Soybean Cyst Nematode Reduces Soybean Yield Without Causing Obvious Aboveground Symptoms

Field experiments were conducted at locations in northern and southern Illinois, central Iowa, and central Missouri from 1997 to 1999 to investigate the effects of Heterodera glycines on soybean growth, development, and yield. A wide range of infestation levels was present at all locations. Two locally adapted cultivars, one resistant to H. glycines, were grown at each location. Cultivars were planted in alternating four-row strips with 76 cm between rows. For each cultivar, 20 1-m-long single-row plots were sampled every 2 weeks starting 4 weeks after planting. Infection by H. glycines reduced plant height and leaf and stem weight on the resistant cultivars in the first 12 weeks after planting, and delayed pod and seed development 12 to 14 weeks after planting. Biomass accumulation was not reduced on the susceptible cultivars until 10 weeks after planting; reduction in pod and seed development occurred throughout the reproductive stages. Susceptible cultivars produced significantly lower yields than resistant cultivars, but the yield reductions were not accompanied by visually detectable symptoms.

A chorismate mutase from the soybean cyst nematode Heterodera glycines shows polymorphisms that correlate with virulence

Parasitism genes from phytoparasitic nematodes are thought to be essential for nematode invasion of the host plant, to help the nematode establish feeding sites, and to aid nematodes in the suppression of host plant defenses. One gene that may play several roles in nematode parasitism is chorismate mutase (CM). This secreted enzyme is produced in the nematode's esophageal glands and appears to function within the plant cell to manipulate the plant's shikimate pathway, which controls plant cell growth, development, structure, and pathogen defense. Using degenerate polymerase chain reaction primers, we amplified and cloned a chorismate mutase (Hg-cm-1) from Heterodera glycines, the soybean cyst nematode (SCN), and showed it had CM activity. RNA in situ hybridization of Hg-cm-1 cDNA to SCN sections confirms that it is specifically expressed in the nematodes' esophageal glands. DNA gel blots of genomic DNA isolated from SCN inbred lines that have differing virulence on SCN resistant soybean show Hg-cm-1 is a member of a polymorphic gene family. Some Hg-cm family members predominate in SCN inbred lines that are virulent on certain SCN resistant soybean cultivars. The same polymorphisms and correlation with virulence are seen in the Hg-cm-1 expressed in the SCN second-stage juveniles. Based on the enzymatic activity of Hg-cm-1 and the observation that different forms of the mutase are expressed in virulent nematodes, we hypothesize that the Hg-cm-1 is a virulence gene, some forms of which allow SCN to parasitize certain resistant soybean plants.

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.

Lack of Predictable Race Shift in Heterodera glycines-Infested Field Plots

Soybean cultivars with different sources of resistance to Heterodera glycines were grown at three locations initially infested with races 2, 3, and 6 in order to investigate H. glycines race shift in field populations. Each spring and fall, soil samples were taken from each plot and race tests were conducted to evaluate effects of cultivar and time of sampling. Field experiments were paired field plots rotated annually with corn since 1991. Cultivars included at the northern and central Missouri sites were Williams 82 (susceptible to H. glycines), Linford (PI 88788 source of resistance), MFA 9043 (Peking) replaced by Morsoy 9345 (Peking and PI 88788) from 1995 to 1997, and Jackson II (Peking + PI 88788) replaced by Asgrow 3431 (Peking and PI 88788) in 1996-97. Cultivars at the southern Missouri site were Essex or Hutcheson (susceptible to H. glycines), Forrest (Peking), Hartwig (PI 437654), and Rhodes (PI 88788 + Peking). In 1995, race tests were performed at four temperature regimes to determine temperature effects on race designations. Race shifts were not predictable based on the source of resistance of the soybean cultivar planted. Variability in female numbers on Lee 74 among tests caused changes in female indices (FI). Furthermore, race designations were influenced by the time of sampling and temperature at which the race tests were conducted. The variability of H. glycines populations in both field and greenhouse situations diminishes the value of race test results when making cultivar recommendations.

