Soybean Cyst Nematode Population Research Articles

Genetics of soybean-Heterodera glycines system

Genetics of resistance to soybean cyst nematode (SCN), Heterodera glycines Ichinohe is very complex. Crosses involving PI 437654, which is resistant to all races of cyst nematodes with other sources of resistance (Peking, PI 88788, and PI 90763) indicated that resistance to race 3 was controlled by four genes, two of which were dominant resistance genes and the other two were recessive resistance genes. For race 5, a four gene model with two recessive and two dominant resistance genes in epistasis has been proposed. For race 14, the results suggested a three gene model with one dominant and two recessive alleles. Several other plant introductions have been isolated which have different genes conditioning resistance. Most of the currently grown soybean varieties derived resistance from Peking and/or PI 88788. Resistance to SCN in these soybean varieties has broken down because of the emergence of several new races and populations of SCN. The use of PI 437654 or Hartwig and other plant introductions with different genes for resistance will broaden genetic diversity and stabilize yield.

Performance of Blends of Soybean Cyst Nematode Resistant and Susceptible Cultivars

Use of soybean cyst nematode (SCN), (Heterodera glycines Ichinohe), resistant cultivars of soybean [Glycine max (L.) Merr.] is common practice to control losses due to this pest. The experiment was conducted to compare blends of resistant and susceptible cultivarswith their yield response when grown in pure monoculture stands. Two SCN‐resistant cultivars Bedford and Bradley, three susceptible cultivars or breeding lines, Essex, York, and N79‐491, and blends of resistant and susceptible cultivars (50:50 ratio) were tested against soybean monoculture in a SCN Race 14‐infested field. The mean yield (over 5 yr) of Bedford, Bradley, Essex, York, and N79‐491 grown in monoculture was 1707, 1703, 903,1063, and 1379 kg ha−1, respectively. Yield of blends were generally similar to their resistant cultivar components in monoculture but they were always greater (P = 0.05) than the susceptible cultivar components. The Bedford and Essex blend resulted in the greatest yield, which was 672 kg ha−1 (51%) greater than the mean of component crop yields in pure stand. The race designation of the SCN population changed from Race 14 to Race 5 when resistant Bedford and Bradley were grown, from Race 14 to 9 when susceptible cultivars were grown, and from Race 14 to 2 in plots containing blends of resistant and susceptible cultivars or lines. Resistant cultivars grown with susceptible cultivars or lines in blends had lower numbers of cysts on the roots 30 d after planting than resistant cultivars grown in pure stands. Blends of resistant and susceptible cultivars can maintain soybean yields at acceptable levels by minimizing the selection pressure on the nematode population for their ability to parasitize resistant cultivars.

Occurrence of Soybean Sudden Death Syndrome in East-Central Illinois and Associated Yield Losses

Sudden death syndrome (SDS) of soybeans, causal organism Fusarium solani, occurred for the first time in epidemic proportions in east-central Illinois in 1993. SDS occurred in 46% of the soybean fields, based on air and ground surveys. Within fields containing plants with SDS, the percent area affected ranged from 1 to 70% with an average of 7.3 and 13.9% for ground and aerial surveys, respectively. Soil samples taken in 25 fields in areas where all plants showed SDS symptoms and in adjacent areas where plants appeared healthy did not differ in cyst populations of soybean cyst nematode (Heterodera glycines). Disease severity and yield components were compared from replicated plots at an experimental farm and in a commercial field and in a nonreplicated commercial field. Diseased plants from infested areas had fewer pods and seeds and less 300-seed weights. Plant yields were 46, 41, and 20% less for plantsin plots with a high incidence of SDS than were the yields for those with a low incidence of SDS in a nonreplicated commercial field, replicated plots at an experimental farm, and a commercial field, respectively. Seed germination was less and the frequency of Phomopsis spp. was greater on seeds harvested from plants in plots with a high occurrence of SDS compared to plots with a low occurrence of SDS

