Soybean Cyst Nematode Heterodera Glycines Research Articles

Modulations in gene expression and mapping of genes associated with cyst nematode infection of soybean.

Infection of the soybean root by the soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) induces a well-documented, yet poorly understood, response by the host plant. The plant response, involving the differentiation of a feeding structure, or "syncytium," facilitates the feeding and reproduction of the nematode to the detriment of the host. We used a genetic system involving a single dominant soybean gene conferring susceptibility to an inbred nematode strain, VL1, to characterize the nematode-host interaction in susceptible line PI 89008. The restriction fragment length polymorphism marker pB053, shown to map to a major SCN resistance locus, cosegregates with resistance among F2 progeny from the PI 89008 x PI 88287 cross. Cytological examination of the infection process confirmed that syncytium development in this genetic system is similar to that reported by others who used noninbred nematode lines. Our study of infected root tissue in the susceptible line PI 89008 revealed a number of genes enhanced in expression. Among these are catalase, cyclin, elongation factor 1alpha, beta-1,3-endoglucanase, hydroxy-methylglutaryl coenzyme A reductase, heat shock protein 70, late embryonic abundant protein 14, and formylglycinamidine ribonucleotide synthase, all of which we have genetically positioned on the public linkage map of soybean. Formylglycinamidine ribonucleotide synthase was found to be tightly linked with a major quantitative trait locus for SCN resistance. Our observations are consistent with the hypothesis proposed by others that feeding site development involves the dramatic modulation of gene expression relative to surrounding root cells.

High-efficiency induction of soybean hairy roots and propagation of the soybean cyst nematode.

Cotyledon explants of 10 soybean [Glycine max (L.) Merr.] cultivars were inoculated with Agrobacterium rhizogenes strain K599 with and without binary vectors pBI121 or pBINm-gfp5-ER possessing both neomycin phosphotransferase II (nptII) and beta-glucuronidase (gus) or nptII and green fluorescent protein (gfp) genes, respectively. Hairy roots were produced from the wounded surface of 54-95% of the cotyledon explants on MXB selective medium containing 200 microg ml(-1) kanamycin and 500 microg ml(-1) carbenicillin. Putative individual transformed hairy roots were identified by cucumopine analysis and were screened for transgene incorporation using polymerase chain reaction. All of the roots tested were found to be co-transformed with T-DNA from the Ri-plasmid and the transgene from the binary vectors. Southern blot analysis confirmed the presence of the 35S-gfp5 gene in the plant genomes. Transgene expression was also confirmed by histochemical GUS assay and Western blot analysis for the GFP. Attempts to induce shoot formation from the hairy roots failed. Infection of hairy roots of the soybean cyst nematode (Heterodera glycines Ichinohe)-susceptible cultivar, Williams 82, with eggs of H. glycines race 1, resulted in the development of mature cysts about 4-5 weeks after inoculation. Thus the soybean cyst nematode could complete its entire life cycle in transformed soybean hairy-root cultures expressing GFP. This system should be ideal for testing genes that might impart resistance to soybean cyst nematode.

Two simple sequence repeat markers to select for soybean cyst nematode resistance coditioned by the rhg1 locus