Is Change in Electrical Potential or pH a Hatching Signal for Heterodera glycines?

ABSTRACT This study explored the possibilities that changes in the egg shell/lipid layer electrical potential or pH communicate external hatching conditions to the Heterodera glycines second-stage juvenile (J2) within the mature egg and that electrophysiology could measure effects of chemicals on emergence. Potentials were measured following application of the emergence inducers (ZnSO(4) and ZnCl(2)), ions that do not affect emergence, or synthetic emergence inhibitors. Results were compared with pH measurements and emergence bioassays. Healthy appearing eggs had negative resting potentials. Application of ZnSO(4) caused a smooth depolarization. However, eggs containing J2 and immature eggs depolarized to a similar degree when ZnSO(4) was added. In addition, ZnSO(4), synthetic emergence inhibitors, and CaCl(2) caused similar depolarization, and some depolarization was measured in dye-permeable eggs and empty shells. Results suggest that change in cation surface charge contributed to depolarization and that Cl penetrated the egg shell/lipid layer without causing potential changes. In bioassays, zinc consistently stimulated emergence to a greater degree than H(2)O, other cations, or buffers, and counteracted emergence inhibitors. Zinc-caused emergence stimulation was independent of pH. In summary, it is concluded that depolarization and pH are not emergence signals and electrophysiology is unlikely to measure effectiveness of emergence stimulators or inhibitors.

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.

Soybean Cyst Nematode Reproduction in the North Central United States

An experiment was conducted in Heterodera glycines-infested fields in 40 north central U.S. environments (21 sites in 1994 and 19 sites in 1995) to assess reproduction of this nematode. Two resistant and two susceptible soybean cultivars from each of the maturity groups (MG) I through IV were grown at each site in 6.1 m by 4 row plots. Soil samples were collected from each plot at planting and harvest and processed at Iowa State University to determine H. glycines initial (Pi) and final (Pf) population densities as eggs per 100 cm³ of soil. Overall, reproduction (Pf/Pi) of H. glycines on susceptible cultivars in all MG was similar. Reproduction was higher on MG III and IV susceptible cultivars than on those in MG I and II. Resistant MG I and II cultivars reduced nematode population densities more consistently than those in MG III and IV. Reproduction of the nematode was similar among sites within the same maturity zone (MZ), defined as the areas of best adaptation of the corresponding MG. Nonetheless, careful monitoring of nematode population densities is necessary to assess changes that occur over time in individual fields.

Infection by Heterodera glycines Elevates Isoflavonoid Production and Influences Soybean Nodulation

High-performance liquid chromatography and Sinorhizobium fredii USDA191 nodC-lacZ gene fusion were used to monitor changes in the isoflavonoid content of soybean roots infected with Heterodera glycines isolate TN1. Isoflavonoid concentrations in infected roots of both H. glycines-resistant Hartwig and susceptible Essex soybean were two to four-fold higher than those of uninfected roots 2 and 3 days after inoculation. The isoflavonoids produced activated the transcription of nodC-lacZ fusion. The most abundant isoflavonoids were identified as daidzein and genistein by HPLC and GC/MS. Heterodera glycines increased the number of nodules formed on Essex roots inoculated with B. japonicum (USDA110) but reduced shoot weight and decreased the net nitrogenase activity of the nodules. Heterodera glycines infection of resistant Hartwig did not affect the total number of nodules or their nitrogen- fixing capacity.