Variation in Parasitic Potential of Heterodera glycines Populations

A great deal of variation in parasitic potential has been observed among the soybean cyst nematode (SCN) Heterodera glycines Ichinohe populations in the USA. To best utilize sources of genetic resistance to SCN, this variation needs to be characterized. Fifty‐eight populations of the SCN were collected from 10 major soybean producing states. These populations were compared for infraspeeific variation in parasitic potential on soybean [Glycine max (L.) Merr.] genotypes ‘Peking’, ‘Pickett’, Plant Introduction (PI) 88788, PI 90763, and ‘Lee 68’. Race structure, genes for parasitism (capability of a nematode population to produce at least one cyst on a resistant genotype), and levels of parasitism (percentage of cysts that developed on resistant genotypes compared with the susceptible cultivar Lee 68) were assessed. Race was present in 45% of locations; it was the most widespread population. Race 6 was present in 19% of the locations. Frequency of occurrence of other races (Races 1, 2, 5, 9, and 14) ranged between 3 and 9%. On the basis of reproduction on genotypes, 83% of the populations tested had genes to parasitize Pickett. All populations from Arkansas, Florida, Georgia, Iowa, North Carolina, and South Carolina produced cysts on this genotype. Average level of parasitism was greatest on Pickett and lowest on PI 90763; 91% of SCN populations tested had <10% level of parasitism on PI 90763. The SCN gene frequencies demonstrated that nematode populations with predominant genes for parasitism on Pickett and Lee 68 were widespread especially in the southern soybean producing states in the USA. Cultivation of varieties with resistance derived from Peking and Pickett in these regions should not be very effective. For management of SCN‐caused yield losses, cultivation of soybean varieties with resistance derived from P190763, PI 88788, and PI 437654 should be encouraged.

A Method for Infesting Small Field Plots with Soybean Cyst Nematode

AbstractField experimentation with soybean cyst nematode (Heterodera glycines Ichinohe) is difficult, because of problems in obtaining plots with desired population densities. Therefore, a technique was developed for infesting small field plots with soybean cyst nematode. In 1990 and 1991, cysts, eggs, and infective juveniles were mixed with soil and applied with a commercial fertilizer spreader. Infected soybean [Glycine max (L.) Merr.] root segments were distributed by hand. Infested soil and roots were incorporated with a tractor‐drawn herbicide incorporator, and plots were planted to susceptible soybean. At‐planting egg densities in 1990 and 1991 averaged 184 and 100 eggs 100 cm−3 soil, respectively, representing 78 and 90% of expected egg densities. Resultant low densities of encysted eggs, which are highly aggregated, probably prevented detection of eggs in some plots. No soybean cyst nematode was detected in uninfested plots at planting, although some contamination was noted at harvest. This method may be appropriate for establishing soybean cyst nematode population densities for some small‐plot research.

Soil and Plant Factors Associated with Sudden Death Syndrome of Soybean

Sudden death syndrome (SDS) of soybean (Glycine max [L.] Merr.) is a soilborne disease of increasing importance in high-yield environments. This study was conducted to determine which combinations of soil fertility parameters and soybean cyst nematode (Heterodera glycines Ichinoe) (SCN) second stage juvenile (J2) population levels were associated with SDS disease severity. Also, the effect of SDS disease severity on leaf nutrient content and on soybean yield and seed germination were determined. These studies were conducted at the Pine Tree Station, in Colt, on a Crowley milt loam (fine, montmorillonitic, thermic Typic Albaqualfs) over 3 yr with the soybean cultivar Lee 74 [...]

Inheritance of Genes for Resistance to Soybean Cyst Nematode Populations

The arbitrary division of continuous distributions of phenotypes on F2 plants into ratios of resistant:susceptible categories may not accurately reflect the number and dominance of genes for resistance to pests. Resistance of soybean, Glycine max L. Merr., lines to soybean cyst nematode (CN), Heterodera glycines Ichinohe, populations has been variously defined, representing relatively few cysts per plant phenotypes. This review considers the genetic principles that are relevant to genetic studies involving CN and soybean. The distribution of a phenotype, such as number of cysts per F2 plant, is determined by the number and dominance of soybean genes for resistance segregating and the frequencies of CN genes for avirulence with which they interact in a gene‐for‐gene manner. An assumption seems to have been that only one soybean genotype conferred resistance, which implied heterogeneous CN populations. The resistant: susceptible ratios commonly used were deduced to be 3d:4N‐3d, with dominant (d) and recessive (r) genes for resistance at N independent loci. An A_ or aa genotype (of d and r genes) at any one locus may condition F2 plant resistance to homozygous (for avirulence) CN (and parasite clones). Then only one genotype or 3r gives susceptibility as 4N−3r:3r ratios. Some of the resistant:susceptible ratios were not greatly different, and might be the same with the effects of linkage. Heterogeneous and homozygous CN populations may not be distinctly different, rather the differences are in the distributions of phenotypes they effect with F2 plants: continuous with heterogeneous but discrete phenotypes, similar to the parental ranges, with homozygous populations. The materials and methods for definitive genetic studies, including allelism and linkage, were discussed.