The soybean cyst nematode (SCN) (Heterodera glycines Inchinoe) is the most economically significant soybean pest. The principal strategy to reduce or eliminate damage from this pest is the use of resistant cultivars. Identifying resistant segregants in a breeding program is a difficult and expensive process which is complicated by the oligogenic nature of the resistance and genetic variability in the pathogen. Fortunately, resistance at one SCN-resistance locus, rhg1, is generally accepted as a necessity for the development of resistant genotypes using any source of resistance and when challenged by any SCN race. Thus, the development of SCN resistant cultivars would be expedited if an effective and rapid system were available to identify breeding lines carrying a resistance allele at the rhg1 locus. In this study we report two simple sequence repeat (SSR) or microsatellite loci that cosegregate and map 0.4 cM from rhg1. Allelic variation at the first of these loci, BARC-Satt309, distinguished most, if not all, SCN-susceptible genotypes from those carrying resistance at rhg1 derived from the important SCN-resistance sources ’Peking’, PI 437654, and PI 90763. BARC-Satt309 was also effective in distinguishing SCN resistance sources PI 88788 and PI 209332 from many, but not all, susceptible genotypes. BARC-Satt309 cannot be used in marker-assisted selection in populations developed from typical southern US cultivars crossed with the important resistance sources PI 88788 or PI 209332 because these genotypes all carry the identical allele at the BARC-Satt309 locus. A second SSR locus, BARC-Sat_168, was developed from a bacterial artificial chromosome (BAC) clone that was identified using the primers to BARC-Satt309. BARC-Sat_168 distinguished PI 88788 and PI 209332 from southern US cultivars such as ’Lee’, ’Bragg’ and ’Essex’. Both BARC-Satt309 and BARC-Sat_168 were used to assay lines from SCN-susceptible×SCN-resistant crosses and proved to be highly effective in identifying lines carrying rhg1 resistance from those carrying the allele for SCN susceptibility at the rhg1 locus.

Efficiency Gained by Screening Segregating Soybean Progenies with Soybean Cyst Nematode Race 2 versus Race 5

Soybean cyst nematode(SCN)(Heterodera glycines Ichinohe) is a major pest of soybean [Glycine max (L.) Merr.] in the USA and has many races. The use of SCN resistant cultivars effects changes in the ability of H. glycines field populations to parasitize these cultivars. The objective of this experiment was to determine if screening for race 2 resistance alone was sufficient to identify resistance for both races 2 and 5. Progenies of crosses involving ‘Hartwig” soybean were tested for resistance to either SCN race 2 or 5 in the F2 generation and for resistance to both races in the F2:3 generation in greenhouse studies. Individual plants were grown in soil infested with 1000 eggs of the appropriate race. Resistant plants were transplanted to the field for seed production. Progenies from the harvested plants were tested with both races in separate tests. All F2:3 progenies resistant to race 2 were also resistant to race 5. Only 64% of F2:3 progenies resistant to race 5 were also resistant to race 2. These results indicate that it is more efficient to screen for race 2 resistance alone than to screen for resistance to either race 5 or both races when resistance to both races is sought from crosses involving Hartwig.

Genetic diversity of soybean germplasm resistant to Heterodera glycines

Information on the genetic background of soybean [Glycine max (L.) Merr.] germplasm resistant to Soybean Cyst Nematode (SCN) (Heterodera glycines Ichinohe) is essential for developing plant breeding strategies, however, pedigree information for SCN resistant germplasm is not always available. Using restriction fragment length polymorphisms (RFLPs) to fingerprint different soybean lines is a useful means of determining genetic relationships and identifying unique genotypes. In this study, 102 soybean genomic probes with five different restriction enzymes were used to detect DNA variations and investigate genetic relationships among 56 soybean plant introductions (PIs) and cultivars. Among 510 probe/enzyme combinations, a total of 1707 hybridization bands were generated, of which 501 were polymorphic. A cluster dendrogram and a principal component analysis (PCA) plot diagram were constructed using DNA fingerprinting of the 56 soybean lines. Based on the clustering and PCA analyses, two major clusters comprising 15 groups were identified for the PIs and cultivars in the dendrogram. Reactions to different race isolates of SCN were distinguishable among different groupings of the clusters and PCA results, and various origins of the PIs and the pedigree information of the cultivars could be associated with the different clusters. Additional information on the appropriate number of probes used to detect genetic diversity more efficiently was elucidated through this research. We believe that the genetic relationships determined among these 56 soybean lines could provide useful information in identifying unique sources of genes for SCN resistance.

