Seasonal Biochemical Changes in Eggs of Heterodera glycines in Missouri

Changes in the carbohydrate (glucose, trehalose, and glycogen) and total protein contents of eggs retained within Heterodera glycines cysts were monitored monthly in a field microplot experiment conducted from March 1993 to March 1995. Treatments included two near-isogenic lines of soybean cv. Clark differing for date of maturity, and one corn hybrid. The soybean lines were planted in microplots infested with H. glycines at a high average initial population density (Pi) (23,810 eggs/100 cm³ soil), and the corn was planted in microplots infested at high (24,640) and low (5,485) Pi. Soil temperatures at 15 cm depth and rainfall were monitored. Carbohydrate contents varied in the same pattern, with the highest levels measured before planting (May) and after harvest (October) in both years. Neither Pi nor soybean isoline had an effect on any measured response, but the carbohydrate contents of eggs from corn and soybean microplots differed during the overwinter (October-May) periods (P < 0.0001). Trehalose accumulation was negatively correlated with soil temperature (r = -0.78 and r = -0.84, P = 0.0001, July through November 1993 and 1994, respectively), which reflects its role as a cryoprotectant. In contrast to the pattern for carbohydrates, total protein was lowest before planting and after harvest, and highest (>20 μg/1,000 eggs) June through October. Protein content was unaffected by plant cultivar or species. Protein and carbohydrate levels in H. glycines eggs showed seasonal changes that appeared to be primarily temperature-dependent.     

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.     

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.     

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 Temperature and Soybean Phenology on Dormancy Induction of Heterodera glycines

Growth room and field experiments were conducted to determine the influence of soil temperature and soybean phenology on dormancy induction of a North Carolina population of Heterodera glycines race 1. Three temperature regimes and two photoperiods to regulate plant phenology were investigated in growth rooms. H. glycines hatch was greatest from the 26 and 22 C (day and night) temperature treatment, intermediate at 22 and 18 C, and least from the decreasing regime (26 and 22 C, 22 and 18 C, and 18 and 14 C). More eggs hatched and greater nematode reproduction occurred on pod-producing soybeans than on those that remained vegetative. In the field study, hatching patterns were not different between depodded and naturally senescing soybeans nor between the different maturity groups of soybean cultivars (groups V through VIII). Egg hatch (9-16%) was greatest in August and September when mean soil temperatures were between 25 and 29 C. Hatch declined to 1% in vitro and was not detectable in the bioassay in November. Greatest nematode numbers were observed on the latest maturing cultivar (group VIII) and fewest on the cultivar which matured earliest (group V). Decreasing temperature appears to be more important than soybean phenology in dormancy induction of H. glycines.     

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.     

Development of Heterodera glycines as Affected by Alachlor and Fenamiphos

A series of greenhouse experiments was conducted to elucidate the postinfection development of Heterodera glycines in response to applications of alachlor and fenamiphos. The rate of H. glycines maturation on a susceptible soybean cultivar was not altered by 1.0 μg alachlor/g soil but was completely inhibited by 1.0 or 1.5 μg fenamiphos/g soil. An alachlor-fenamiphos combination allowed development after an initial 300-degree-day delay. Nematode maturation on the resistant soybean cultivar Centennial with 1.0 μg alachlor/g soil was similar to that observed on an untreated resistant control. Twice as many females matured on Centennial plants growing in alachlor-treated soil as on untreated Centennial plants. Fenamiphos in combination with alachlor (1.0 μg a.i./g soil) allowed development on Centennial at half the rate of the resistant control. This antagonism between alachlor and fenamiphos on development may help to explain late season population resurgence of H. glycines observed with field application of these pesticides.     

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.     

Genetic Analysis of Esterase Polymorphism in the Soybean Cyst Nematode, Heterodera glycines

The genetic basis of esterase polymorphism in Heterodera glycines was investigated through controlled matings and analysis of F₁ and F₂ progeny. Three nematode lines, each fixed for a different esterase phenotype, were isolated and purified through repeated directional selection and inbreeding. Each phenotype was characterized by its distinct pair of closely spaced bands of esterase activity. Single-female single-male crosses were conducted according to a modified agar-plate mating technique. F₁ progeny were homogeneous, exhibiting both parental esterase phenotypes (codominant heterozygotes) but no hybrid bands. Approximately 1,500 F₂ progeny segregated in a 1:2:1 ratio for expression of the esterase phenotypes of the female parental line, the heterozygote, and the male parental line. Apparently the three esterase phenotypes correspond to three codominant alleles of a single esterase locus. Reciprocal crosses gave similar results, suggesting no maternal inheritance.     

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.     

Complete Characterization of the Race Scheme for Heterodera glycines

One hundred thirty-eight isolates of Heterodera glycines from nine states in the United States, People''s Republic of China, and Indonesia were tested on the four standard soybean race differentials. A total of 12 variants were found, including the five races described previously. The seven variants that did not correspond to one of the described races and reports from other areas of populations that could not be classified are evidence that the present race classification system needs to be fully characterized. Eleven additional races are described; this expands the total to 16 races, the maximum possible using the four prescribed differentials and a + or - rating for each. The seven new races are designated as 6, 9, 10, 13, 14, 15, and 16. This complete characterization of the race scheme will allow for immediate communication of the discovery of the remaining four races plus the identification of previous undescribed races.     

