Chemical-Mediated Toxicity of N-Viro Soil to Heterodera glycines and Meloidogyne incognita

N-Viro Soil (NVS) is an alkaline-stabilized municipal biosolid that has been shown to lower population densities and reduce egg hatch of Heterodera glycines and other plant-parasitic nematodes; but the mechanism(s) of nematode suppression of this soil amendment are unknown. This study sought to identify NVS-mediated changes in soil chemical properties and their impact upon H. glycines and Meloidogyne incognita mortality. N-Viro Soil was applied to sand in laboratory assays at 0.5%, 1.0%, 2.0%, and 3.0% dry w/w with a nonamended treatment as a control. Nematode mortality and changes in sand-assay chemical properties were determined 24 hours after incubation. Calculated lethal concentration (LC₉₀) values were 1.4% w/w NVS for second-stage juveniles of both nematode species and 2.6 and >3.0% w/w NVS for eggs of M. incognita and H. glycines, respectively. Increasing rates of NVS were strongly correlated (r² = 0.84) with higher sand solution pH levels. Sand solution pH levels and, to a lesser extent, the production of ammonia appeared to be the inorganic chemical-mediated factors responsible for killing plant-parasitic nematodes following amendment with NVS.     

Factors Affecting the Suppression of Heterodera glycines by N-Viro Soil

Previous laboratory research demonstrated that N-Viro Soil (NVS), an alkaline-stabilized municipal biosolid, suppressed plant-parasitic nematodes. This study continued to explore the use of NVS as a nematode management tool specifically addressing factors that could influence its use. N-Viro Soil from different locations, the components of NVS (de-watered biosolids and fly ash admixtures), and sterilized NVS were applied to sand microcosms to determine effects on nematode survival sand solution pH and ammonia concentrations. This study confirmed the previous finding that an important mechanism of Heterodera glycines suppression by NVS was the generation of alkaline soil conditions. Only the fly ash admixture that resulted in an increase in pH to 10.0 suppressed H. glycines to the same level as NVS. Alkaline-stabilization of biosolids was necessary to achieve nematode suppression. Biosolids applied at rates <3% dry w/w did not suppress H. glycines to the same level as equivalent amounts of NVS. Sand solution pH levels after biosolid application, regardless of rate, were approximately 8.5 whereas 1% and 4% w/w NVS amendment resulted in pH levels of 10.3 and 11.6, respectively. NVS from different processing facilities were all effective in suppressing H. glycines. The NVS source that produced the highest concentration of ammonia did not reduce H. glycines survival to the same level as those sources generating pH levels above 10.1. Microbes associated with NVS appeared not to be responsible for the nematode suppressiveness of the amendment; there was no difference in nematode suppression between autoclaved and nonautoclaved NVS. The role that ammonia plays in the suppression of H. glycines by NVS is still unclear.     

Multiple Pest Interactions in Soybean: Effects on Heterodera glycines Egg Populations and Crop Yield

Population changes of Heterodera glycines eggs on soybean in small field plots were influenced by the lepidopterous insect pest, Helicoverpa zea; however, few effects on eggs due to the presence of annual weeds were detected. Soybeans defoliated 15-35% by H. zea during August remained green and continued to produce new flowers and pods later into the season than soybeans without H. zea, resulting in higher numbers of H. glycines eggs at harvest on insect-defoliated soybeans. Final H. glycines populations also were influenced by soil population density (Pi) of the nematode at planting. Fecundity of H. glycines was generally greater at the undetected and low Pi than at high Pi levels. Soybean yields were suppressed 12, 22, and 30% by low, moderate, and high H. glycines Pi, respectively. When weed competition and H. zea feeding damage effects were added, yields were suppressed 34, 40, and 57% by the three respective nematode Pi levels. Effects among the three pests on soybean yield were primarily additive.     

Temperature Effects on Race Determination in Heterodera glycines

Currently there are 16 possible races for Heterodera glycines, and these are differentiated based on ability of a nematode population to develop on a set of four differential soybean genotypes. Because results are based on numbers of nematode females that develop to a specific stage rather than on the reproductive capability of these females, race determinations based on female indices may not represent results obtained after several reproductive cycles of H. glycines. Counting numbers of eggs and juveniles, and then developing corresponding indices, would allow reproduction to be considered in making race determinations. Our objectives were to compare the present race identification scheme for H. glycines based on female indices with those using egg and juvenile indices and to examine the effect of temperature on race designations using female, egg, and juvenile indices. Race designations for H. glycines populations from two locations in Illinois were determined at 20, 27, and 30 °C in a water bath. The numbers of females, eggs, and juveniles (at 19 days) were recorded, and an index based on each life stage was calculated. Race determinations based on female, egg, or juvenile indices were inconsistent when conducted at 20 °C, which demonstrates that this temperature is not suitable for identifying races of H. glycines. However race designations at 27 and 30 °C were consistent for all three indices. This indicates that counting females, eggs, or juveniles should be equally reliable when race determinations are conducted at these two temperatures, and choice of method would depend on investigator preference or research objective.     