Differential Display Analysis of the Early Compatible Interaction Between Soybean and the Soybean Cyst Nematode

The marked cellular changes during feeding site formation of the soybean cyst nematode (Heterodera glycines) indicate major changes in soybean gene expression. We used differential display of mRNA to detect host gene expression changes during the early compatible interaction between soybean and H. glycines. Fifteen cDNA clones corresponding to mRNAs with different abundances in H. glycines-infected versus uninfected roots were identified. Differential display results indicated that abundances of five mRNAs increased in infected roots, whereas abundances of 10 mRNAs decreased. Transcripts for nine of these 15 cDNAs could be detected on RNA blots, and their hybridization signals confirmed the differential display results for eight of these nine cDNAs. Sequence analyses identified five cDNAs with decreased mRNA levels in infected roots as corresponding to two putative aldolase genes, a transcription-factor TFIIA homologue, the soybean small GTP-binding protein gene sra1, and the soybean auxin down-regulated gene ADR12. RNA blot analyses of other auxin down-regulated genes revealed a decrease in their mRNA abundances in H. glycines-infected roots as well.

Monosomic Alien Addition Lines Derived from Glycine max (L.) Merr. and G. tomentella Hayata: Production, Characterization, and Breeding Behavior

Utilization of exotic germplasm (16 wild perennial species of the subgenus Glycine Willd.) to broaden the genetic base of soybean [G. max (L.) Merr.] has been impeded because of extremely low intersubgeneric crossability. Our objective here is to report the production, identification, and breeding behavior of monosomic alien addition lines (MAALs) each with 2n = 40 chromosomes of soybean and one chromosome from G. tomentella Hayata (accession PI 483218, 2n = 78). Glycine tomentella contains genes controlling several economically useful traits such as resistance to soybean rust (Phakopsora pachyrhizi Sydow) and soybean cyst nematode (Heterodera glycines Ichinohe) and tolerance to salt and drought. We have isolated 287 plants with 2n = 41 chromosomes from BC3 and BC4 progenies. On the basis of distinguishing morphological features, these lines were grouped into 22 MAALs and were designated as MT‐I to MT‐XXII. MT denotes max and tomentella. The addition of an extra chromosome of G. tomentella to the 2n soybean complement modified several morphological traits including flowering habit, plant height, degree of pubescence, seed fertility, number of seed per pod and plant, pod and seed color, and seed yield. The female transmission of an extra G. tomentella chromosome in MAALs averaged 36.5% and male transmission averaged 11.7%. This study sets a stage whereby the germplasm treasure harbored in exotic germplasm, the wild perennial relatives of the soybean, which was unexplored and not utilizable in the past, can now be reached by geneticists and plant breeders.

Soil Variables Associated with Sudden Death Syndrome in Soybean Fields in Iowa.

Sudden death syndrome, caused by Fusarium solani f. sp. glycines, has increased in prevalence in soybean production regions in the North-Central United States. Little is known about soil factors and environmental conditions that influence disease severity in this pathosystem. We studied associations between biological, chemical, and physical soil variables and severity of foliar symptoms of sudden death syndrome in nine commercial soybean fields in Iowa during 1995 and 1996. Disease was patchy in all fields, and soil samples were collected in each field along a transect that ran from a symptomless area through a diseased area. There were 25 sampling stops along each transect, separated by distances of 1.5 to 2.5 m. At each stop, soil samples were collected and soil strength, soil moisture, and foliar disease severity (at plant growth stage R6) were measured. Soil samples were assayed for population densities of F. solani f. sp. glycines, cysts of the soybean cyst nematode (Heterodera glycines), and for chemical variables (soluble salts, pH, organic matter, cation exchange capacity, and concentrations of P, K, Ca, Mg, Mn, and Fe). Cross-correlation analyses were carried out to test for associations between soil variables and disease severity in individual fields, while discriminant analysis was used to assess the effects of soil variables across all fields. Disease severity showed consistent associations with F. solani f. sp. glycines populations (strong effect) and H. glycines cyst counts (minor effect). Available K was identified as a possible disease-enhancing factor, but the magnitude of its effect was dependent on the overall K-concentrations in the fields. For example, as the median K-concentration increased, the correlation between K and disease decreased. None of the other soil variables showed consistent associations with disease. The results suggest that localized presence or absence of F. solani f. sp. glycines is the chief reason for the patchiness of sudden death syndrome in affected fields. Thus, manipulation of soil nutrient status or fertility level appears to have limited potential for reducing disease in the high-yield soybean production environment of Iowa. Instead, producers should focus on preventing the establishment or reducing populations of F. solani f. sp. glycines and H. glycines in their fields.