Unique Immunogenic Proteins in Heterodera glycines Eggshells

Polyclonal antibodies were raised against Heterodera glycines eggshells to determine the feasibility of developing an immunoassay for H. glycines eggs. An indirect enzyme-linked immunosorbent assay (ELISA) was developed from anfisera collected 10 weeks after the initial injection. From serial dilutions of sonicated eggshells or whole eggs, a sensitivity of detection to 5 ng/ml sonicated eggshells or 1 egg of H. glycines was determined. The method of eggshell preparation had no effect on the antibodies produced; however, the antibodies cross-reacted with sonicated J2 of H. glycines and eggs of Meloidogyne incognita and H. schachtii. Most of the proteins in both life stages of H. glycines and eggs of M. incognita and H. schachtii had similar migration properties when separated on SDS-PAGE gels and stained with Coomassie blue. Western blot analysis, with antisera adsorbed with homogenized J2 of H. glycines, showed proteins that were specifically localized to eggshells of H. glycines. Monoclonal antibodies might provide a useful immunoassay where polyclonal antibodies lack sufficient specificity.

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 cm(3) 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 mug/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.

Effects of Zinc Fertilization of Corn on Hatching of Heterodera glycines in Soil

Experiments were conducted to determine the effects of zinc fertilizers on hatching and soil population densities of Heterodera glycines. In vitro egg hatching in solutions of reagent-grade zinc sulfate and zinc chloride and fertilizer-grade zinc sulfate was significantly greater than hatching in deionized water, whereas zinc chelate fertilizer significantly inhibited egg hatching relative to deionized water. In greenhouse experiments, no differences in cumulative percentage egg hatch were detected in soil naturally infested with H. glycines amended with fertilizer-grade zinc sulfate and zinc chelate at rates equivalent to 0, 1.12, 11.2, and 112 kg Zn/ha and subsequently planted with corn (Zea mays L.). In a field experiment, no significant differences in H. glycines egg population densities and corn yields were detected among plots fertilized with 0, 11.2, and 22.4 kg Zn/ha rates of zinc chelate. Yields of H. glycines-susceptible soybean planted in plots 1 year after zinc fertilization of corn plots also were not significantly affected. Zinc compounds significandy affected H. glycines egg hatching in vitro, but had no effect on hatching in natural soils.

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.

Distinguishing between Viable and Nonviable Heterodera glycines Eggs

Greenhouse tests were conducted to determine the persistence of nonviable Heterodera glycines eggs in soil. Laboratory and greenhouse tests were conducted to determine the usefulness of the vital stains acridine orange and tetrazolium red for differentiating viable and nonviable eggs of H. glycines. Nonviable (boiled) egg preparations were compared with preparations that were not boiled for their persistence in methyl bromide-fumigated soil. Boiled eggs persisted longer (>200 days) than untreated eggs, perhaps due to disinfestation of the egg suspensions by boiling. Neither stain was a good indicator of egg viability as inferred from infectivity of juveniles in a bioassay of the same egg preparations exposed to the stains.

Phytoparasitic Nematode Populations in Festuca arundinacea Field Plots in Southwestern Missouri

Field plots of tall rescue (Festuca arundinacea) at two locations on the same experimental farm in southwestern Missouri were sampled (one in 1987-88, the other in 1988-89) to inventory root-parasitic nematodes and to determine whether cultivars or endophyte (Acremonium coenophialum) infection frequencies (EIF) affected nematode population densities within single growing seasons. Plots were planted with seven tall rescue cultivars: Kentucky-31, Kenhy, Johnstone, Martin, Mozark, Missouri-96, and Forager. Kentucky-31 seed with high and low EIF were planted in separate plots. Plant-parasitic nematodes were extracted from soil samples, identified to genus, and enumerated four and three times per year for the 1987-1988 and 1988-1989 studies, respectively. Several plant-parasitic genera were identified from both fields, including Helicotylenchus, Heterodera, Hoplolaimus, Paratylenchus, Pratylenchus, Tylenchorhynchus, and members of genera grouped in the family Tylenchidae. Densities of five of these seven groups of nematodes differed among tall fescue cultivars in the 1987-88 study, but only two out of eight groups did so in the 1988-89 study. Irrespective of tall rescue cultivar, EIF had no consistent impact on nematode densities. The putative suppressive effect of endophyte infection on infection by plant-parasitic nematodes is not detectable within single growing seasons and deserves long-term study in field situations.