Dormancy of Heterodera glycines in Missouri

A 2-year study was conducted in field microplots to determine the relative importance of soybean phenology and soil temperature on induction of dormancy in Heterodera glycines in Missouri. Four near-isogenic soybean lines differing for maturity date were planted in microplots infested with a race 5 isolate of H. glycines. Soil temperature was monitored at a depth of 15 cm. Eggs of H. glycines, extracted from cysts collected monthly from each microplot, were used in hatching tests and bioassays to determine dormancy. Egg hatching and second-stage juvenile (J2) infectivity rates decreased sharply from their highest levels in midsummer (July-August) to a low level by October of each year and remained low (< 10% hatching and < 0.2 J2/cm root) until May or June of the following year. The patterns of numbers of females and eggs in the bioassays were similar. The decreases were not related to soil temperature and did not differ consistently among soybean isolines. The monophasic changes in all nematode responses with peak midsummer rates suggest that H. glycines produces one primary generation per year in central Missouri. Changes in hatching rates and the timing of minimum and maximum rates suggested that H. glycines eggs exhibit more than one type of dormancy.     

Development of Heterodera glycines as Affected by Fusarium solani,the Causal Agent of Sudden Death Syndrome of Soybean

The effects of the blue form of Fusarium solani, the causal agent of sudden death syndrome (SDS), on Heterodera glycines were examined in the greenhouse. Roots of soybean cv. Coker 156 were inoculated with either H. glycines alone or F. solani + H. glycines in combination. Population levels of H. glycines were reduced 47% in the presence of F. solani. Life-stage development of H. glycines increased 3% in 30 days in the presence of F. solani. Fusarium solani colonized epidermal and cortical cells adjacent to developing juveniles of H. glycines and the nematode-induced syncytia within the soybean root tissue. At 40 days after inoculation, F. solani was isolated from 37% of the cysts in soil recovered from the F. solani + H. glycines combination treatment. Fusarium solani significantly affected H. glycines population density, life-stage development, and succeeding populations.     

Effect of Rotation Crops on Heterodera glycines Population Density in a Greenhouse Screening Study

Crop rotation is a common means of reducing pathogen populations in soil. Several rotation crops have been shown to reduce soybean cyst nematode (Heterodera glycines) populations, but a comprehensive study of the optimal crops is needed. A greenhouse study was conducted to determine the effect of growth and decomposition of 46 crops on population density of H. glycines. Crops were sown in soil infested with H. glycines. Plants were maintained until 75 days after planting, when the soil was mixed, a sample of the soil removed to determine egg density, and shoots and roots chopped and mixed into the soil. After 56 days, soil samples were again taken for egg counts, and a susceptible soybean (‘Sturdy’) was planted in the soil as a bioassay to determine egg viability. Sunn hemp (Crotalaria juncea), forage pea (Pisum sativum), lab-lab bean (Lablab purpureus), Illinois bundleflower (Desman-thus illinoensis), and alfalfa (Medicago sativa) generally resulted in smaller egg population density in soil or number of cysts formed on soybean in the bioassay than the fallow control. Sunn hemp most consistently showed the lowest numbers of eggs and cysts. As a group, legumes resulted in lower egg population densities than monocots, Brassica species, and other dicots.     

Susceptibility of Soybean Introductions to Races 1, 2, 3, and 4 of Heterodera glycines

Thirteen soybean plant introduction (PI) lines, selected for their apparent susceptibility to Heterodera glycines, were compared with cultivar Lee 74 as hosts of H. glycines races 1, 2, 3, and 4. Race 3 produced the highest average number of females of the four races. Compared to Lee 74, more (P = 0.05) females of H. glycines race 1 were extracted from eI 274420, PI 274423, and PI 317333; PI 86457 had more females of H. glycines race 2; and PI 86443, PI 86457, PI 261467, PI 274420, PI 274421, and PI 274423 had more females of H. glycines race 3. Similar numbers of females of H. glycines race 4 developed on all of the soybean lines and Lee 74. PI 274421, PI 274420, or PI 196159 could provide a more or equally susceptible host for H. glycines races 1, 2, 3, and 4 than Lee 74. One of these three lines could be substituted for Lee as the standard susceptible cultivar in the race determination test.     

