Soybean Cyst Nematode Population Density Research Articles

A Survey of Soybean Cyst Nematode Population Densities and Phenotypes in Illinois: 2018 and 2020

In Illinois, soybean cyst nematode (SCN; Heterodera glycines) has been estimated to cause over $15 million in losses as recently as 2021. In addition, populations of SCN insensitive to PI 88788-derived resistance, the most commonly used SCN resistance source in commercially available soybeans, continues to increase in soybean production areas in the United States. Despite this, awareness of this pathogen and its current status in the state is relatively lacking. A survey was conducted in 2018 and 2020 to assess the presence, population level, and virulence phenotypes in Illinois soybean fields. Soil samples were collected and processed for SCN eggs after harvest each year, and samples with sufficient numbers of SCN eggs were virulence tested using the Hg type test. A total of 95 samples were collected, representing 46 counties. Eighty-seven percent of fields sampled contained SCN eggs, with 38% containing population densities over 2,000 eggs/100 cm3. All samples included in the Hg type test had a female index > 10 on indicator line PI 88788, indicating insensitivity to this resistance source. Extrapolated across soybean production acres, these data indicate that approximately 4.4 million acres of soybeans contain levels of SCN requiring SCN management, and over 54% of acres using cultivars derived from PI 88788 are not fully protected from this pathogen.

Fluopyram activates systemic resistance in soybean

The soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is a significant yield-limiting factor in soybean production in the Midwestern US. Several management practices are implemented to mitigate yield losses caused by SCN, including using SDHI (succinate dehydrogenase inhibitors) fungicides delivered as seed treatments. A set of studies was conducted to evaluate the effect of two seed-applied succinate dehydrogenase inhibitors (SDHI) compounds, fluopyram and pydiflumetofen, on SCN population densities, plant injury, and plant growth. Cyst counts in untreated control and pydiflumetofen treated plants were 3.44 and 3.59 times higher than fluopyram, respectively, while egg counts were 8.25 and 7.06 times higher in control and pydiflumetofen. Next-generation sequencing was later employed to identify transcriptomic shifts in gene expression profiles in fluopyram and pydiflumetofen -treated seedlings. RNA expression patterns of seed treatments clustered by sampling time (5 DAP vs. 10 DAP); therefore, downstream analysis was conducted by timepoint. At 5 DAP, 10,870 and 325 differentially expressed genes (DEG) were identified in fluopyram and pydiflumetofen, respectively. These same treatments generated 219 and 2 DEGs at 10 DAP. Multiple DEGs identified in soybean seedlings treated with fluopyram are linked to systemic resistance, suggesting a potential role of systemic resistance in the suppression of SCN by fluopyram, in addition to the known nematicidal activity. The non-target inhibition of soybean succinate dehydrogenase genes by fluopyram may be the origin of the phytotoxicity symptoms observed and potentially the source of the systemic resistance activation reported in the current study. This work helps to elucidate the mechanisms of suppression of SCN by fluopyram.

Assessing Direct and Residual Effects of Cover Crops on the Soybean Cyst Nematode, Heterodera glycines.

Greenhouse experiments were conducted to determine if cover crops directly decrease population densities of the soybean cyst nematode (SCN), Heterodera glycines, and/or have residual effects on reproduction of the nematode on soybean (Glycine max). Population densities of SCN were not significantly decreased by nine cover crop plants or three cover crop mixes compared with a non-planted soil control in a repeated 60-day-long greenhouse experiment. When susceptible soybeans were grown in the soils after cover crop growth, fewer SCN females formed after three annual ryegrass (Lolium multiflorum) cultivars (Bounty, King, and RootMax), the Daikon radish (Raphanus sativus var. longipinnatus) cultivar CCS779, Kodiak mustard (Brassica juncea), and a mix containing cereal rye, crimson clover (Trifolium incarnatum), plus Daikon radish (cultivars not stated) compared with following the non-planted control. In another repeated experiment, cover crops were grown for 56 days in SCN-infested soil in the greenhouse then exposed to Iowa winter conditions for 28 days to simulate winter termination of the plants. One treatment, a cover crop mix containing 'Bounty' annual ryegrass plus 'Enricher' Daikon radish, had a decrease in SCN population density greater than the non-planted control at the end of the experiment. Significantly fewer SCN females formed on soybeans grown after several cover crops, including the three annual ryegrass cultivars that had the suppressive residual effects in the first experiment. In summary, there were no cover crop treatments that consistently decreased SCN population densities across experiments, and only one cover crop treatment in one experiment significantly reduced SCN population densities more than a non-planted soil control. However, there was a somewhat consistent, adverse, residual effect of cover crops on reproduction of SCN on susceptible soybeans after growth of multiple cover crops.

