistant and Susceptible Cultivars on Reproduction Potentials of Heterodera glycines and Globodera tabacum solanacearum

The reproductive potentials of Heterodera glycines (mixture of races 3 and 4 and unidentified races) and a tobacco cyst nematode Globodera tabacum solanacearum were studied in the field. The experiments involved four cultivars of soybean Glycine max and four cultivars of Nicotiana tabacum. The reproductive potential of the H. glycines population was high on Essex and Lee 74 soybean but low on Forrest and Bedford over the 3 years (1982-84) of continuous cropping. The reproductive potential of H. glycines was 12% on Forrest and 6% on Bedford in 1982 but increased to 37 and 35% in 1983 and to 71 and 41% in 1984, respectively, on these two cultivars. The reproductive potential of G. tabacum solanacearum was high on McNair 944 and Coker 319 tobacco cultivars and low on VA 81 and PD 4 over the 3 years of cropping. The reproductive potential of G. tabacum solanacearum on VA 81 and PD 4 was 18 and 17% in 1982, 7 and 16% in 1983, and 5 and 5% in 1984, respectively. The changes in reproductive potentials of H. glycines and G. tabacum solanacearum may be related to inherent genetic variability in the systems that control reproduction of the two cyst nematodes and nature of resistance incorporated in the soybean and tobacco cultivars.     

Genetics of Soybean-Heterodera glycines Interactions

The soybean cyst nematode, Heterodera glycines, is one of the most economically important pathogens of soybean. Effective management of the nematode is often dependent on the planting of resistant soybean cultivars. During the past 40 years, more than 60 soybean genotypes and plant introductions (PI) have been reported as resistant to H. glycines. About 130 modern soybean cultivars registered in the United States are resistant to certain races of H. glycines. Several resistance genes have been identified and genetically mapped; however, resistance levels in many soybean cultivars are not durable. Some older cultivars are no longer resistant to certain H. glycines populations in many production areas, especially if a soybean monoculture has been practiced. Past soybean registration reports show that all resistant cultivars developed in public institutions from the mid-1960s to the present have been derived from five PIs. This narrow genetic background is fragile. To further complicate the issue, soybean-H. glycines genetic interactions are complex and poorly understood. Studies to identify soybean resistance genes sometimes have overlapped, and the same genes may have been reported several times and designated by different names. Nevertheless, many potential resistance genes in existing germplasm resources have not yet been characterized. Clearly, it is necessary to identify new resistance genes, develop more precise selection methods, and integrate these resistance genes into new cultivars. Rational deployment of resistant cultivars is critical to future sustained soybean production.