Resistance to Root-knot, Reniform, and Soybean Cyst Nematodes in Selected Soybean Breeding Lines

Soybean breeding lines and reported sources of nematode resistance were evaluated in repeated greenhouse tests for resistance to North Carolina populations of the soybean cyst nematode Heterodera glycines, reniform nematode Rotylenchulus reniformis, and the root-knot nematode species Meloidogyne incognita, M. arenaria, and M. arenaria. Lines from the soybean breeding program in Missouri that had 'Hartwig' soybean as a parent were the most resistant to races 1-4 of the soybean cyst nematode and the population of reniform nematode evaluated here. Numerous cysts of an inbred soybean cyst nematode race 4 population were produced on several of these Hartwig descendants, however, and accession $92-1603 had a cyst index of 29.2%. These accessions were also susceptible to M. arenaria and M. arenaria. Soybean lines N87-539 and N91-245 from the breeding program in North Carolina had strong resistance to an inbred soybean cyst nematode race 1 population and to M. arenaria, respectively. Soybean germplasm from the Georgia breeding program demonstrated the strongest resistance to the root-knot nematode species tested. Lines from the Georgia program, including G80-1515, G83-559, G93-9106, and G93-9223, that incorporated both root-knot and soybean cyst nematode resistance had the best overall resistance to the nematode populations evaluated. Resistance reported in the soybean lines was generally upheld. In a few cases, differences in the origin and culture of the nematode populations used in this study may have led to discrepancies between reported and observed resistance.

Possible Transfer of Resistance to Heterodera glycines from Glycine tomentella to Glycine max

Eight wild perennial Glycine species (G. argyrea, G. canescens, G. curvata, G. cyrtoloba, G. latifolia, G. microphylla, G. tabacina, and G. tomentella) were evaluated for resistance to isolates of races 1, 3, and 14 of Heterodera glycines. In a second experiment, reproduction of isolates of races 3, 5, and 14 of H. glycines on five of the wild perennial species was determined. Seventy-one derived fertile lines (2n = 40) that were hybrids between G. max cv Clark 63 and G. tomentella also were evaluated for resistance to isolates of races 3, 5, and 14. All of the wild perennial Glycine species were resistant (Female Indices [FI] less than 10) to all of the isolates that were tested on them. In most cases no females matured. The soybean cvs. Clark 63 and Altona, which were tested at the same time as the hybrids, were susceptible to all isolates of H. glycines tested. When the tests were combined and a single FI calculated with the average number of females on Lee 74, one derived fertile line was resistant to race 3, three derived fertile lines were resistant to race 5, and five derived fertile lines were resistant to race 14. Thus, transfer of resistance to H. glycines from G. tomentella to G. max apparently occurred.

Soil Infestation Density Affects the Results of Heterodera glycines Race Tests

Production of females by stock populations of races 1, 2, 3, 4, 5, 6, 9, and 14 of Heterodera glycines on 'Lee 74', 'Pickett', 'Peking,' PI 88788, and PI 90763 soybean cultivars and lines at inoculum densities of 100, 1,000, 4,000, 5,000, and 10,000 eggs and second-stage juveniles/pot (2.2, 21.8, 87.1, 108.9, and 217.9/cm(3) soil) was tested to determine the effects of soil population density on race identification using the 16-race scheme. Tests of all races were repeated 1 to 3 times during the year, except for the race 9 test, which was conducted once. Only races 3 and 9 had the same race designation at all infestation densities in repeated tests. Races 2 and 4 consistently were identified at all except the lowest infestation densities. Race 5 was identified as race 15 at the 100 infestation density in one test, and as race 1 at the 1,000 density in another test. Race 6 had significant numbers of females on Peking and P190763 that resulted in female indices of 34 on Peking and 14 on P190763 when the infestation density was 10,000. Race 14 was consistently identified between infestation densities of 4,000 and 10,000 but was identified as race 6 or 9 at the 100 infestation density and as race 4 or 14 at the 1,000 infestation density. Race 1 was identified as race 5 in a few instances because Pickett was not resistant to this population. The results of this series of experiments suggest that an infestation density of 4,000 eggs and second-stage juveniles/pot is best for race identification. Races were very poorly differentiated at the lowest density, differentiation was inconsistent at the 1,000 infestation density, and densities higher than 4,000 had reduced numbers of females on Lee 74 and relatively high numbers on the differentials which resulted in poor race differentiation with some races.