Rotation and Fertilization Effects on Corn and Soybean Yields and Soybean Cyst Nematode Populations in a No‐Tillage System

AbstractRotations have long been used to improve crop yields. No‐tillage production acreage is increasing, and production information on rotation effects on yields and soybean cyst nematode populations is needed for this system. Field experiments were initiated in 1985 and continued through 1992 to evaluate P‐K fertilization effects on yield of a corn‐soybean [Zea mays L.‐Glycine max (L.) Merr.] rotation and on populations of soybean cyst nematode (Heterodera glycines Ichinohe) (SCN). The research was conducted on a Loring silt loam soil (a fine‐silty, mixed, active, thermic Oxyaquic Fragiudalf), using no‐tillage management. The experimental design was a split‐plot for corn evaluation and split‐split‐plot for soybean evaluation. Individual treatments were replicated five times. Main plots were broadcast P‐K rates of 0–0, 15–28, 29–56, 44–84, and 59–112 kg ha−1. The split‐plots were continuous corn, continuous soybean, a corn‐soybean rotation, and a soybean‐corn rotation. The split‐split‐plots were two soybean cultivars possessing different levels of SCN resistance. Yields of both crops were increased by the fertilization. In this study, corn yields were increased 14% and soybean yields increased 11% with the rotation. Rotating the two cultivars produced similar yields and nonrotated yields of resistant TN 4‐86 were higher than susceptible Essex, indicating an effect of SCN. The primary benefit of corn in the rotation was to reduce SCN populations. However, within Essex rotations the SCN population recovery was rapid, while populations remained relatively low in the TN 4‐86 rotations. The SCN populations were greater when rotations were fertilized with the two lowest P‐K rates, compared with populations within the unfertilized check and the highest P‐K rate. Soybean producers should consider a no‐tillage system of rotating corn with SCN‐resistant soybean cultivars, fertilized at high P and K rates for yield improvement and reduced SCN populations.

Characterization of cDNAs encoding serine proteinases from the soybean cyst nematode Heterodera glycines

Three cDNAs encoding serine proteinases (HGSPI-III) were isolated from a cDNA library constructed from feeding females of Heterodera glycines. The library was screened with three separate serine proteinase gene fragments amplified from cDNA of H. glycines using consensus oligonucleotide primers. Each predicted protein contains a secretion signal sequence, a propeptide and a mature protein of 226–296 amino acids. One of the predicted enzymes, HGSP-II has 41% identity to a chymotrypsin-like enzyme from the mollusc, Haliotis rufescens, and analysis of key residues involved in substrate binding also suggests a chymotrypsin-like specificity. HGSP-I and HGSP-III show greatest homology to kallikreins but sequence analysis does not allow prediction of their substrate preferences. Southern blot analysis suggests that HGSP-II and HGSP-III are encoded by single-copy genes in contrast to HGSP-I which may have two or more homologues. The regions encoding the mature proteinases were cloned into an expression vector and recombinant protein produced in Escherichia coli. Both HGSP-I and HGSP-II were shown, after refolding, to cleave the synthetic peptide N-CBZ-Phe-Arg-7-amido-4-methylcoumarin, and this activity could be inhibited by the cowpea trypsin inhibitor, CpTI. HGSP-III showed no activity against the synthetic substrates tested. The information gained from these studies indicates that serine proteinases are an important group of enzymes in H. glycines and further characterization will aid the development of a proteinase inhibitor-based approach for transgenic plant resistance to plant parasitic nematodes.