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

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

Multivariate Analysis of Selected Edaphic Factors and Their Relationship to Heterodera glycines Population Density

The influence of selected soil physical and chemical factors on population density of Heterodera glycines was investigated in 1988 and 1989 in two different locations of a soybean (Glycine max) field. Soil variables of a Norfolk loamy sand were measured after planting soybeans susceptible to H. glycines. Cyst and egg populations were determined after harvest. Nematode population density was found to be clustered. Up to 91% of the eggs were parasitized by a sterile fungus. Principal component analysis with orthogonal VARIMAX rotation grouped 12 variables into five uncorrelated factors in 1988 and three in 1989. In 1988, the factor "pH and Mg" was positively correlated (P < 0.001) with cyst and egg population density. Also, the factor "fine texture and Cu" was negatively correlated (P < 0.05) with egg population density. In 1989, the factor "pH, Mg and Cu" was positively correlated (P < 0.05) with levels of cysts and percentage of parasitized eggs, but not with total egg number. Across 2 years, factors containing soil pH and Mg were positively associated with cyst nematode population density. Copper appeared to be negatively associated with populations of H. glycines.     

Population Dynamics of Heterodera glycines in Conventional Tillage and No-Tillage Soybean/Corn Cropping Systems

The effects of no-tillage (NT), conventional tillage (CT), and crop rotation on soybean yield and population dynamics of Heterodera glycines were compared during a 7-year study in a silty clay loam soil with 6% organic matter. Either H. glycines-resistant ''Linford'' soybean or susceptible ''Williams 82'' soybean was rotated with corn and grown on 76-cm-wide rows in both tillage systems. Soybean was planted in 1994, 1996, 1998, 1999, and 2000. Yield of Linford was significantly greater than Williams 82 in all years. Soybean yield was affected by tillage in 1999 and 2000. No-tillage production tended to support more reproduction (R = number of eggs at harvest/number of eggs at planting) on both cultivars. The largest R for Williams 82 were in 1998: 58.35 for NT plots and 11.78 for CT plots. For Linford, the largest R were 12.09 for NT plots in 1996, and 3.71 for CT in 1999. When corn was planted, R decreased more in NT. When soybean was planted in years subsequent to 1994, numbers of eggs at harvest (Pf) were greater for Williams 82 NT than for Williams 82 CT or Linford in both tillage systems; however, crop rotation with corn negated these population increases. The soil became suppressive to H. glycines in 1999 and was suppressive in 2000. After the 3 years of continuous soybean, Pf per 250 cm[sup3] soil were 2,870 for Williams 82 NT, 791 for Williams 82 CT, 544 for Linford NT, and 990 for Linford CT in 2000, compared with Pf of 13,100 for Williams 82 NT, 15,000 for Williams CT, 2,360 for Linford NT, and 2,050 for Linford CT in 1994. Describing population dynamics solely on the basis of R was not adequate, but also required independent examination of initial populations following overwintering and Pf after the growing season. Planting soybean either NT or CT in rotation with corn did not result in long-term increases in numbers of H. glycines eggs.     

Seasonal Biochemical Changes in Eggs of Heterodera glycines in Missouri

Changes in the carbohydrate (glucose, trehalose, and glycogen) and total protein contents of eggs retained within Heterodera glycines cysts were monitored monthly in a field microplot experiment conducted from March 1993 to March 1995. Treatments included two near-isogenic lines of soybean cv. Clark differing for date of maturity, and one corn hybrid. The soybean lines were planted in microplots infested with H. glycines at a high average initial population density (Pi) (23,810 eggs/100 cm³ soil), and the corn was planted in microplots infested at high (24,640) and low (5,485) Pi. Soil temperatures at 15 cm depth and rainfall were monitored. Carbohydrate contents varied in the same pattern, with the highest levels measured before planting (May) and after harvest (October) in both years. Neither Pi nor soybean isoline had an effect on any measured response, but the carbohydrate contents of eggs from corn and soybean microplots differed during the overwinter (October-May) periods (P < 0.0001). Trehalose accumulation was negatively correlated with soil temperature (r = -0.78 and r = -0.84, P = 0.0001, July through November 1993 and 1994, respectively), which reflects its role as a cryoprotectant. In contrast to the pattern for carbohydrates, total protein was lowest before planting and after harvest, and highest (>20 μg/1,000 eggs) June through October. Protein content was unaffected by plant cultivar or species. Protein and carbohydrate levels in H. glycines eggs showed seasonal changes that appeared to be primarily temperature-dependent.     

