Velvetbean in Rotation with Soybean for Management of Heterodera glycines and Meloidogyne arenaria

The effect of previous crops - soybean (Glycine max) or velvetbean (Mucuna deeringiana) - and aldicarb on yield and nematode numbers for selected soybean cultivars was studied in a field infested with a mixture of Meloidogyne arenaria and Heterodera glycines. Soybean following velvetbean yielded 959 kg/ha more than soybean following soybean. Nematicide treatment resulted in increased yield, and there was no interaction between nematicide treatment and previous crop. Cultivars interacted significantly with nematicide treatment but not with previous crop for yield. Velvetbean reduced numbers of H. glycines but not M. arenaria. Cultivars interacted with previous crop, and the previous crop x nematicide x cultivar interaction was significant for both M. arenaria and H. glycines. We concluded that velvetbean is effective in reducing yield losses caused by mixed populations of M. arenaria and H. glycines, regardless of genetic resistance of soybean cultivar.

Survey for Heterodera glycines in Maryland

Soybean cyst nematode, Heterodera glycines, was detected in 116, or 25%, of 463 soil samples collected in eight of the nine counties east of the Chesapeake Bay in Maryland.

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.

Soybean Planting Date and Maturity Effects on Heterodera glycines and Macrophomina phaseolina in Southeastern Kansas

Heterodera glycines reproduction and damage potential were evaluated for H. glycines-resistant and -susceptible soybean cultivars from maturity groups (MG) III, IV, and V on two planting dates in each of two years (1990-1991). Infection by H. glycines reduced yields of late-planted susceptible cultivars to a lesser degree than early planted cultivars in 1990 but not in 1991. The MG V susceptible cultivar yielded better than earlier-maturing susceptible cultivars even when yields of resistant cultivars were similar across maturity groups. Soybean yields were a function of nematode densities on roots in 1990, and nematode soil densities and root colonization by the charcoal rot fungus, Macrophomina phaseolina, in 1991. Harvest densities of H. glycines were lower for late-planted than for early-planted susceptible soybeans in 1990; however, nematode population increase was more rapid on roots of late-planted soybeans in 1991. Soybean maturity group did not have a significant effect on nematode populations in either year of the study, but colonization rates of M. phaseolina were lower for MG V cultivars than for earlier-maturing cultivars. Delayed soybean planting and cultivar maturity selection do not appear to be viable management options for H. glycines in southeastern Kansas.

Differential Sensitivity of Meloidogyne spp. and Heterodera glycines to Selected Nematicides

Differential sensitivity of Meloidogyne arenaria, M. hapla, M. incognita, M. javanica, and Heterodera glycines races 1 and 5 to the nonfumigant nematicides aldicarb, ethoprop, and fenamiphos was evaluated using a 48-hour root-penetration bioassay. Generally, H. glycines was more tolerant of the nematicides, especially ethoprop, than were the Meloidogyne species. Among Meloidogyne species, M. incognita was most sensitive to aldicarb and fenamiphos, but its reaction to ethoprop was similar to the other three Meloidogyne species.

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.

Reproduction of Plant-parasitic Nematodes on Winter Rapeseed (Brassica napus ssp. oleiferas)

The reproduction of isolates of five plant-parasitic nematode species on the winter rapeseed cultivars Bridger, Gorzanski, H-47, Lindora, and Viking was evaluated. Each cultivar was a good host for Helicotylenchus pseudorobustus, Meloidogyne hapla, and M. incognita, All rapeseed cultivars were poor hosts for Pratylenchus scribneri, in comparison with a susceptible reference host. Heterodera glycines females rarely developed on any cultivar, but low numbers of juveniles invaded roots and males occasionally reached maturity.

Effects of Eight Herbicides on In Vitro Hatching of Heterodera glycines

Laboratory studies were conducted to evaluate effects of selected herbicides on hatching of free eggs of the soybean cyst nematode, Heterodera glycines. The herbicides used were Atrazine (atrazine), Basagran (bentazon), Bladex (cyanazine), Blazer (acifluorfen), Command (clomazone), Lasso (alachlor), Sonalan (ethalfluralin), and Treflan (trifluralin). Treatments comprised two concentrations of commercial herbicide formulations and deionized water and 3.14 mM zinc sulfate as negative and positive controls, respectively. Eggs were extracted from females and cysts, surface disinfested, and incubated in herbicide or control solutions at 25 +/- 2 C in darkness. Hatched second-stage juveniles were counted every other day for 24 days. Hatching of H. glycines eggs in 50 and 500 mug/ml Blazer was 42 to 67% less than that in deionized water and 6l to 78% less than that in zinc sulfate solution. Zinc sulfate significantly increased hatching activity in 50 mug/ml but not 500 mug/ml Blazer. The other herbicides tested at various concentrations had no significant effect on egg hatching. The specific component of Blazer inhibiting egg hatching is unknown. Suppression of hatching by Blazer indicates that this postemergence soybean herbicide may have a potential role in managing H. glycines.