Interactive Effects of Soybean Cyst Nematode, Arbuscular-Mycorrhizal Fungi, and Soil pH on Chlorophyll Content and Plant Growth of Soybean

Nutritional deficiency chlorosis, especially iron-deficiency chlorosis, and the soybean cyst nematode (SCN) limit soybean yield. Arbuscular mycorrhizal fungi (MF) generally have beneficial effects on plant growth. The interactive effects of SCN, MF, and soil pH on leaf chlorophyll content (LCC) and growth of soybean were examined in a greenhouse experiment. The experiment was a randomized complete block design with three factors: SCN population densities, MF inoculation, and soil pH levels. SCN reduced LCC, and the effect increased with increasing time during 5 to 9 weeks after planting, especially in the high-pH (pH 8) soil. MF increased LCC in low-pH (pH 5.6) soil regardless of SCN population density and in the high-pH soil without SCN. However, MF reduced LCC if both pH and SCN population density were high. The high-SCN population density (inoculation with 10,000 eggs per 100 cm3 of soil) reduced soybean shoot weight in all soils regardless of MF. MF increased shoot weight at pH 6.9 and pH 8 but not at pH 5.6. When MF was present, shoot weight was generally highest at pH 6.9. At high SCN when MF was absent, plant growth was better in pH 5.6 than pH 6.9 and pH 8 soils. This study demonstrates that SCN causes greater damage to soybean when interacting with high pH, and MF had a beneficial effect on soybean growth regardless of SCN infection in all pH soils, in spite of the negative effect of MF on LCC at approximately 5 to 9 weeks after planting in high-pH soil at high SCN population density.

Resistance Gene Pyramiding and Rotation to Combat Widespread Soybean Cyst Nematode Virulence

Soybean cyst nematode (SCN) is an important pathogen of soybean causing >$1 billion in yield losses annually in the United States. Planting SCN-resistant soybean cultivars is the primary management strategy. Resistance genes derived from the plant introduction (PI) 88788 (rhg1-b) and PI 548402 (Peking; rhg1-a and Rhg4) are the main types of resistance available in commercial cultivars. The PI 88788 rhg1-b resistance allele is found in the majority of SCN-resistant cultivars in the north central United States. The widespread use of PI 88788 rhg1-b has led to limited options for farmers to rotate resistance sources to manage SCN. Consequently, overreliance on a single type of resistance has resulted in the selection of SCN populations that have adapted to reproduce on these resistant cultivars. Here we evaluated the effectiveness of rotating soybean lines with different combinations of resistance genes to determine the best strategy for combating the widespread increase in virulent SCN and limit future nematode adaptation to resistant cultivars. Eight SCN populations were developed by continuous selection of a virulent SCN field population (Heterodera glycines [HG] type 1.2.5.7) on a single resistance source or in rotation with soybean pyramiding different resistance gene alleles derived from PI 88788 (rhg1-b), PI 437654 (rhg1-a and Rhg4), PI 468916 (cqSCN-006 and cqSCN-007), and PI 567516C (Chr10). SCN population densities were determined for eight generations. HG type tests were conducted after the eighth generation to evaluate population shifts. The continued use of rhg1-b or 006/007 had limited effectiveness for reducing SCN type 1.2.5.7 population density, whereas rotation to the use of rhg1-a/Rhg4 resistance significantly reduced SCN population density but selected for broader SCN virulence (HG type 1.2.3.5.6.7). A rotation of rhg1-a/Rhg4 with a pyramid of rhg1-b/006/007/Chr10 was the most effective combination at both reducing population density and minimizing selection pressure. Our results provide guidance for implementation of a strategic SCN resistance rotation plan to manage the widespread virulence on PI 88788 and sustain the future durability of SCN resistance genes.