Velvetbean and Bahiagrass as Rotation Crops for Management of Meloidogyne spp. and Heterodera glycines in Soybean

Soybean (Glycine max) yield often is limited by the phytoparasitic nematodes Meloidogyne spp. and Heterodera glycines in the southeastern United States. We studied the effects of rotation with bahiagrass (Paspalum notatum), velvetbean (Mucuna pruiens), or continuous soybean, aldicarb, and soybean cultivar on yield and population densities in two fields infested with a mixture of Meloidogyne spp. and H. glycines. Velvetbean and bahiagrass reduced population levels of both nematode species to near zero prior to planting soybean. At harvest, both nematode populations were equal in soybean following bahiagrass and continuous soybean but were lower following velvetbean. Both bahiagrass and velvetbean as previous crops were equal in producing significantly (P < 0.003) higher yield than continuous soybean. Velvetbean increased subsequent soybean yield by 98% and bahiagrass increased subsequent soybean yield by 85% as previous crops compared to continuous soybean. The major differences between the two rotation crops were yield response of the nematode-susceptible cultivars and at-harvest nematode populations. Velvetbean tended to mask genetic differences among cultivars more so than bahiagrass. Velvetbean also produced a more long-term effect on nematode populations, with numbers of both Meloidogyne spp. and H. glycines lower in soybean following velvethean than following bahiagrass or continuous soybean.

Managing Soybean Resistance to Heterodera glycines

Heterodera glycines is an economically important pest of soybean in the United States. The steps necessary to manage this nematode are to sample for inoculum density, identify the race present, and then select appropriate control measures. Planting resistant cultivars is the most widely used management practice, and this tactic has provided enormous economic benefits. However, the nematode has adapted to each source of resistance deployed. The time required for the nematode to adapt to resistant cultivars is lengthened by including nonhost crops and susceptible cultivars in rotations with resistant cultivars. Searching for new sources of resistance and developing more techniques to prolong the effectiveness of resistance genes are necessary to maintain profitability for soybean producers.

Variation in Efficacy of Isolates of the Fungus ARF Against the Soybean Cyst Nematode Heterodera glycines

An unnamed fungus, designated ARF, that parasitizes eggs and sedentary stages of cyst nematodes is a potential biological control agent of Heterodera glycines. The objectives of this study were to determine whether ARF isolates differ in their ability to suppress nematode numbers in soil and to compare the efficacy of ARF in heat-treated and native soil. The effectiveness of 11 ARF isolates was compared by introducing homogenized mycelium into heat-treated soil. Soybean seedlings were transplanted into pots containing fungus-infested soil and inoculated with H. glycines. After 30 or 60 days, the number of nematodes and the percentage of parasitized eggs were determined. Three isolates (907, 908, and TN14), which were previously reported to be weak egg parasites in vitro, consistently suppressed nematode numbers by 50% to 100%. Of the isolates previously reported to be aggressive egg parasites, four (903, BG2, MS3, and TN12) reduced nematode numbers by 56% to 69% in at least one experimental trial, but the other four had no effect on nematode numbers. When the efficacy of isolate TN14 was tested in heat-treated and native soil, nematode suppression was greater in the heat-treated soil in only one of two trials. In both soil treatments, nematode numbers were reduced by more than 60%. We conclude that virulence toward nematode eggs in vitro is a poor indicator of effectiveness of an ARF isolate in soil, and that the presence of soil microbes may reduce, but does not completely inhibit, activity of isolate TN14.