Distribution, Density, and Diversity of Heterodera glycines in Missouri

Between May, 1988, and July, 1992, the University of Missouri Plant Nematode Diagnostic Laboratory collected data on Heterodera glycines populations in Missouri by offering diagnostic services to soybean farmers. A greenhouse bioassay (free), egg count ($10/sample), and race determination test ($25) were conducted by request on soil samples submitted to the lab. Each test was offered for a specific purpose: the bioassay for fields not known to be infested; the egg count for population monitoring in fields with known infestations; and the race test for fields with a history of resistant cultivars. Of 818 samples submitted for bioassay on a H. glycines-susceptible soybean cultivar, 13 (1.6%) contained brown cysts but no white females, and 364 (45%) contained white females after 35 days in the greenhouse. Of 6,193 egg counts, 39% were either free of H. glycines or contained fewer than 500 eggs/250 cm(3) soil, the action threshold for Missouri. The remaining 61% ranged from 500-400,000 eggs/250 cm(3) soil (mean = 10,617). Eleven races were detected, with races 3 (45%) and 1 (23%) the most common. The data show that H. glycines is widespread in Missouri (with confirmed infestations in 80 of 114 counties), that most infested fields have population densities above the action threshold, and that there is considerable genetic diversity among H. glycines field populations.

Flow Cytometric Analysis and Sorting of Heterodera glycines Eggs

A nondestructive technique was developed to characterize and separate eggs of soybean cyst nematode, Heterodera glycines, by developmental stage using flow cytometry. Eggs from cysts cultured on susceptible soybean roots were suspended in 0.1% xanthan gum or 59% sucrose and loaded into either a Coulter EPICS 752 or EPICS 753 flow cytometer. Eggs were analyzed and sorted according to forward angle and 90 degrees light scatter, flow cytometric parameters that are relative measures of object size and granularity, respectively. Mature eggs containing vermiform juveniles were less granular and slightly larger than eggs in earlier stages of embryogeny, allowing for separation of mature eggs from immature eggs. The effectiveness of flow cytometric sorting was evaluated by comparing the developmental stages of subpopulations of unsorted and sorted eggs. Of a subpopulation of unsorted eggs, 62% contained vermiform juveniles, whereas 85 to 95% of sorted subpopulations of larger, less granular eggs contained vermiform juveniles. Suspending H. glycines eggs in 0.1% xanthan gum or 59% sucrose for flow cytometric analysis had no effect on subsequent egg hatch in vitro. This technique is an efficient and effective means to collect large, relatively homogeneous quantities of H. glycines eggs in early or late embryogeny, and would likely be useful for analyzing and sorting eggs of other nematode species for use in developmental, genetic, or physiological research, or for identification and collection of parasitized eggs.

Pratylenchus, paratylenchus, helicotylenchus, and other nematodes on soybean in missouri

Eighteen species in eight genera of plant-parasitic nematodes were identified from soil samples collected from soybean fields throughout Missouri. The genera represented were Helicotylenchus, Heterodera, Hoplolaimus, Meloidogyne, Paratylenchus, Pratylenchus, Tylenchorhynchus, and Xiphinema. Three fields, each with high densities of Helicotylenchus pseudorobustus, Pratylenchus hexincisus, or Paratylenchus projectus, were planted in 1989 with six soybean cultivars, with plots of each cultivar either not treated or treated with 5.43 kg/ha aldicarb, to determine whether economically important relationships existed. In none of the sites were nematode densities affected by either aldicarb treatment or cultivar, nor were seed yields related to nematode densities; however, mean seed yield was significantly lower in the P. projectus site. In 1990, seed yield was negatively correlated (r = -0.34, P < 0.05) with P. projectus density at planting. Based on the present and previous studies, H. pseudorobustus and P. hexincisus do not appear to be of economic interest on soybean, but P. projectus probably deserves more study.