Impact of Wheat on Soybean Cyst Nematode Population Density in Double-Cropping Soybean Production.

Double-cropping is defined as producing more than one crop on the same parcel of land in a single growing season. It is reported to have many benefits when incorporated in cropping systems, including improving soil health. In some double-cropping systems, soybean is planted following winter wheat. The soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is a major soybean pathogen, and several reports suggest suppressive effects of wheat on SCN populations. Field trials were conducted from 2017 to 2018 to investigate the effect of wheat on SCN populations in double-cropping soybean. Nine fields with three levels of initial SCN populations (low, moderate, and high) were selected in Illinois. Wheat was planted in strips alternating with strips-maintained weed-free and under fallow over winter and early spring. Soybean was planted in all strips after wheat harvest. SCN egg densities were acquired at four time points: wheat establishment, post-wheat/pre-soybean, mid-soybean (R1 growth stage or beginning of flowering), and post-soybean harvest. Wheat strips reduced SCN egg densities compared with fallow strips at the R1 stage (−31.8%) and after soybean harvest (−32.7%). Double-cropping soybean with wheat has the potential to suppress SCN field populations and is a system with the potential to provide additional farm income. This study is meant to be a first step toward a better understanding of the mechanisms that govern the suppression of SCN by wheat.

Robotic agricultural instrument for automated extraction of nematode cysts and eggs from soil to improve integrated pest management

Soybeans are an important crop for global food security. Every year, soybean yields are reduced by numerous soybean diseases, particularly the soybean cyst nematode (SCN). It is difficult to visually identify the presence of SCN in the field, let alone its population densities or numbers, as there are no obvious aboveground disease symptoms. The only definitive way to assess SCN population densities is to directly extract the SCN cysts from soil and then extract the eggs from cysts and count them. Extraction is typically conducted in commercial soil analysis laboratories and university plant diagnostic clinics and involves repeated steps of sieving, washing, collecting, grinding, and cleaning. Here we present a robotic instrument to reproduce and automate the functions of the conventional methods to extract nematode cysts from soil and subsequently extract eggs from the recovered nematode cysts. We incorporated mechanisms to actuate the stage system, manipulate positions of individual sieves using the gripper, recover cysts and cyst-sized objects from soil suspended in water, and grind the cysts to release their eggs. All system functions are controlled and operated by a touchscreen interface software. The performance of the robotic instrument is evaluated using soil samples infested with SCN from two farms at different locations and results were comparable to the conventional technique. Our new technology brings the benefits of automation to SCN soil diagnostics, a step towards long-term integrated pest management of this serious soybean pest.

Dynamics of Population Density and Virulence Phenotype of the Soybean Cyst Nematode as Influenced by Resistance Source Sequence and Tillage.