Reproductive Advantage in Soybean Associated with Resistance to Soybean Cyst Nematode: Effects of Tolerance and Nematode Density

Natural selection is used by plant breeders to increase the frequency of desirable genotypes in a population. When soybean [Glycine max (L.) Merr.] populations segregating for resistance to soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) are grown on SCN‐infested soil, resistant genotypes produce more seed than susceptible genotypes. In a bulk‐harvested population this differential reproduction results in an increased frequency of resistant genotypes in the population. This research was conducted to determine the effects of tolerance to SCN and the frequency of resistant genotypes in the population on the effectiveness of natural selection for resistance to SCN. A second objective was to relate initial SCN population densities and parental pure‐stand yield ratios to the effectiveness of natural selection. Four 1:1 viable seed mixtures of resistant (R) and susceptible (S) genotypes were evaluated in four SCN‐infested fields for 2 yr. Two of the S genotypes were moderately tolerant and two were intolerant to SCN. The R genotypes in mixtures with moderately tolerant, S genotypes had an average reproductive advantage (ratio of R/S seed number) of 2.2 compared to 4.2 for R genotypes in mixtures with intolerant, S genotypes. In a second experiment, five ratios, 1:9, 3:7, 1:1, 7:3, and 9:1, of R to S genotypes were compared for two of the genotypic mixtures (‘Gordon’/‘ Bragg’ and Gordon/‘Wright’). The effectiveness of natural selection was not related to the frequency of the R genotype in the mixture. On an individual‐plot basis across the four mixtures the linear regression of the pure‐stand R/S yield ratios predicted 47% of the variation in reproductive advantage of R genotypes compared to 22% for log10 of initial density of SCN eggs. Effectiveness of natural selection for SCN resistance was dependent on the initial population density of SCN, the specific environmental conditions, and the tolerance level of the genotypes in the soybean population.

Genes for Resistance to Soybean Cyst Nematode Populations

Relatively few soybean (Glycine max (L.) Merr.) lines have been reported to be resistant to soybean cyst nematode (CN, Heterodera glycines Ichinohe) populations and races. The criterion for resistance was relatively few cysts per plant, a phenotype of the cyst nematode‐soybean association determined by genes of both organisms. Thus the frequencies of nematode genes for avirulence influenced the phenotypes observed. The vast majority of lines are considered to be susceptible to CN, implying that they have no genes for resistance. The objective was to show that some CN susceptible soybean lines do have genes for resistance to some CN. Previously developed CN inbreds were further inbred by transferring single cysts for several CN generations. Artificial selection among inbreds resulted in fewer cysts on specific susceptible soybean lines, apparently due to increased frequencies of some CN genes for avirulence. Twelve susceptible soybean lines apparently have genes for resistance to specific CN inbreds since relatively few cysts per plant were observed. Crosses involving six of these lines produced continuous distributions of numbers of cysts on F2 plants, indicating the segregation of at least two genes for CN resistance in each cross. The possible genetic similarities of some of these lines and some CN resistant lines are discussed, including the potential linkage effects of CN genes for avirulence on the different phenotypes.

Selection Pressure on Soybean Cyst Nematode from Soybean Cropping Sequences

It has been recommended that soybean [Glycine max (L.) Merr.] cultivars resistant to soybean cyst nematode (SCN), Heterodera glycines Ichinohe, be rotated with susceptible cultivars in order to delay or avoid development of populations capable of damaging resistant cultivars. The objective of this study was to determine if evidence for stabilizing selection of SCN populations with use of susceptible cultivars existed following 10 yr of such cropping sequences. Cropping sequences included susceptible ‘Tracy’ and race 3 resistant ‘Centennial’ grown consecutively with and without nematicide. Sequences of 2 yr of Centennial and 1 yr of Tracy, a blend of 80% Centennial and 20% Tracy, and a breeding line resistant to SCN races 3 and 4 planted consecutively were the other treatments. ‘Lee’, ‘Pickett 71’, ‘Peking’, PI 88788, and PI 90763 were grown in the greenhouse in soil from each field plot in 1985. Cyst indices (percent of cysts on a soybean line compared to number on Lee) were calculated for these race differentials. Race 3 was predominant following Tracy planted consecutively and the breeding line resistant to races 3 and 4. Race 2 was predominant in one of three treatments involving rotation with 2 yr of Centennial and 1 yr of Tracy. Cyst indices on Pickett 71 were large for cropping sequences continuing Centennial and the blend of Centennial and Tracy. Cyst indices were variable on Peking, PI 88788, and PI 90763 for sequences involving Centennial, but indices were within a standard deviation of the description for race 4. No evidence for stabilizing selection sufficient to prevent race shifts in SCN populations through use of susceptible cultivars was obtained.