Effect of crop rotation on soil population densities of Fusarium solani and Heterodera glycines and on the development of sudden death syndrome of soybean

The effect of crop rotation on the development of sudden death syndrome (SDS) of soybean and on the soil population densities of Fusarium solani and the soybean cyst nematode ( Heterodera glycines) was observed for four years in a field naturally infested with both pathogens. In 1989 all plots were planted to a soybean cultivar susceptible to both pathogens (cv. Lee 74). In 1990 and 1991, the plots were planted to one of the following crops: soybean cv. Lee 74, soybean cv. Hartz 6130 (resistant to both pathogens), sorghum, fescue, or wheat (not double-cropped). In 1992 all plots were planted to soybean cv. Lee 74. The parameters measured to determine rotation effects were severity of SDS over the season, the soil population densities of both pathogens, soybean yields and soybean seed weights. Rotation to any crop except soybean significantly reduced the H. glycines population densities in 1990 and 1991. In 1991, the H. glycines population densities began to rise in the nematode-resistant soybean treatment, but were lower than those in the susceptible soybean treatment. Averaged over all years, the F. solani population densities were significantly lower when sorghum or wheat were grown than when soybean or fescue were grown. Considering the whole season, the F. solani population densities were generally the highest at harvest. Negative correlations between SDS and yield were significant, as were positive correlations between SDS and population densities of F. solani. In 1992, SDS was very low in all treatments, but yields were highest in plots planted to non-soybean crops and lowest in plots planted to the susceptible soybean.

Influence of the soybean cyst nematode ( Heterodera glycines) on the incidence of brown stem rot in soybean and adzuki bean

Experiments were carried out in pots to study the influence of the soybean cyst nematode ( Heterodera glycines) on the incidence of brown stem rot (BSR) in soybeans ( Glycine max) and adzuki beans ( Vigna angularis) caused by the soilborne fungi, Phialophora gregata f.sp. sojae and P. g. f.sp. adzukicola. The presence of H. glycines increased the incidence of BSR in susceptible soybean (cv. Nakate-Hikarikuro) and adzuki bean (cv. Erimoshozu) for both pathogens. The disease-enhancing effects of H. glycines were not shown in the soybean cultivar Peking, which is resistant to the nematode. In a soybean cultivar resistant to BSR (cv. BSR302), the incidence of BSR was low, even under the presence of H. glycines. In split-root experiments using the two BSR susceptible cultivars, the disease-enhancing effects of H. glycines were not observed in either soybeans or adzuki beans, whose roots were inoculated separately with P. gregata and H. glycines.

Genetic analysis of parasitism in the soybean cyst nematode Heterodera glycines.

A genetic analysis of parasitic ability in the soybean cyst nematode Heterodera glycines was performed. To identify and characterize genes involved in parasitism, we developed three highly inbred H. glycines lines, OP20, OP25 and OP50, for use as parents for controlled crosses. Through these crosses, we have identified genes in the inbred parents that control reproduction of the nematode on hosts that carry resistance genes. These genes, designated as ror-* for reproduction on a resistant host, segregate in a normal Mendelian fashion as independent loci. Host range tests of F1 generation progeny indicated that at least one parasitism gene in both the OP20 and OP50 lines for host PI 88788 was dominant. Parasitism genes in OP50 for hosts "Peking" and PI 90763 are recessive. Two types of single female descent populations, a single backcrossed BC1F2-derived and a double backcrossed BC2F1-derived, were established on the susceptible soybean cultivar "Lee 68." Host range tests for parasitism in these lines demonstrated the presence of two independent genes in OP50, one for host PI 88788 designated ror-1 and one for host PI 90763 designated ror-2. OP20 carries two independent genes for parasitism on PI 88788, designated as alleles kr3 and kr4.

Characterization of two cDNAs encoding cysteine proteinases from the soybean cyst nematode Heterodera glycines.