The soybean cyst nematode (SCN), Heterodera glycines, is the most damaging pathogen of soybean. Use of resistant cultivars is an effective strategy to manage SCN, but it also selects for virulent populations over time. A 12-year field experiment was initiated in 2003 to study how tillage and 11 different sequences of four cultivars impact SCN population dynamics and virulence. An SCN-susceptible cultivar and three resistant cultivars (R1, R2, and R3 derived from cultivars PI 88788, Peking, and PI 437654, respectively) were used. Tillage had minimal effect on SCN population density. Compared with no till, conventional tillage resulted in a faster increase of SCN virulence to Peking when the SCN was selected by R2 and virulence to PI 88788 by R3. Among the three SCN-resistant cultivars, R1 supported the greatest population density, R2 supported intermediate population density, and R3 supported the least SCN population density. The SCN populations selected by R1 overcame the resistance in PI 88788 but not in Peking and PI 437654. R2 selected SCN populations that overcame the resistance in Peking but not in PI 88788 and PI 437654. In contrast, R3 selected SCN populations that overcame both PI 88788 and Peking sources of resistance. There was no increase of virulence to PI 437654 in any cultivar sequence. R1 in rotation with R2 or R3 had a negative effect on female index on Peking. Susceptible soybean reduced SCN virulence to Peking, indicating that there was fitness cost of the Peking virulent SCN type. These results suggest that rotation of Peking with PI 88788 is a good strategy for managing the SCN, and susceptible cultivar and no till may reduce SCN virulence selection pressure in some rotations.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Effects of ILeVO Seed Treatment on Heterodera glycines Reproduction and Soybean Yield in Small-Plot and Strip-Trial Experiments in Iowa.

ILeVO (fluopyram) is a fungicide seed treatment for soybean sudden death syndrome (SDS) that also has nematicidal activity. ILeVO is sold with a base of insecticide Poncho (clothianidin), nematode-protectant VOTiVO (Bacillus firmus), and Acceleron fungicides (metalaxyl, fluxapyroxad, and pyraclostrobin). Yield and reproduction of the soybean cyst nematode (SCN) (Heterodera glycines) on soybean plants grown from seed treated with ILeVO plus the base were compared with those treated with only the base in 27 small-plot experiments and 12 strip-trial experiments across Iowa from 2015 to 2017. To increase the likelihood that yield results were related to effects on SCN, data were used only from 26 small-plot experiments and 12 strip trials in which symptoms of SDS were low or nonexistent. An SCN reproductive factor (RF) was calculated for each experimental unit by dividing the SCN population density at harvest by the population density at planting. ILeVO significantly reduced SCN RF by 50% in one strip-trial experiment and by 36 to 60% in four small-plot experiments but yields were not increased by ILeVO in any of those five experiments. Soybean yields were 2.8 to 3.7 bushels/acre (bu/ac) (188.3 to 248.8 kg/ha) greater with ILeVO in three small-plot experiments but SCN RF was not reduced in those experiments. Also, yield was 1.9 bu/ac (127.8 kg/ha) greater with ILeVO in one strip-trial experiment in 2016 but SCN samples were not collected at harvest from the study to assess the possible effects of ILeVO on SCN reproduction. When strip-trial data from 2015 and 2016 were combined, there was a small but significant 0.8 bu/ac (52.2 kg/ha) yield increase with ILeVO. Overall, the effects of ILeVO on SCN reproduction and soybean yield were variable in these field studies.

Field and greenhouse evaluations of soil suppressiveness to Heterodera glycines in the Midwest corn-soybean production systems.

Soil suppressive to the soybean cyst nematode (SCN), a major yield-limiting pathogen of soybean, plays an important role in biological control. Field and greenhouse experiments were conducted to evaluate the effects of tillage, crop sequence, and biocide application on SCN suppression in corn-soybean cropping systems in Minnesota. The experiment was a split-plot design with no-tillage and conventional tillage as main plots, and six crop-biocide treatments (CRCS, CSCS, SSSS, SSSS + streptomycin, SSSS + captan, and SSSS + formaldehyde – the four letters represent crops in 2009 to 2012, respectively; C is corn, R is SCN-resistant soybean, and S is SCN-susceptible soybean) as subplots with four replicates. Soil samples were taken from each plot at planting, midseason, and harvest each year for SCN egg counts, and soybean yield was determined. In addition, soil samples collected from each plot at midseason were assayed for suppressiveness to SCN. Tillage had minimal effect on SCN population density and soybean yield. Annual rotation with corn reduced SCN population density, but also reduced soil suppressiveness as SCN egg population density increased in the following SCN-susceptible soybean compared with soybean monoculture. Rotation with SCN-resistant soybean and corn was the most effective in reducing SCN population density. The bactericide streptomycin did not affect SCN populations but the fungicide captan increased SCN population density. The biocide formaldehyde was the most effective in reducing the level of suppressiveness to SCN. The greenhouse study confirmed that the soil was suppressive to SCN, but failed to detect effects of tillage, crop sequence, and biocide field treatments. This study demonstrated that the soil in the fields was suppressive to the SCN, and biological agents, especially fungal antagonists, were involved in nematode suppression.