Effects of Monocropping Resistant and Suspectible Soybean Cultivars on Cyst Nematode Infested Soil1

Studies of soybean [Glycine max (L.) Merr.] cultivar sequence and use of nematicides were conducted on a soil uniformly infested with the soybean cyst nematode (SCN), Heterodera glycines Ichinohe. Assumption that new races of SCN would develop rapidly if only SCN resistant cultivars were grown had led to specific recommendations that SCN resistant cultivars not be grown > 2 yr without rotating with a SCN susceptible cultivar or a nonhost crop. No field data were available to support the recommendation. This study was initiated in 1976 to more critically evaluate the effects of SCN on soybean productivity in a long‐term monocropping program. Resistant to SCN race 3, ‘Centennial’, and one that is susceptible, ‘Tracy’, were grown continuously with and without a nematicide. The nematicide treatment was dropped after 6 yr. The two cultivars were also grown in a sequence of 2 yr of Centennial and 1 yr of Tracy and in a blend of 80% Centennial with 20% Tracy. A breeding line resistant to SCN races 3 and 4 constitutedhn additional treatment. Sorghum [Sorghum bicolor (L.) Moench.] replaced the nematicide treatment in the seventh or seventh and eighth years to be followed by Tracy. Nematicide treatment did not enhance seed yield for either the susceptible or the resistant cultivar. No treatment was superior to growing the SCN race 3 resistant cultivar Centennial continuously over the 10‐yr period. No advantages were determined for including a SCN susceptible cultivar in the program. Beginning with the third year of this study and seventh year of continuous soybean production, the SCN population appears to have stabilized at a low level.

Effect of Rotating 'Forrest' and 'Bedford' Soybean on Yield and Soybean Cyst Nematode Population Dynamics1

The soybean cyst nematode (Heterodera glycines Ichinohe) is a serious pest that might be managed by the judicious use of resistant cultivars. Soybean [Glycine max (L.) Merr. ‘Bedford’] was rotated with ‘Forrest’ for 5 yr on a Tiptonville silt loam (Typic Arguidolls) to determine the effect of selection pressure on the reproductive ability of H. glycines and the effect of rotations on seed yield. At the end of the field experiment in 1984, nematode populations were tested in the greenhouse for compatibility with five soybean lines having genes for resistance to H.glycines. Population levels of H. glycines at harvest when Forrest and Bedford were monocultured were not significantly different after the second year. A 2‐yr rotation of Forrest and Bedford reduced cyst populations levels on Bedford at harvest relative to Bedford monoculture, but no rotation scheme reduced nematode population density after growing Forrest. Yields were higher for Bedford than for Forrest in the first 2 yr of the study, but thereafter no rotation was consistently superior. Cyst reproduction in the greenhouse was highest on Forrest and lowest on PI90763. Bedford best differentiated the field‐selected nematode populations in the greenhouse experiment. Reproduction on Bedford was lower if Forrest was grown in the last year of the field experiment than if Bedford was grown, indicating that the frequency of virulence gene(s) was not maintained in the nematode population in the absence of selection pressure.

Decrease in soybean cyst nematode population by highly resistant soybean cultivars

Decrease in soybean cyst nematode population by highly resistant soybean cultivars

Effect of Secondary Selection on Cyst Nematode Reproduction on Soybeans1

“Resistance” of soybeans [Glycine max L. (Merr.)] to soybean cyst nematodes (SCN, Heterodero glycines Ichinohe) is determined by the relative number of nematode females or cysts that can develop in the roots of soybeans. Soybean cyst nematode (SCN) populations have been selected for ability to reproduce on soybeans with several sources of resistance and were used to identify genetic differences in both SCN and soybeans. The objective of this experiment was to determine reproductive changes in SCN populations initially selected on ‘Cloud’ or on ‘PI 89772’ after secondary selection on the other, genetically‐different, source of resistance. Response to the secondary selection was rapid; apparently SCN genes for reproductive ability were not fixed by the primary selection. Four cycles of selection or SCN generations on Cloud, of the population initially selected on PI 89772, significantly increased its ability to reproduce on Cloud while concurrently decreasing it on PI 89772. When the SCN population initially selected on Cloud was then selected on PI 89772, it could reproduce relatively better on PI 89772 but less well on Cloud. Thus, SCN genes for ability to reproduce on specific soybeans are not independent. Williams, the “susceptible” check, may have at least one of the same genes for resistance as Cloud. Evidence from SCN populations selected from four other field populations indicates that at least some of the SCN genes for reproduction on genetically different soybeans probably are alleles. By judicious deployment of the soybean genes, we may be able to keep SCN populations below damage‐threshold levels.

Reproduction of Differentially Selected Soybean Cyst Nematode Populations on Soybeans 1

Reproduction of Differentially Selected Soybean Cyst Nematode Populations on Soybeans ¹

Reproduction of Tennessee Soybean Cyst Nematode Population on Cultivars Resistant to Race 4

Reproduction of Tennessee Soybean Cyst Nematode Population on Cultivars Resistant to Race 4

Effect of the mulching with plastic film on the soybean cyst nematode population in a soybean field

Effect of the mulching with plastic film on the soybean cyst nematode population in a soybean field