Survey of Plant-parasitic Nematodes in Missouri Cotton Fields

During September 1990, 30 cotton fields in each of three Missouri counties were surveyed for plant-parasitic nematodes. Soil samples for nematode analysis consisted of a composite of 20 cores collected in a zig-zag pattern within a 1-ha block in each field. Cores were taken from within weed-free cotton rows. Nine genera of plant-parasitic nematodes were found (Rotylenchulus, Helicotylenchus, Hoplolaimus, Meloidogyne, Paratylenchus, Pratylenchus, Tylenchorhynchus, Heterodera, and Trichodorus), and five species were identified: Meloidogyne incognita, Rotylenchulus reniformis, Hoplolaimus galeatus, Pratylenchus vulnus, and P. scribneri. This is the first report of R. reniformis, H. galeatus, P. vulnus, and P. scribneri in Missouri cotton fields and the first report of R. reniformis and P. vulnus in Missouri. The known cotton pathogens M. incognita, R. reniformis, and H. galeatus were found in 30%, 3%, and 2% of the fields sampled, respectively. The correlation between sand content of the soil sample and the number of vermiform M. incognita in the sample was not significant, with r(2) = 0.13. Select fields where H. galeatus and R. reniformis were found in 1990 were sampled more intensely in 1991. The 1-ha block sampled in 1990 was sampled in 1991, along with three other 1-ha blocks uniformly distributed within the field. In addition, a 1-ha block was sampled in each of eight nearby fields, within 2 km of the first field. The nine plant-parasitic nematode genera identified in the 1990 survey were observed again in 1991. Within-field distribution of M. incognita, R. reniformis, and H. galeatus was not uniform. When M. incognita, R. reniformis, or H. galeatus were present in a field, the same species was found in 38%, 25%, or 50% of nearby fields, respectively.

Response of Soybean Cultivars to Aldicarb in Heterodera glycines-infested Soils in Missouri

Soybean cultivars in maturity groups III, IV, and V that were resistant or susceptible to Heterodera glycines were evaluated for their response to in-furrow applications (5.43 kg a.i./ha) of aldicarb. Tests were conducted over three years (1988-1990) in various H. glycines-infested locations in Missouri for a total of 16 environments. In 7 environments, overall yields were higher (P </= 0.05) in aldicarb-treated than in nontreated plots. In one environment, soybean yields in aldicarb-treated plots were lower (P </= 0.05) than in nontreated plots. Cultivar x aldicarb interactions were detected in only one environment. Resistant cultivars yielded higher than susceptible cultivars in 13 of 15 environments. Final populations (Pf) of H. glycines were generally unaffected by aldicarb treatment; however, in 3 environments Pf were significantly higher, whereas in 2 other environments Pf were significantly lower in aldicarb-treated plots than in nontreated plots.

Applications of nematode community structure research to agricultural production and habitat disturbance

Nematode communities in agricultural habitats are diverse, usually comprising tens of species and a large number of individuals. The study of nematode community structure can be approached in a number of ways which can be categorized under a synecological approach. Although the plant-parasitic species are of most obvious interest to plant hematologists, these are only a part of the nematode fauna that affects agroecosystems. The application of community structure research to investigation of those effects can help elucidate their importance. This information is not only intrinsically valuable, but it is necessary in the development of sustainable agricultural production systems.

Effects of Heterodera glycines and Meloidogyne incognita on Early Growth of Soybean

Greenhouse and field microplot studies were conducted to compare soybean shoot and root growth responses to root penetration by Heterodera glycines (Hg) and Meloidogyne incognita (Mi) individually and in combination. Soybean cultivars Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were selected for study. In the greenhouse, pot size and number of plants per pot had no effect on Hg or Mi penetration of Coker 237 roots; root weight was higher in the presence of either nematode species compared with the noninoculated controls. In greenhouse studies using a sand or soil medium, and in field microplot studies, each cultivar was grown with increasing initial population densities (Pi) of Hg or Mi. Interactions between Hg and Mi did not affect early plant growth or number of nematodes penetrating roots. Root penetration was the only response related to Pi. Mi penetration was higher in sand than in soil, and higher in the greenhouse than in the field, whereas Hg penetration was similar under all conditions. At 14 days after planting, more second-stage juveniles were present in roots of susceptible than in roots of resistant plants. Roots continued to lengthen in the greenhouse in the presence of either Mi or Hg regardless of host genotype, but only in the presence of Mi in microplots; otherwise, responses in field and greenhouse studies were similar and differed only in magnitude and variability.