Microbial communities in soils with different population densities of soybean cyst nematode

The relationship of abiotic factors with soybean cyst nematode (SCN), the most damaging pathogen of soybean, has been studied extensively, but characterization of microbial communities in infested soils and on soybean roots under field conditions remains unknown. This study investigated the relationship of different SCN population density levels with soil physical, chemical and microbial properties in soils and on soybean roots. A commercial soybean field with different levels of SCN densities (low, medium, high) was sampled, and microbial communities were characterized using both dilution plating assays and molecular approaches. Population densities of species of Trichoderma, actinomycetes, Pseudomonas and other bacteria were significantly higher (P ≤ 0.05) in soils with low SCN (SCN-C) compared with high (SCN-A) and medium levels (SCN-B). Diversity indices of Trichoderma communities on roots based on denaturing gradient gel electrophoresis (DGGE) were significantly higher in soils with SCN-C compared with SCN-A and SCN-B. Cluster analyses and canonical correspondence analysis (CCA) generally separated microbial communities into three groups based on different SCN levels, and CCA showed that low SCN population levels were positively correlated with high levels of nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), cation exchange capacity (CEC), base saturation (BS), the populations of Trichoderma and bacteria. Moreover, diverse Trichoderma, Pseudomonas, Bacillus and actinomycetes were found on roots in SCN-C soils based on dilution plating and DGGE, suggesting that the presence of these microbes on soybean roots might be associated with improved plant health and reduction of SCN.

Increase in Soybean Cyst Nematode Virulence and Reproduction on Resistant Soybean Varieties in Iowa From 2001 to 2015 and the Effects on Soybean Yields

Management of the soybean cyst nematode (SCN) relies heavily on use of SCN-resistant soybean varieties to limit nematode reproduction and minimize yield loss. For Iowa, almost all SCN-resistant soybean varieties contain SCN resistance genes from a breeding line named Plant Introduction (PI) 88788. Iowa State University conducts experiments to evaluate numerous SCN-resistant and three to four SCN-susceptible soybean varieties in up to nine field experiments across Iowa each year. Data on SCN population density, virulence (SCN race and HG type), soybean yield, precipitation, and growing degree days from more than 25,000 four-row plots in field experiments conducted from 2001 to 2015 were analyzed to determine how these factors affected SCN reproduction and yield. SCN population densities were positively correlated with temperatures and negatively associated with precipitation during the growing seasons, indicating that SCN reproduction was greatest in hot, dry years. Over the years, virulence of SCN populations on PI 88788 increased in the fields in which the experiments were conducted, resulting in increased end-of-season SCN population densities and reduced yields of SCN-resistant soybean varieties with the PI 88788 source of resistance. These results indicate that soybean yield loss caused by SCN on resistant varieties with the common PI 88788 source of resistance likely will increase as virulence of SCN populations increases unless new sources of resistance become widely available and used in the future.

Host-Derived Artificial MicroRNA as an Alternative Method to Improve Soybean Resistance to Soybean Cyst Nematode.