An Alternative Field Method for Screening Soybean Genotypes for Resistance to Heterodera glycines

The soybean cyst nematode (Heterodera glycines) has become an increasingly severe problem in soybean production areas in Brazil. The development and use of resistant cultivars is the most efficient method of minimizing losses due to this pathogen. Our objective was to test the efficiency of an alternative method for screening soybean genotypes for resistance to H. glycines in field plots. The alternative method was compared to the standard method of sowing the test genotypes in fields found to be infested during the previous crop season. In the alternative method, the test genotypes are sown in the furrow following the uprooting of 45-day-old infected plants. The alternative method resulted in twice the cyst population and fewer escapes, and more consistent results than the standard method. The major advantage of the alternative method is that it permits screening in a more homogeneous distribution of H. glycines in the soil.

Esterase Allozymes of Soybean Cyst Nematode, Heterodera glycines, from China, Japan, and the United States

Individual females from 19 populations of Heterodera glycines from China, Japan, and the United States were analyzed for esterase allozyme polymorphism. Eight esterase electrophoretic phenotypes were resolved. Four putative loci, est-1, est-2, est-3, and est-4, were identified, having one, one, two, and one allele, respectively. The four loci expressed six genotypes in the four Chinese populations. Loci est-2, est-3, and est-4 were identified in five Japanese populations and expressed five genotypes, whereas only loci est-2 and est-3 were identified in 10 populations from the United States and expressed four genotypes. Putative alleles at each locus were defined as characters for data analysis. Phylogenetic analysis using parsimony (PAUP) was utilized to determine relationships among the 19 populations. More loci and alleles in populations from China and phylogenetic similarities among populations from Japan and the United States are consistent with a founder effect resulting from dissemination of progenitor H. glycines from China to Japan and subsequent introductions of founder populations from Japan to the United States.

Survey of Heterodera glycines Races and Other Plant-parasitic Nematodes on Soybean in North Carolina

A survey of soybean-production areas in the Piedmont, Coastal Plain and Tidewater regions of North Carolina was conducted from 1994 to 1996. Heterodera glycines was detected in 55 of 77 fields sampled in 15 counties. The host race of H. glycines was determined for 39 of the populations collected. Of all populations collected, 4% were race 1, 40% race 2, 16% race 4, 7% race 5, and 4% race 9; the remaining 29% could not be accurately categorized. None of the populations evaluated had high levels of reproduction on the resistant cultivar Hartwig. The southern root-knot nematode Meloidogyne incognita was detected in 26% of the fields. Helicotylenchus spp. were detected in all fields sampled, Tylenchorhynchus spp. were found in 62%, Paratrichodorus spp. in 56%, and Pratylenchus spp. in 72% of fields sampled. Mesocriconema spp., Xiphinema spp., and Hoplolaimus spp. were detected in less than 20% of the fields sampled.

Influence of herbicide application to soybeans on soybean cyst nematode egg hatching

The hatching of Heterodera glycines eggs in soybean root exudates collected after postemergence application of three herbicides, and the hatching potential of H. glycines eggs from females feeding on herbicide-treated plants, were measured in vitro. Hatching in all root exudate solutions (RES) was greater than in deionized water but less than in 0.003 M ZnSO solution. Filtering RES with a 0.22-mum-filter increased H. glycines hatching in RES. Application of acifluorfen, bentazon, and lactofen to foliage of soybean plants inhibited hatching of H. glycines eggs from the same plants. Hatching in RES from the different herbicide-treated soybeans was similar. Application of crop oil concentrate and non-ionic surfactant adjuvant to foliage did not affect hatching of H. glycines eggs from soybean plants.