Effects of Interactions among Heterodera glycines, Meloidogyne incognita, and Host Genotype on Soybean Yield and Nematode Population Densities

The effects of host genotype and initial nematode population densities (Pi) on yield of soybean and soil population densities of Heterodera glycines (Hg) race 3 and Meloidogyne incognita (Mi) race 3 were studied in a greenhouse and field microplots in 1983 and 1984. Centennial (resistant to Hg and Mi), Braxton (resistant to Mi, susceptible to Hg), and Coker 237 (susceptible to Hg and Mi) were planted in soil infested with 0, 31, or 124 eggs of Hg and Mi, individually and in all combinations, per 100 cm(3) soil. Yield responses of the soybean cultivars to individual and combined infestations of Hg and Mi were primarily dependent on soybean resistance or susceptibility to each species separately. Yield of Centennial was stimulated or unaffected by nematode treatments, yield of Braxton was suppressed by Hg only, and yield suppressions caused by Hg and Mi were additive and dependent on Pi for Coker 237. Other plant responses to nematodes were also dependent on host resistance or susceptibility. Population densities of Mi second-stage juveniles (J2) in soil were related to Mi Pi and remained constant in the presence of Hg for all three cultivars. Population densities of Hg J2 on the two Hg-susceptible Cultivars, Braxton and Coker 237, were suppressed in the presence of Mi at low Hg Pi.

Effects of Environments, Meloidogyne incognita Inoculum Levels, and Glycine max Genotype on Root-knot Nematode-Soybean Interactions in Field Microplots

Five soybean cultivars (Braxton, Gordon, Jeff, Bragg, and Wright) resistant to Meloidogyne incognita (Mi) and three susceptible cultivars (Coker 156, GaSoy 17, and Coker 237) were grown at two locations for four seasons in microplots with increasing initial soil population densities (Pi) of Mi. The resistant cultivars and Coker 156 yielded better than GaSoy 17 and Coker 237 at all Pi. Yield response was dependent on environmental conditions and at one location was stimulated on Braxton, Gordon, Jeff, and Bragg by low Pi. Although Mi reproduced well on all cultivars, the pattern of reproduction differed. Population densities of Mi leveled off after 90 days on GaSoy 17 and Coker 237 but were still increasing after 120 days on the resistant cultivars; population densities were lower on resistant than on the susceptible cultivars. The population density of Mi on Coker 156 after 120 days was intermediate between those on the other susceptible and on the resistant cultivars. Mi population densities followed the same pattern under varying environmental conditions.

Plant-parasitic nematode communities in dogwood, maple, and peach nurseries in tennessee

Nursery blocks (48 dogwood, 27 red maple, and 17 peach) distributed among 20 Tennessee nurseries were sampled for nematodes in March, July, and October 1981. Plant-parasitic nematodes were extracted from soil, counted by genera, and identified to species after fixation. A total of 57 species in 24 genera were found, with 1-16 species occurring in a site. The species most commonly detected were Paratylenchus projectus and Xiphinema americanum, which were found in 88% and 78% of the sites, respectively. Relationships existed between distribution and densities of some species present in more than 10% of the sites and certain soil factors (pH, bulk density, texture, and organic matter content). Plant-parasitic nematode community diversity was related to tree age, percentage of weed ground cover, and number of weed species. Site similarities in community ordinations were dependent on the individual nurseries sampled, tree age, and soil type, but clusters of sites of similar tree ages and soil types were not exclusive.