The soybean cyst nematode (SCN), Heterodera glycines, is one of the most important pests limiting soybean production worldwide. Novel approaches to managing this pest have focused on gene silencing of target nematode sequences using RNA interference (RNAi). With the discovery of endogenous microRNAs as a mode of gene regulation in plants, artificial microRNA (amiRNA) methods have become an alternative method for gene silencing, with the advantage that they can lead to more specific silencing of target genes than traditional RNAi vectors. To explore the application of amiRNAs for improving soybean resistance to SCN, three nematode genes (designated as J15, J20, and J23) were targeted using amiRNA vectors. The transgenic soybean hairy roots, transformed independently with these three amiRNA vectors, showed significant reductions in SCN population densities in bioassays. Expression of the targeted genes within SCN eggs were downregulated in populations feeding on transgenic hairy roots. Our results provide evidence that host-derived amiRNA methods have great potential to improve soybean resistance to SCN. This approach should also limit undesirable phenotypes associated with off-target effects, which is an important consideration for commercialization of transgenic crops.

Preceding Crops Affected Soybean Iron‐Deficiency Chlorosis and Vesicular‐Arbuscular Mycorrhizal Fungi in Soybean Cyst Nematode‐Infested Fields

Iron‐deficiency chlorosis (IDC) and soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) are important factors limiting soybean [Glycine max (L.) Merr.] yield. Vesicular‐arbuscular mycorrhizal fungi (MF) are generally beneficial to crops. The effect of preceding crops with known variation in MF colonization was tested in two SCN‐infested fields in Waseca and Lamberton in Minnesota to determine the effect of MF and other soil biotic and abiotic factors on IDC. No crop effect on soybean yield was detected. There was no difference in SCN population density following nonhost crops at Lamberton, but sunn hemp (Crotalaria juncea L.) resulted in lower SCN population density than corn (Zea mays L.) at Waseca. Chlorosis rating and colonization frequency of soybean roots by MF were lowest following canola (Brassica napus L.) and sugar beet (Beta vulgaris L.), the two nonmycorrhizal crops. Soybean had lowest number and weight of N‐fixing nodules following soybean, highest following corn, and intermediate following other crops. Iron‐deficiency chlorosis was positively correlated with MF and negatively correlated with nodulation and soil Fe at both sites. The correlations of SCN, other soil elements, and soil penetration resistance with IDC were either insignificant at both sites or inconsistent across the sites. No consistent correlation of soil factors with MF was detected. This study suggests that a causal relationship between MF and IDC might exist, but it requires further exploration.

Influence of Winter Annual Weed Management and Crop Rotation on Soybean Cyst Nematode (Heterodera glycines) and Winter Annual Weeds: Years Four and Five

Certain winter annual weeds have been documented as alternative hosts to soybean cyst nematode (SCN), and infestations by such species are common in no-till production fields in the midwestern United States of Indiana, Ohio, and Illinois. The objective of this research was to determine the influence of crop rotation and winter annual weed management on winter weed growth, SCN population density, and crop yield. Two crop rotations (SS and soybean–corn rotation) and six winter annual weed-management systems (autumn-applied herbicide, spring-applied herbicide, autumn + spring applied herbicides, autumn-seeded Italian ryegrass, autumn-seeded wheat, and a nontreated check) were evaluated in long-term, no-tillage systems at West Lafayette, IN, and Vincennes, IN. In the fourth and fifth years of these experiments, the 2-yr corn–soybean rotation generally resulted in increased soybean yield, decreased winter annual weed growth, and reduced SCN population density compared with SS. Autumn or spring herbicide applications or both were a more effective option than cover crops at reducing winter annual weed density. Cover-crop systems generally did not differ from the nontreated check in winter weed density. Between years three and five, winter annual weed SCN hosts in nontreated check plots increased approximately threefold to levels as high as 102 and 245 plants m−2 at West Lafayette, IN, and Vincennes, IN, respectively, which are infestation levels at or above those commonly observed in production fields. However, controlling winter annual weeds did not influence crop yields or SCN population density. The results of these studies suggest that winter weed management, even at the high levels of weed infestation present in these studies, appears to have little value as a tool for SCN management in corn and soybean production systems in the midwestern United States.