Effects of Some Pesticides on the Growth of ARF18 and Its Pathogenicity to Heterodera glycines

The effects of 22 pesticides on the mycelial growth and pathogenicity of the biocontrol fungus ARFI8 to Heterodera glycines were tested in vitro. The chemicals were added to agar at 10, 100, and 1,000 ppm a.i.; a block of agar containing the fungus was added to each test concentration; and fungal growth was measured. Subsequently, a block of the fungus on the pesticide-containing agar was used to determine the ability of the fungus to parasitize eggs of H. glycines. Aldicarb, bentazone, and chlorothalonil had little or no effect on fungal growth, whereas benomyl and thiophanate methyl completely inhibited growth of the fungus at 10 ppm. The relative insensitivity of ARF18 to certain pesticides would permit selected use of those pesticides with ARF18 in an integrated control program if the effects on the fungus in the field are similar to results from petri dish studies.

Inoculation Method for Studying Early Responses of Glycine max to Heterodera glycines

An inoculation technique was developed for studying molecular responses of soybean to the soybean cyst nematode (Heterodera glycines). Effect of inoculum age (0-7 days after eggs were released from cysts) and inoculation site (meristem, elongation, or differentiation zone) on infection were tested on four soybean genotypes. Two genotypes (PI 437654 and cv. Peking) were resistant and two (cv. Essex and cv. Hutcheson) were susceptible to race 3 of H. glycines. Inoculum consisting of second-stage juveniles (J2) was prepared by gently agitating nematode eggs at 75 revolutions per minute at 28 degrees C for various intervals. Infection rates were monitored cytologically. The most consistent infection rate was obtained with 48-hour-old inoculum containing more than 80% J2. More than 100 juveniles/root were observed after inoculation with the 48-hour-old inoculum placed at the root elongation zone, in both resistant and susceptible soybeans. Horizontal orientation of roots during inoculation, the use of concentrated J2 inoculurn (500 J2 in 125 mul/root), and restriction of inoculum to the root elongation zone facilitated synchronous root infection.

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.

Pathogenicity of Pratylenchus penetrans, Heterodera glycines, and Meloidogyne incognita on Soybean Genotypes

The pathogenicity of Heterodera glycines, Meloidogyne incognita, and Pratylenchus penetrans on H. glycines-resistant 'Bryan,' tolerant-susceptible 'G88-20092,' and intolerant-susceptible 'Tracy M' soybean cultivars was tested using plants grown in 800 cm(3) of soil in 15-cm-diam. clay pots in three greenhouse experiments. Plants were inoculated with 0, 1,000, 3,000, or 9,000 H. glycines race 3 or M. incognita eggs, or vermiform stages of P. penetrans/pot. Forty days after inoculation, nmnbers of all three nematodes, except H. glycines on Bryan, generally increased with increasing inoculum levels in Experiment I. Heterodera glycines and M. incognita significantly decreased growth only of Tracy M. At 45 and 57 days after inoculation with 6,000 individuals/pot in experiments II and III, respectively, significantly more P. penetrans and M. incognita than H. glycines were found on Bryan. However, H. glycines and M. incognita population densities were greater than P. penetrans on G88-20092 and Tracy M. Growth of Tracy M infected by H. glycines and M. incognita and growth of G88-20092 infected by M. incognita decreased in Experiment III. Pratylenchus penetrans did not affect plant growth. Reduction in plant growth differed according to the particular nematode species and cultivar, indicating that nematodes other than the species for which resistance is targeted can have different effects on cultivars of the same crop species.

Population Changes in Heterodera glycines and Its Bacterial Parasite Pasteuria sp. in Naturally Infested Soil

