A Revised Classification Scheme for Genetically Diverse Populations of Heterodera glycines

Heterodera glycines, the soybean cyst nematode, is a major yield-limiting pathogen in most soybean production areas worldwide. Field populations of H. glycines exhibit diversity in their ability to develop on resistant soybean cultivars. Since 1970, this diversity has been characterized by a bioassay used to assign a race classification to a population. The value of the race scheme is reflected in the number and quality of resistant soybean cultivars that have been developed and released by soybean breeders and nematologists working in concert. However, the race scheme also has been misapplied as a means of studying H. glycines genotypes, in part due to the use of the term "race." For fungal and bacterial pathogen species, "race" can theoretically be applied to individuals of a population, thus allowing inference of individual genotypes. Application of a race designation to an individual egg or second-stage juvenile (J2) of H. glycines is not possible because a single J2 cannot be tested on multiple hosts. For other nematode species, "race" is defined by host ranges involving different plant species, whereas the H. glycines race test involves a set of lines of the same plant species. Nonetheless, because H. glycines populations vary in genetic diversity, and this variation has implications for management strategies, a mechanism is needed for documenting and discussing population differences. The HG Type scheme described herein avoids the implication of genetic uniformity or predictability in contrast to the way the race scheme has been used.     

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.     

Genetic Diversity for Resistance to Heterodera glycines Race 5 in Soybean

Heterodera glycines is a serious pest of soybean in the United States. Plant introductions 90763 and 424595 are reported to be resistant to H. glycines race 5; however their genetic relationship for resistance is unknown. Crosses between these two lines and the susceptible cultivar Essex were studied in the F₁, F₂, and F₃ generations to determine the number of genes involved in inheritance of resistance. The plants were screened using conventional techniques based on the index of parasitism. The data were subjected to analyses using chi-square test to determine goodness of fit between observed and expected genetic ratios. The cross PI 424595 x Essex segregated 1 resistant:63 susceptible in the F₂ generation, which indicated the presence of three recessive genes controlling resistance to race 5. In the cross PI 90763 x Essex, resistance was conditioned by one dominant and two recessive genes. The cross between PI 424595 and PI 90763 segregated into 13 resistant:3 susceptible. The data fit a four-gene model with two recessive and two dominant genes with epistasis. PI 90763 has a dominant gene, whereas PI 424595 has a recessive gene; both share two additional recessive genes for resistance to race 5. This information is important to geneticists and soybean breeders for the development of cultivars resistant to H. glycines.     

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.     

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.     

An Innovative Method for Counting Females of Soybean Cyst Nematode with Fluorescence Imaging Technology

Use of resistant cultivars is one of the major tactics for combating soybean cyst nematode, Heterodera glycines Ichinohe, which is the most destructive pathogen affecting soybean seed production. However, developing new H. glycines-resistant soybean cultivars is a very labor-intensive process, partially due to the lack of a quick method for counting the H. glycines females that develop on soybean roots. We have developed a fluorescence image-based system for counting females on excised seedling roots cultured on nutrient media in petri dishes. In this system, the females fluoresced when exposed to a wavelength of 570 nm. The fluorescent images were captured with a digital camera, transferred to a computer, and displayed on a monitor. The image of an entire sample was viewed at once, and the fluorescing females were counted manually. This system significantly improved the efficiency and accuracy of counting females developed on cultured seedling roots compared to a microscope counting method. The potential for applications in the screening of nematode-resistant crops is discussed.     

An Alternative Method for Evaluating Soybean Tolerance to Heterodera glycines in Field Plots

Alternate planting dates and periodic destruction of the previous year''s soybean crop as well as 1-year bare fallow were used to establish a range of population densities ofHeterodera glycines for the subsequent year. Soybean cultivar Coker 156 (susceptible, moderately tolerant) was compared to cultivars Essex (susceptible, intolerant) and Bedford (resistant) to evaluate tolerance at different H. glycines population densities established through the previous year''s treatments. Yield of Coker 156 was consistently intermediate between yields of Bedford and Essex in 1986 and 1987. Yield of Essex was negatively correlated (P = 0.05) with preplant egg numbers of H. glycines in 1987, whereas yield of Bedford and Coker 156 were not related to nematode density. Reproduction of H. glycines was greater (P = 0.05) on the moderately tolerant Coker 156 than on either of the other cultivars.     