Population Dynamics of Heterodera glycines Life Stages on Soybean

Population fluctuations of Heterodera glycines differ in fields with high and low initial population densities. In a field with low initial numbers of nematodes, the numbers of cysts and eggs in soil remained low through 100 days from planting then increased during the remainder of the growing season. In a field with high initial nematode populations, numbers increased at 30 days, decreased to low numbers at 100 days, and then resurged to maximum populations at harvest. Numbers of juveniles were greatest at 100 days in the low initial population density field and at planting in the high initial population density field. The initial numbers of eggs in the soil gave the best correlation to soil and root nematode populations 15 and 30 days later. Juveniles in the soil at planting gave the largest correlation coefficients with nematode populations in the roots at 15 days in the field with the low initial population density. Eggs and juveniles in the soil at harvest were poorly related to numbers that overwintered.     

Soybean Photosynthesis and Yield as Influenced by Heterodera glycines,Soil Type and Irrigation

The effects of soil types and soil water matric pressure on the Heterodera glycines-Glycine max interaction were examined in microplots in 1988 and 1989. Reproduction of H. glycines was restricted in fine-textured soils as compared with coarse-textured ones. Final population densities of this pathogen in both years of the study were greater in nonirrigated soils than in irrigated soils. The net photosynthetic rate of soybean (per unit area of leaf) was suppressed only slightly or not at all in response to infection by H. glycines and other stresses. Relative soybean-yield suppression in response to H. glycines was not affected by water content in fine-textured soils, but slopes of the damage functions were steepest in sand, sandy loam, and muck soils at high water content (irrigated plots). Yield restriction of soybean in response to this pathogen under irrigation was equal to or greater than the yield suppression under dry conditions. Although yield potential may be elevated by irrigation when soil-water content is inadequate, supplemental irrigation cannot be used to circumvent nematode damage to soybean.     

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.     

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.     

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.     

DNA Restriction Fragment Length Polymorphism in Races of the Soybean Cyst Nematode,Heterodera glycines

Restriction endonuclease digests of total DNA from races 3, 4, and 5 of the soybean cyst nematode, Heterodera glycines, have been analyzed on agarose gels. DNA fragment patterns of race 4 were completely different from those patterns obtained for races 3 and 5 by all eight restriction enzymes tested. Differences in long and short restriction DNA fragments generated by the enzyme Msp I or its isoschizomer, Hpa II, were detected between race 3 and 5 digestion profiles. Rapid DNA isolation followed by its digestion with either Msp I or Hpa II enzymes and visualization of repetitive DNA fragments in agarose gels provided a diagnostic assay for the populations of the three races examined in this study.     

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.     

Effects of Temperature on Development of Heterodera glycines on Glycine max and Phaseolus vulgaris

Soybean cyst nematode resistant ''Fayette'' and susceptible ''Williams 79'' soybeans (Glycine max) and resistant ''WIS (RRR) 36'' and susceptible ''Eagle'' snap beans (Phaseolus vulgaris) were used in determining the effects of host and temperature on the development, female production, sex ratios, and host response to Heterodera glycines. Temperatures were maintained constant at 16, 20, 24, 28, and 32 C using water-filled tanks. The most rapid development and greatest female production occurred between 20 and 28 C. The equation DS = 5(10⁻⁶)x²y² - 3(10⁻⁴)x²y - 2.8(10⁻³)x² - 1.94(10⁻²)y² + 0.4288x + 1.0220y - 12.7185, where DS = developmental stage, X = time, and Y = temperature, predicted the developmental stage of the nematode and accounted for 84% of the variation. Male : female ratios did not differ within this range and were generally less than one. At all temperatures the resistant soybean produced the greatest number of necrotic responses to H. glycines infection, followed by the resistant snap bean. The susceptible soybean and snap bean produced the fewest necrotic responses.     

Changes in the Heterodera glycines Female Index as Affected by Ten-year Cropping Sequences

The objective of this experiment was to measure the change in female index (FI) of Heterodera glycines from bioassays on Bedford, Peking, PI 89772, and PI 90763 soybean (Glycine max) for 12 cropping sequence treatments over a 10-year period. Cropping sequences included continuous plantings of Forrest, Peking, and D72-8927 soybean (all resistant to race 3); Bedford, Nathan, and D75-10710 soybean (all resistant to races 3 and 14); a Bedford-corn (Zea mays) rotation; a rotation of Bedford, Essex (susceptible), and Forrest; and a 70:30 blend of Bedford and Forrest. The FI from bioassays with PI 89772 and PI 90763 decreased over time from 24.3 to 1.6 with treatments involving continuous Bedford, Nathan, and D75-10710 and the Bedford-corn rotation. The FI increased in bioassays using Bedford with treatments involving Bedford, Nathan, D75-10710, the Bedford-Forrest blend, and the two rotations. Results of this field experiment confirm greenhouse experiments in which reciprocal changes occur in FI on PI 89772 and PI 90673 compared with FI on Bedford.