Soybean Photosynthesis and Yield as Influenced by Heterodera glycines,Soil Type and Irrigation

The effects of soil types and soil water matric pressure on the Heterodera glycines-Glycine max interaction were examined in microplots in 1988 and 1989. Reproduction of H. glycines was restricted in fine-textured soils as compared with coarse-textured ones. Final population densities of this pathogen in both years of the study were greater in nonirrigated soils than in irrigated soils. The net photosynthetic rate of soybean (per unit area of leaf) was suppressed only slightly or not at all in response to infection by H. glycines and other stresses. Relative soybean-yield suppression in response to H. glycines was not affected by water content in fine-textured soils, but slopes of the damage functions were steepest in sand, sandy loam, and muck soils at high water content (irrigated plots). Yield restriction of soybean in response to this pathogen under irrigation was equal to or greater than the yield suppression under dry conditions. Although yield potential may be elevated by irrigation when soil-water content is inadequate, supplemental irrigation cannot be used to circumvent nematode damage to soybean.     

A Revised Classification Scheme for Genetically Diverse Populations of Heterodera glycines

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

Field Interrelationships among Heterodera glycines,Pratylenchus scribneri,and Three Other Nematode Species Associated with Soybean

Field experiments were conducted in 1982 and 1983 to assess interactions between Heterodera glycines and Pratylenchus scribneri on soybean in southern Illinois. Soybean cyst nematode susceptible cultivar Williams 79 and resistant cultivar Fayette were treated or not treated with aldicarb 15G. Initial population densities were 35 H. glycines cysts containing eggs, 100 P. scribneri, 30 Helicotylenchus pseudorobustus, 225 Paratylenchus projectus, and 85 Tylenchorhynchus martini per 250 cm³ soil in 1982, whereas in 1983 populations were 11 H. glycines cysts, 330 P. scribneri, and 620 H. pseudorobustus. In both years H. glycines populations increased on nontreated Williams 79, decreased on both treated and nontreated Fayette, and remained at initial levels on treated Williams 79. Recovery of P. scribneri per gram dry root was different between nontreated cultivars in 1982 but not in 1983. Aldicarb treatment suppressed soil and root populations of P. scribneri on both cultivars in both years. Populations of H. pseudorobustus, P. projectus, and T. martini at harvest indicated little population increase on either nontreated cultivar. In 1982 H. glycines caused yield suppression but P. scribneri did not, as differences in yield occurred between cultivars but not between aldicarb treatments. In 1983, however, there were no yield differences between cultivars, but aldicarb application resulted in yield increase in both cultivars. In 1983 the yield increase resulting from P. scribneri control was approximately 25%. No synergistic effect on yield was observed between H. glycines and P. scribneri.     

Overwinter Population Dynamics of Heterodera glycines

The purpose of this research was to compare the overwinter survival of populations of Heterodera glycines at different latitudes in the United States and the effect of changing latitudes before and after the initiation of dormancy. Soil samples infested with H. glycines were collected in August or October in 1992 to 1994 from soybean fields in two to four states (combinations of Arkansas, Florida, Minnesota, Missouri, and Wisconsin). The samples were mixed thoroughly, divided into subsamples, shipped to an overwinter location, and buried until time for processing. To determine survival, cysts, eggs, and second-stage juveniles were extracted from replicated subsamples and counted each month from December to May. Survival generally was between 50% and 100%, and often was best in the state of origin. In Florida, survival was at the 50 to 100% level in soil from most locations, and in Wisconsin was near 100%. Survival of H. glycines in Arkansas and Missouri varied more than at the other locations. In a separate test, survival in microplots in Arkansas, in a more natural environment than that of buried samples, was 70 to 94% for field isolates from Arkansas, Minnesota, and Missouri and 100% for isolates of races 1, 3, and 14 that had been maintained in a greenhouse for several years. Survival appears to be better than previous tests had indicated. High survival rates require cultivars with high levels of resistance and long-term rotations for management.     

Relationship Between Time of Infection with Heterodera glycines and Soybean Yield

Experiments were conducted to determine the relationship between time of infection by Heterodera glycines and soybean growth in the greenhouse and yield of plants grown in the field. Soybean cultivar Essex seedlings growing in the greenhouse were inoculated with H. glycines at 2, 4, or 6 weeks after planting. Seedling growth was inhibited by H. glycines infection at 2 or 4 weeks after planting but not at 6 weeks. Infection of Essex by H. glycines in the field was delayed 2-6 weeks by nematicides. Yields were significantly increased when H. glycines infection was delayed 2 weeks by nematicide treatment. Essex yields were highest when infection was delayed 6 weeks, equalling the yield of the H. glycines-resistant cultivar Asgrow 5474. The effect of H. glycines on soybean growth in the greenhouse and yields in the field declined when infection was delayed 6 weeks. Thus, soybean sensitivity to H. glycines seemed to diminish with age of the soybean plants.     

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 ZnSO₄ solution. Filtering RES with a 0.22-μm-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.     

Distribution of Races of Heterodera glycines in the Central United States

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

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 cm³ 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.