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.     

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.     

Population Development of Heterodera glycines and Soybean Yield in Soybean-maize Rotations Following Introduction into a Noninfested Field

An 11-year field study was initiated in 1979 to monitor population development of Heterodera glycines. Fifty cysts of a race 5 population were introduced into plots in a field with no history of soybean production and that had been in sod for 20 years. Soybean cultivars either susceptible or resistant to H. glycines were grown either in monoculture or rotated with maize in a 2-year rotation. During the first 5 years, resistant cultivars with the Peking source of resistance were planted. After year 5, monocuhure of Peking resistance resulted in 18 cysts/250 cm³ of soil, whereas populations resulting from the continuous cropping of susceptible soybean resulted in 45 cysts/250 cm³. Some plots in all treatments, including control plots, were contaminated at the end of year 5. Crop rotation delayed population development of H. glycines. During years 6 through 11 cv. Fayette (PI88.788 source of resistance) was planted. In year 6 numbers of cysts declined to 1/250 cm³ of soil in the treatment consisting of monocultured Fayette. At the end of year 10, cysts were below the detection level in all treatments in which Fayette was planted. Yield of susceptible soybean in monoculture with or without H. glycines infestation was lower beginning in year 6 when compared to yield of soybean grown in rotation and remained lower throughout the duration of the experiment except for 1987 (year 9). Yields of susceptible and resistant soybean were different each year except for drought years in 1980 and 1988. From 1979 to 1982 differences in yield were due to lower yield potential of resistant cultivars. Except for the drought year, yield of cv. Fayette was greater than susceptible Williams 82 during years 6 through 11.     

Variation in Resistance of Soybean Lines to Races of Heterodera glycines

The objective of this study was to determine the interrelationships of Heterodera glycines races based on their resistance to soybean (Glycine max) cultivars and lines against which they were tested. Greenhouse tests determined the numbers of females of each of eight races of H. glycines that developed on 277 to 522 soybean cultivars and lines. A Female Index (number of females on a test cultivar as a percentage of the number on ''Lee 74'') was calculated and used in frequency distributions, correlations, and duster analyses of the resistance reactions to the different races in an attempt to determine relationships among cultivars. Frequency distribution patterns of all cultivars and lines tested against each race were skewed in favor of resistance, and in some cases bimodality was observed. The majority of correlations between pairs of races were highly significant. Cluster analyses based on the correlations divided eight races into four clusters that explained 73% of the variation in resistance. Cluster 1 was comprised of races 2, 4, and 14; Cluster 2 was comprised of races 6 and 9; Cluster 3 was comprised of races 1 and 3; and Cluster 4 was comprised of race 5. The information obtained in this study could increase the efficiency of testing resistant soybean breeding lines for resistance to H. glycines.     

Evaluation of Pea and Soybean as Trap Crops for Managing Heterodera glycines

Trap crops that stimulate nematode egg hatching but not reproduction have been reported as an effective means for managing certain nematodes. Studies were carried out at two field sites each year in 1998 and 1999 to evaluate the potential of trapping the soybean cyst nematode (Heterodera glycines) with soybean and pea in the corn year to manage the nematode in Minnesota. The trap crops were planted on the same day as corn at each site and later killed with the herbicide glyphosate. Nematode egg densities were determined at planting, 1 and 2 months after planting, and at harvest. Treatments included four seeding rates (0, 124,000, 247,000, and 494,000 seeds/ha) of resistant soybean as a trap crop and four kill dates (3, 4, 5, and 6 weeks after planting). No effects of the trap-crop and kill-date treatments on H. glycines population density, corn yield, and the followingyear soybean yield were observed at the two locations. In a second study, the experiment included four trap-crop comparisons (resistant soybean at 494,000 seeds/ha, susceptible soybean at 494,000 seeds/ha, pea at 1,482,000 seeds/ha, and no trap crop) and five kill dates (3, 4, 5, 6 weeks after planting, and no-kill). At the Waseca site, egg density at harvest was lower where resistant soybean was grown for 6 weeks and where pea was grown for 5 and 6 weeks compared with where no trap crop was grown. Maintaining pea plants for more than 5 weeks, however, reduced corn yield by 20% at the Waseca site. At the Lamberton site, egg density at harvest was lower where the susceptible soybean was grown for 5 weeks compared with where no trap crop was grown. Even with significant reduction of eggs in some treatments, use of soybean and pea as trap crops in the corn year was not an effective means for managing H. glycines.     