A two-year soil sampling study was conducted on four microplots naturally infested with Heterodera glycines and an undescfibed species of Pasteuria. The objectives of the study were to investigate the population dynamics of both organisms and to assess the potential of Pasteuria sp. as a biological control agent of H. glycines. Seasonal fluctuations were observed in numbers of cysts, eggs per cyst, second-stage juveniles (J2) of H. glycines, number of Pasteuria endospores attached per J2, and percentages of endospore-encumbered J2. Percentages of endospore-encumbered J2, Y, increased with the mean numbers of endospores per J2, X, according to the equation Y = 87.0(1 - e(-0.53X)). In contrast, numbers of J2 per 250 cm(3) soil, Y, decreased with the numbers of endospores per J2, X, according to the exponential decay model Y= 67.4 + 220.1e(-1.2X). The equilibrium J2 density (67.4 +/- 3.3) derived from this function was consistent with the predictions of the Lotka-Volterra model of population dynamics based on the equation 0.0195ln(y) - 0.000336y = 0.000049x - 0.00285ln(x) + 0.06589, where x and y represent the biweekly means of J2 densities and the percentages of endospore-encumbered J2, respectively. In all cases, predicted equilibrium densities of J2 were below the damage threshold reported from field studies. These results indicate that, given sufficient time following introduction into a field, Pasteuria may increase to levels that would be effective as one component in an integrated pest management proglmn to control H. glycines.

Effect of Soybean Cyst Nematode (Heterodera glycines) on Yield of Resistant and Susceptible Soybean Cultivars Grown in Ohio

Soybean (Glycine max) producers in Ohio rarely use soybean cyst nematode (Heterodera glycines, SCN)-resistant cultivars because of concerns over limited yield potential and lack of resistance to Phytophthora sojae. A two-year study was initiated to determine grain yield and nematode population increase on soybean cyst nematode-resistant cultivars in maturity groups II and III in production fields. Sites differed in soil texture, nematode densities, and P. sojae infestation at a number of locations in Ohio. Soil was assayed for nematode densities before planting and at harvest. Yields of resistant cultivars averaged 0% to 18% higher than those of susceptible cultivars in fine-textured soils with average preplant populations ranging from 463 to 14,330 SCN eggs/100 cm(3) soil. In coarse-textured soils, yields of susceptible cultivars were 21% to 56% less than the resistant cultivars with average preplant densities ranging from 1,661 to 15,558 SCN eggs/100 cm(3) soil. The reproductive index ranged from 0.1 to 5.5 for resistant cultivars and 0.4 to 112 for susceptible cultivars. In 1993, yield of P. sojae-susceptible, nematode-resistant 'Asgrow A 3431' was as high as yield of the P. sojae-resistant, nematode-susceptible cultivar 'Resnik' in a Phytophthora-infested field. The nematode-resistant cultivars Madison Experimental 131527 and Asgrow A3431 had higher yields than AgVenture AV1341 and susceptible cultivars Resnik and Kenwood when compared over five nematode-infested sites. Nematode-resistant cultivars were found to be excellent alternatives to currently grown susceptible cultivars for managing SCN where group III cultivars are used. However, better cultivar alternatives may be needed for sites with combined Phytophthora root rot and cyst nematode problems.

Soil Moisture Control and Direct Seeding for Bioassay of Heterodera glycines on Soybean

Soil moisture control during evaluations of Heterodera glycines-Glycine max interactions has not been reported routinely as a standardized procedure. A novel soil moisture replacement system was examined in controlled environmental chambers for use in bioassays for female development. The system is compact, lightweight, and has a contained reservoir for moisture supply to multiple test units. Varied soil moisture treatment levels were sustained at or near replacement rates over extended periods of testing. Direct seeding of selected soybean cultivars consistently resulted in 100% seed germination. Subsequent shoot and root growth was successfully restricted to accommodate the size of the system with minimal shoot pruning. Numbers of mature H. glycines females extracted from the roots of susceptible soybean cultivars were consistently high. Inoculum levels of either 500 or 1,000 eggs/plant routinely resulted in numbers of females at more than 30% of the initial inoculum. No evidence of nematode contamination of uninfested plants was found at any level of observation. Results demonstrate a potential for the standardization of two additional variables in determining races and for screening cultivars or lines for resistance to H. glycines.

Tannic Acid Effects on Hatching of Heterodera glycines in Vitro

Effects of tannic acid on hatching of Heterodera glycines eggs were determined in vitro using three batches of eggs obtained from greenhouse cultures in Florida or from naturally infested field soil in Minnesota. A quadratic model fits the percentage egg hatch. Hatch increased with increasing tannic acid concentrations from 0 to about 39 mg/liter, then declined with further increases in concentration. Tannic acid did not induce hatching of dormant eggs obtained from the field.