Soil Suppressiveness Against the Disease Complex of the Soybean Cyst Nematode and Sudden Death Syndrome of Soybean

The ecology of the complex of soybean cyst nematode (SCN) and sudden death syndrome (SDS) of soybean was investigated under soybean monoculture in two field experiments from 2003 to 2007. Initially, susceptible soybean 'Spencer' was planted while inoculating Fusarium virguliforme into nonfumigated or preseason-fumigated plots (methyl bromide, MB, at 450 kg/ha), and SCN and SDS were monitored. In one field, SCN population densities declined in nonfumigated but increased in fumigated plots. After years of limited SDS in 2003 and 2004, SDS developed later in nonfumigated than fumigated plots. In 2006 in the greenhouse, nondisturbed or disturbed soil cores (10-cm diameter, 30-cm depth) from field plots received two two-level factors: (i) nonfumigated or fumigated (1,070 kg/ha MB); and (ii) noninoculated or inoculated with 9,000 second-stage juveniles of SCN. At harvest, nonfumigated cores from nonfumigated plots had fewer nematodes and less SDS regardless of disturbance or inoculation than the corresponding fumigated cores and any cores from fumigated plots. In the second field, SCN became detectable after 2003 during the monoculture in nonfumigated plots and lagged in fumigated plots; both treatments had low levels of SDS. Exploiting the suppressiveness of the first field could allow for biological control of SDS and SCN in soybean production.

Influence of Winter Annual Weed Removal Timings on Soybean Cyst Nematode Population Density and Plant Biomass

Soybean cyst nematode (SCN) is one of the most yield limiting pathogens in U.S. soybean production. Henbit and purple deadnettle are winter annual weeds shown to facilitate SCN reproduction after crop harvest in the eastern Corn Belt. These weeds, along with volunteer soybean that germinates in autumn after harvest, are common to postharvest soybean production fields and provide an opportunity for SCN reproduction and population increase outside of the typical soybean production season. The objective of this experiment was to determine if autumn removal of these weeds and volunteer soybean can influence the winter weed seedbank, plant biomass, and SCN population densities. Microplots were established with or without Lamium spp. and volunteer soybean, and four winter weed removal timings (none, October, December, and May). Dry weights of autumn Lamium spp. were reduced 50% in October when grown in competition with volunteer soybean. SCN juveniles were found in henbit roots at higher densities in October (42 per gram of root) than December (5 per gram of root) and were also found in the roots of volunteer soybean (14 per gram of root) in October. SCN egg population densities were 50% lower in August after the summer fallow period. The results of this experiment suggest that autumn removal of winter annual weeds and volunteer soybean did not reduce SCN populations.

Plant Growth and Soybean Cyst Nematode Response to Purple Deadnettle (Lamium purpureum), Annual Ryegrass, and Soybean Combinations

A greenhouse study was conducted to determine the influence of soybean cyst nematode (SCN) –susceptible and –resistant plant combinations on SCN population densities and plant growth. Purple deadnettle, annual ryegrass, SCN-resistant and -susceptible soybean were planted in pots alone or in combination at one plant pot−1. Annual ryegrass and purple deadnettle reduced soybean growth. Pots with SCN-resistant plants had lower numbers of SCN cysts and eggs than pots with SCN-susceptible plants. However, an SCN-susceptible species grown with any of the SCN-resistant plants resulted in higher cyst counts than pots with only SCN-resistant plants. From an SCN management standpoint, this research suggests that there may be no incentive to using annual ryegrass as a cover crop over planting other SCN-resistant crops to reduce SCN population density.