Two cDNAs encoding cysteine proteinases were isolated from a cDNA library constructed from feeding females of Heterodera glycines. The library was screened with a cysteine proteinase gene fragment originally amplified from cDNA of H. glycines. Database searches predict that 1 cDNA (hgcp-I) encodes a cathepsin L-like proteinase, while the second (hgcp-II) encodes a cathepsin S-like enzyme. Both predicted proteins contain a short secretion signal sequence, a long propeptide and a mature protein of 219 amino acids. Southern blot analysis suggests that the cathepsin S-like enzyme, HGCP-II, is encoded by a single-copy gene in contrast to the cathepsin L-like proteinase, HGCP-I which may have 2 homologues. The regions encoding the mature proteinases were cloned into an expression vector and recombinant protein produced in E. coli. HGCP-I was shown, after refolding, to cleave the synthetic peptide Z-Phe-Arg-AMC, and this activity could be inhibited by the engineered rice cystatin Oc-I delta D86. HGCP-II showed no activity against the synthetic substrates tested. The knowledge gained from these studies will improve our understanding of plant nematode proteinases and aid the development of a rational proteinase inhibitor-based approach to plant nematode resistance.

Nematode pathogenesis and resistance in plants.

Nematodes comprise a large phylum of animals that includes plant and animal parasites as well as many free-living species (Maggenti, 1981). Plant parasitic nematodes are obligate parasites, obtaining nutrition only from the cytoplasm of living plant cells. These tiny roundworms (generally 4 mm long and barely visible to the human eye) damage food and fiber crops throughout the world and cause billions of dollars in losses annually (Sasser and Freckman, 1987). Some plant parasitic nematodes are ectoparasites, living outside their host. These species cause severe root damage and can be important virus vectors (Brown et al., 1995). Other species spend much of their lives inside roots as migratory or sedentary endoparasites. Migratory parasites move through the root, causing massive cellular necrosis. However, it is the sedentary endoparasites of the family Heteroderidae that cause the most economic damage worldwide. This group is the focus of the review. The Heteroderidae can be divided into two groups: the cyst nematodes, which include the genera Heterodera and Globodera; and the root-knot nematodes (genus Meloidogyne). The soybean cyst nematode (Heterodera glycines) is the most economically important pathogen of soybean in the United States. Potato cyst nematodes (Globodera pallida and G. rosfochiensis) cause losses in potato-growing areas worldwide (Ross, 1986). Root-knot nematodes, so-called for the characteristic root galls or root knots that they form on many hosts (Figure lA), infect thousands of plant species and cause sevele losses in yield of many crops throughout the world (Mai, 1985). Symptoms of diseased plants infected by these groups of nematodes include stunted growth, wilting, and susceptibility to other pathogens. Nematodes in these three genera have complex interactions with their host plants that generally last more than a month and result in major morphological and developmental changes in both organisms. During the infection, elaborate developmental and morphological changes occur in host root cells, especially in those that become the feeding cells that provide the sole source of nutrients for the nematode. There has been a recent burst of activity in the investigation of the molecular changes that mediate the host-parasite interaction. This ac-

ダイズ連作に伴うダイズシストセンチュウの圃場内の汚染拡大、ダイズへの着生およびダイズ収量に対する有機物施用の影響

ダイズ連作に伴うダイズシストセンチュウの圃場内の汚染拡大、ダイズへの着生およびダイズ収量に対する有機物施用の影響

Genetic mapping of soybean cyst nematode race-3 resistance loci in the soybean PI 437.654