Nematicidal Activity of Fatty Acid Esters on Soybean Cyst and Root-knot Nematodes

Researchers have indicated that the C fatty acid, pelargonic acid (nonanoic acid), has considerable nematicidal activity that could be increased by derivitization and improved emulsification. Microemulsions of methyl and ethylene glycol esters of pelargonic acid developed by Mycogen Corporation (San Diego, CA) were tested for nematicidal activity against root-knot and soybean cyst nematodes. All treamaents were compared to a deionized water control and a microemulsion "blank" (minus active ingredient). Methyl pelargonate reduced gall numbers at concentrations >/=0.8 mul a.i./liter, and ethylene glycol pelargonate reduced gall numbers at >/=6.4 mul a.i./liter in a laboratory bioassay of Meloidogyne javanica on roots of tomato seedlings. Microscopic observation of treated M. javanica second-stage juveniles suggested that methyl pelargonate was toxic to nematodes at concentrations as low as 0.2 mul a.i./liter. Cysts of Heterodera glycines per gram of root were significantly reduced by weekly soil drenches of methyl pelargonate at 6.4, 3.2, and 1.6 mul a.i./liter compared to controls in one greenhouse experiment. Weekly soil drenches of methyl pelargonate at 4.8 or 3.2 mu1 a.i./liter also significantly reduced the number of eggs produced by M. incognita on soybean in a greenhouse test. In both greenhouse tests with soybean, rates of methyl pelargonate >/=4.8 mul a.i./liter had considerable phytotoxicity. No significant interaction of chemical treatment and different soil mixtures affected the nematode numbers produced or plant vigor observed. Soil drenches with microemulsions of methyl pelargonate at 3.2 mul a.i./liter applied weekly, or as two initial applications, were effective as a nematicide for root-knot and soybean cyst nematodes with negligible effects on plant vigor.

Response of Meloidogyne spp., Heterodera glycines, and Radopholus similis to Tannic Acid

Tannins, which are water-soluble polyphenols, are toxic to numerous fungi, bacteria, and yeasts. Our objectives were to study the efficacy of tannic acid in control of Meloidogyne arenaria on tomato and its effects on the behavior of M. arenaria, M. incognita, Heterodera glycines, and Radopholus similis. Three concentrations of tannic acid, 0.1, 1.0, and 10 g/500 cm(3) of soil, were applied preplant (powder) and at-plant (powder and drench) into soil infested with M. arenaria. Tannic acid at the 1.0-g rate reduced galling compared with the untreated control, regardless of methods of application. The 0.1-g rate resulted in no reduction in galling when applied preplant but reduced galling when applied as a drench and in one of two experiments when applied at-plant. The 10-g rate was phytotoxic to tomato seedlings except when applied 7 days preplant. In the latter case, root galling was suppressed to very low numbers. In behavior studies on water agar, Meloidogyne second-stage juveniles were attracted to areas with an increasing tannic acid gradient. Radopholus similis was repelled from the tannic acid gradient in one of two experiments. There was no effect on H. glycines. The response of M. arenaria second-stage juveniles to different concentrations of tannic acid dissolved in alginate was tested. Movement behavior of the second-stage juveniles were observed at 1,000 and 10,000 mug/ml of tannic acid, but not at 10 and 100 mug/ml.

Genetics of Soybean-Heterodera glycines Interactions

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

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.