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.     

Effect of Within-field Variation in Soil Texture on Heterodera glycines and Soybean Yield

The influence of soil texture on Soybean yield in the presence of Heterodera glycines was investigated by comparing yields of susceptible cultivars with a resistant cultivar for 2 years. Soybean yield was negatively correlated with increasing sand content (P = 0.05). Yields of susceptible cultivars were suppressed with increasing sand content. Final nematode population densities were lowest in plots with greatest sand content. Soybean infection by SCN, as determined by the number of cysts 30 days after planting, was not consistently related to soil texture over 2 years. Initial nematode population density was positively related to soybean yield the first year and negatively related to soybean yield the second, probably a result of greater yield suppression by H. glycines in plots with greater sand content.     

Characterization of the Virulence Phenotypes of Heterodera glycines in Minnesota

Knowledge of the virulence phenotypes of soybean cyst nematode, Heterodera glycines populations is important in choosing appropriate sources for breeding resistant cultivars and managing the nematode. We investigated races of 59 H. glycines populations collected from 1997 to 1998 and races and HG Types of 94 populations collected in 2002 from soybean fields across southern and central Minnesota. In the 1997 to 1998 samples, race 3 was predominant and represented 78% of the populations. The remaining populations were 11.9% race 1, 1.7% race 4, 6.8% race 6, and 1.7% race 14. In the 2002 samples, the populations were classified as 15.3% race 1, 77.6% race 3, 2.4% race 5, 3.5% race 6 and 1.2% race 9. Percentage of 1997 to 1998 populations with female indices (FI) higher than 10 were 10.2% on Pickett 71, 3.4% on Peking, 13.6% on PI 88788, 3.4% on PI 90763, 1.7% on PI 209332, and 1.7% on PI 437654. Percentage of 2002 populations with FI >10 was 1.1% on Peking, 17.0% on PI88788, 14.9% on PI 209332, 33.0% on PI 548316, 11.7% on Pickett 71, and 0% on the other three indicators, PI 90763, PI 437654, and PI 89772. The line PI 548316 was relatively susceptible to the Minnesota H. glycines populations and may not be recommended for breeding resistant cultivars in the state. There was no noticeable change of frequencies of virulence phenotypes in response to the use of resistant cultivars during 1997 to 2002 in Minnesota except that FI increased on the PI 209332.     

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.     

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.     

Distribution of Races of Heterodera glycines in the Central United States

A total of 62 populations of Heterodera glycines were collected in 10 states along the Mississippi and Missouri rivers, and 206 populations were collected in Arkansas. Among the 62 populations, races 2, 3, 4, 5, 6, 9, and 14 were found south of 37°N latitude, and races 1 and 3 were found north of 37°N latitude. In Arkansas samples, races 2, 4, 5, 6, 9, and 14 comprised 87% of the populations. In both groups of samples, H. glycines populations with genes that enabled the population to parasitize cv. Pickett occurred the most frequently, followed by those with genes for parasitism of cv. Peking, then PI88.788, and the fewest with genes for parasitism of PI90.763. The diversity of races in this study raises questions about the effectiveness of race-specific cultivars for the management of soybean cyst nematodes. The greater diversity of races of H. glycines in the southern United States may be because of a longer history of planting resistant cultivars.