Short- and long-term tillage effects on Heterodera glycines reproduction in soybean monoculture in west Tennessee

Soybean cyst nematode, Heterodera glycines has been documented as the pathogen most consistently causing yield loss in US soybean production fields almost since its discovery in the US. No-tillage has been adopted in much of the soybean production areas to preserve soil and nutrients. The impact of this production practice change on soybean cyst nematode has been evaluated since the introduction of the practice over thirty years ago and mixed results on the impact have been recorded. This study was initiated to determine if tillage practices and timing of changes impacted soybean cyst nematode reproduction. The study was imposed on a long-term tillage study and treatments were changed to measure short-term as well as long-term effects of tillage on soybean cyst nematode. Significant differences in soybean cyst nematode population density were found between treatments. However, significant differences found could be attributed to short-term changes in tilled and no-tilled treatments. Soybean cyst nematode reproduction was almost twice as high in treatments which changed from no-tillage to disc than tilled treatments changed to no-tillage. No significant differences were found in yield among the treatments. Grain yield was reflective of cultivar grown and environmental conditions.

Soybean Cyst Nematode, Heterodera glycines, Populations Adapting to Resistant Soybean Cultivars in Kentucky.

Soybean cyst nematode (SCN), Heterodera glycines, is the most limiting biotic factor of soybean (Glycine max) production in Kentucky (KY). Unpublished results of a survey of commercial soybean fields in KY in the late 1980s indicated that H. glycines (HG) type 0 (race 3) was the most common HG type in the state. HG type 0 populations cannot reproduce (female index [FI] <10% compared with reproduction on a standard susceptible cultivar) on PI88788, which is the basis of H. glycines resistance in >90% of the soybean cultivars grown in the United States. Recent reports from Illinois (4), Missouri (3), and North Carolina (2) indicate that most populations of H. glycines in those states are now able to reproduce on resistant soybean cultivars derived from PI88788. Because cultivars derived from PI88788 are grown almost exclusively in KY to manage H. glycines, a limited survey was needed to update information on H. glycines populations. Most soybean fields in KY are grown in a 1-year rotation with corn (Zea mays), a nonhost crop for H. glycines. Therefore, the survey targeted fields that had most recently been in corn. Otherwise, fields were arbitrarily selected for sampling. Composite samples were collected in the fall of 2006 or the spring of 2007 and consisted of 20 soil cores (10 to 15 cm deep × 2.5 cm in diameter) collected following a zigzag pattern. Samples were mixed and stored at 4°C until processing. H. glycines cysts were extracted from soil by a sucrose centrifugation and flotation technique (1). Eggs were liberated by crushing cysts caught on a sieve (250-μm-diameter pores) with a rubber stopper. Liberated eggs were then collected, stained with acid fuchsin, and counted. Up to three samples from each county surveyed were sent to the University of Missouri Nematology Laboratory, Columbia, MO for HG type testing (3). A total of 139 samples, representing 19 major soybean-production counties in KY, were analyzed for H. glycines levels. H. glycines eggs were recovered from 106 (76%) samples and 16 (84%) counties. SCN population densities ranged from 38 to 4,275 eggs per 250 cm3 of soil. HG type tests were conducted on 20 populations from eight counties. HG types 1.2.5.7 (race 2) and 2.5.7 (race 1 or 5) were identified, with HG type 2.5.7 (race 1) being the most common (60% of populations screened). No HG type 0 populations were detected. All populations tested had a FI ≥10% on three of nine indicator lines (PI88788, PI209322, and PI548316). FIs on these indicators ranged from 15 to 80, 11 to 81, and 23 to 88%, respectively. Sixty percent of populations tested had FIs ≥30% on PI88788. Of the populations screened, 25 and 35% had FIs ≥10% on PI548402 (Peking) and Pickett, respectively. All populations tested had FIs = 0 on PI437654. Survey results indicate that cultivars deriving their H. glycines resistance from PI88788 may have reduced effectiveness in suppressing current H. glycines populations in KY. Consequently, producers may need to grow soybean cultivars derived from non-PI88788 resistance sources to successfully manage H. glycines in the future.