Identification of Cyst Nematodes of Agronomic and Regulatory Concern with PCR-RFLP of ITS1

The first internally transcribed spacer region (ITS1) from cyst nematode species (Heteroderidae) was compared by nucleotide sequencing and PCR-RFLP. European, Asian, and North American isolates of five heterodefid species were examined to assess intraspecific variation. PCR-RFLP patterns of amplified ITS1 DNA from pea cyst nematode, Heterodera goettingiana, from Northern Ireland were identical with patterns from Washington State. Sequencing demonstrated that ITS1 heterogeneity existed within individuals and between isolates, but did not result in different restriction patterns. Three Indian and two U.S. isolates of the corn cyst nematode, Heterodera zeae, were compared. Sequencing detected variation among ITS1 clones from the same individual, between individuals, and between isolates. PCR-RFLP detected several restriction site differences between Indian and U.S. isolates. The basis for the restriction site differences between isolates from India and the U.S. appeared to be the result of additional, variant ITS1 regions amplified from the U.S. isolates, which were not found in the three India isolates. PCR-RFLP from individuals of the U.S. isolates created a composite pattern derived from several ITS1 types. A second primer set was specifically designed to permit discrimination between soybean (H. glycines) and sugar beet (H. schachtii) cyst nematodes. Fok I digestion of amplified product from soybean cyst nematode isolates displayed a uniform pattern, readily discernible from the pattern of sugar beet and clover cyst nematode (H. trifolii).

Field Efficacy of Verticillium lecanii, Sex Pheromone, and Pheromone Analogs as Potential Management Agents for Soybean Cyst Nematode

A soybean cyst nematode sex pheromone (vanillic acid), chemical analogs of the pheromone, and the fungus Verticillium lecanii were applied in alginate prills (340 kg/ha) to microplots and small-scale field plots as potential management agents for Heterodera glycines on soybean. In 1991 microplot tests, treatment with V. lecanii, vanillic acid, syringic acid plus V. lecanii, or vanillic acid plus V. lecanii lowered midseason cyst numbers compared with the untreated susceptible cultivar control, autoclaved V. lecanii treatment, or aldicarb treatment, At-harvest cyst numbers were lowest with V. lecanii and with vanillic acid treatments. Aldicarb treatment reduced midseason cyst numbers in 1992. There were no differences among seed yields either year. In the field trials, numbers of cysts were reduced one or both years with aldicarb, ferulic acid, syringic acid, vanillic acid, or 4-hydroxy-3-methoxybenzonitfile treatments, or with a resistant cultivar, compared to an untreated susceptible cultivar. Highest yields were recorded after treatment with 4-hydroxy-3-methoxybenzonitrile (1991), methyl vanillate (1992), and aldicarb (1992). These studies indicate that some chemical analogs of vanillic acid have potential for use in soybean cyst nematode management schemes.

Differences in Hatching of Heterodera glycines Egg-mass and Encysted Eggs in vitro

Hatching studies with Heterodera glycines typically have been conducted with a mixture of egg-mass and encysted eggs. Laboratory research was conducted to compare hatching of H. glycines eggs from external egg masses with that of eggs extracted from within females and cysts (encysted eggs). Egg-mass eggs were collected by soaking infected soybean roots in 0.5% sodium hypochlorite, and encysted eggs were collected from females and cysts dislodged from the same roots with a stream of water. Eggs were incubated at 25 degrees C in deionized water, 3.0 mM ZnSOsolution, or one of three synthetic H. glycines hatch inhibitors, mad hatched juveniles were counted every other day for 22 days. Samples of eggs collected at the beginning and end of all experiments were analyzed to determine extent of embryo development. Egg-mass eggs hatched more rapidly than encysted eggs during the first 16 days, but not thereafter. Throughout the experiments, hatch of egg-mass eggs in deionized water was greater than that of encysted eggs. From day 8 to day 22, egg-mass eggs were less sensitive than encysted eggs to the hatch inhibitor 2-(2'-carboxyethyl)-5-[carboxy(hydroxy)methylidenyl]cyclopentanone. A greater proportion of egg-mass eggs contained vermiform juveniles than did encysted eggs at the beginning of the experiments, but not at the end. Results indicated that H. glycines egg-mass and encysted eggs have different hatching behaviors that cannot be explained entirely by differences in embryological development.