Resistance to the soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is difficult to evaluate in soybean [Glycine max (L.) Merr.] breeding. PI 437.654 has resistance to more SCN race isolates than any other known soybean. We screened 298 F6∶7 recombinant-inbred lines from a cross between PI 437.654 and 'BSR101' for SCN race-3 resistance, genetically mapped 355 RFLP markers and the I locus, and tested these markers for association with resistance loci. The Rhg 4 resistance locus was within 1 cM of the I locus on linkage group A. Two additional QTLs associated with SCN resistance were located within 3cM of markers on groups G and M. These two loci were not independent because 91 of 96 lines that had a resistant-parent marker type on group G also had a resistant-parent marker type on group M. Rhg 4 and the QTL on G showed a significant interaction by together providing complete resistance to SCN race-3. Individually, the QTL on G had greater effect on resistance than did Rhg 4, but neither locus alone provided a degree of resistance much different from the susceptible parent. The nearest markers to the mapped QTLs on groups A and G had allele frequencies from the resistant parent indicating 52 resistant lines in this population, a number not significantly different from the 55 resistant lines found. Therefore, no QTLs from PI 437.654 other than those mapped here are expected to be required for resistance to SCN race-3. All 50 lines that had the PI 437.654 marker type at the nearest marker to each of the QTLs on groups A and G were resistant to SCN race-3. We believe markers near to these QTLs can be used effectively to select for SCN race-3 resistance, thereby improving the ability to breed SCN-resistant soybean varieties.

Field Population Dynamics of Soybean Cyst Nematode on Resistant and Susceptible Soybeans and Their Blends

Soybean cyst nematode (Heterodera glycines Ichinohe) (SCN) become an increasing problem in the upper midwest. In order to develop appropriate practices to manage SCN populations it is important to understand the population dynamics of SCN under field conditions. Soybean cyst nematode population dynamics were determined in naturally infested commercial soybean [Glycine max (L.) Merr.] fields in southern Minnesota on a monthly basis during 1991 and 1992 for an adapted susceptible (S) and a SCN Race 3 resistant (R) cultivar and three blends (25% R:75% S; 50% R:50% S; 75% R: 25% S). Population dynamics differed between sites and years. In 1991, fields with similar initial cyst densities were compared. Densities of second‐stage juveniles (J2) peaked in August at one site and gradually declined throughout the growing season at the other. Total cyst production was lower at the site where J2 densities declined. In 1992, fields with high and low initial egg densities were compared. SCN population densities increased when initial densities of J2 and eggs were low and decreased when initial J2 and egg densities were high. Physiological race of the nematode influenced population dynamics on resistant plants. The cyst index under field conditions for the resistant cultivar was 10 (moderately resistant) against Race and 32 (moderately susceptible) against Race 1. The blends generally lowered SCN population densities, but not significantly. Final egg density and cyst production for the resistant cultivar were significantly lower than the susceptible cultivar at each site. The use of SCN resistant cultivars alone or in a 75:25% mixture with a susceptible cultivar (25%) is recommended as a SCN population management strategy to delay shifts from one race to another thus prolonging the effectiveness of resistant cultivars.

Impact of Crop Rotation and Tillage System onHeterodera glycinesPopulation Density and Soybean Yield

The long-term effects of no-till planting practices and rotation on the population dynamics of the soybean cyst nematode (Heterodera glycines) and soybean yield were investigated in field experiments over a period of 8 yr. The experiment was a 2 X 4 factorial, comparing no-till vs. conventional tillage practices in four cropping patterns (continuous soybean, a 1-yr rotation of corn and soybean, a rotation of 2 yr of corn followed by soybean, and a corn-wheat/soybean double-cropping system). Treatments were arranged so that each combination occurred every year after 1986. Soybean after 1 yr of corn had higher yields (P = 0.0001) than soybean after soybean. Two years of corn between soybean crops resulted in soybean yields higher than those after 1 yr of corn in only 2 out of 6 yr. The yields of soybean in the corn, wheat/soybean double-cropping system, however, were generally similar to monoculture soybean. No-till practices had positive or no effects on soybean yield early in the study, but yields of no-till soybean were lower (P = 0.01) than conventionally tilled soybean after several years because weed pressure was greater in no-till plots. Population densities of H. glycines were greater (P < 0.10) in conventionally tilled plots than in no-till plots in 1988 and 1990-1992. Numbers of H. glycines fluctuated in an unpredictable manner from year to year, possibly because of unidentified biological control agents or excessive moisture in certain years. H. glycines population densities declined in a predictable manner when a nonhost was planted

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