Interrelationship of Heterodera glycines and Fusarium solani in Sudden Death Syndrome of Soybean

Experiments were established in field microplots to examine the association between Heterodera glycines and the blue form of Fusarium solani in sudden death syndrome of soybean (SDS). Foliar disease symptoms occurred on more plants per plot, appeared 3 to 7 days earlier, and were more severe on plants grown in plots infested with F. solani + H. glycines than on those inoculated with F. solani only. Yields were suppressed only in treatments that included the nematode. Numbers of H. glycines cysts and second-stage juveniles were significantly lower in plots infested with F. solani + H. glycines than with the nematode alone. Fusarium solani was able to infect cysts and eggs.

Location of Heterodera glycines-induced Syncytia in Soybean as Affected by Soil Water Regimes

Locations of syncytia induced by the soybean cyst nematode (SCN), Heterodera glycines race 3, were compared in roots of 'Essex', a susceptible soybean (Glycine max (L.) Merr.) cultivar, at three soil water regimes. The plants were grown in wet (-5 to -20 kPa), moderately wet (-30 to -50 kPa), and moderately dry (-60 to -80kPa) autoclaved Captina silt loam soil (Typic Fragiudult). In the moderately dry soil, syncytia were found only in the stele, but in moderately wet and wet soils, syncytia occurred primarily in the cortex and occasionally in the stele. The location of syncytia in the cortical tissue of roots growing in wet and moderately wet soils may account for the tolerance of susceptible soybean cultivars grown under well-irrigated conditions where there is less interference with water transport through roots. Cell-wall perforations and dense cytoplasm were characteristic of syncytial cells observed in root tissues of all treatments.

The future of nematology: integration of new and improved management strategies

The potential for managing plant-parasitic nenlatodes by combining two or more control strategies in an integrated program is examined. Advantages of this approach include the use of partially effective strategies and protection of highly effective ones vulnerable from nematode adaptation or environmental risk. Strategies can be combined sequentially from season to season or applied simultaneously. Programs that have several strategies available but that are limited in the true integration of control components are used as examples of current management procedures and the potential for their improvement. These include potato cyst nematodes in northern Europe, soybean cyst nematode in North Carolina, and root-knot nematodes on vegetable and field crops in California. A simplified model of the impact of component strategies on the nematode damage function indicates the potential for combining control measures with different efficacies to give acceptable nematode population reduction and crop protection. The likelihood for additive, synergistic, or antagonistic effects from combining strategies is considered with respect to the biological target and component compatibility.

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.

Penetration Rates by Second-stage Juveniles of Meloidogyne spp. and Heterodera glycines into Soybean Roots

The rates of soybean root penetration by freshly hatched second-stage juveniles (J2) of Meloidogyne arenaria, M. hapla, M. incognita, M. javanica, and Heterodera glycines races 1 and 5 were examined over a period of 1 to 240 hours. Heterodera glycines entered roots more quickly than Meloidogyne spp. Penetration by most nematodes was accomplished within 48 hours. The increases in penetration after 48 hours were insufficient to warrant further assessments. Penetration of J2 into roots of soybean seedfings in a styrofoam container was as good or better than in a clay pot. Thus, rapid and accurate root-penetration assessments can be made at 48 hours after inoculation.

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.

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.

Crop Rotation Studies with Velvetbean (Mucuna deeringiana) for the Management of Meloidogyne spp

Results from a greenhouse experiment at Cabrils, Spain, with two velvetbean (Mucuna deeringiana) accessions (Florida and Mozambique) growing in sterilized sandy loam and inoculated with Meloidogyne arenaria race 2, M. incognita race 1, and M. javanica revealed that the legume was not a host for these nematodes. In contrast, roots of 'Clemson Spineless' okra (Hibiscus esculentum), 'Summer Crookneck' squash (Cucurbita pepo), and 'Davis' soybean (Glycine max) were galled by all three root-knot nematodes. Greenhouse experiments at Auburn, Alabama, using soils infested with Heterodera glycines (race 14) + M. incognita or with H. glycines + M. arenaria (race 2) showed that, in contrast to Davis soybean, a Mexican and the Florida velvetbean accessions were not hosts for the nematodes. An experiment with 'Florunner' peanut (Arachis hypogaea) and the Florida velvetbean in a field infested with M. arenaria (race 1), near Headland, Alabama, showed that significant juvenile populations of the nematode at peanut harvest time were present only in plots with peanut. A microplot rotation experiment demonstrated that 'Black Beauty' eggplant (Solanum melongena) following the Florida velvetbean had heavier shoots and lower numbers of M. arenaria juveniles in the roots and in the soil than eggplant after Summer Crookneck squash or Davis soybean.

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.

Effect of Soybean Tip Removal on Penetration and Development of Heterodera glycines

On a few occasions, soybeans with broken root tips were included in tests to evaluate resistance to Heterodera glycines. Although females developed on these plants, the numbers tended to be lower than on similarly treated intact roots. To test the possibility that removal of the root meristem affected nematode development, a culture system using pruned soybeans was devised that permitted access to the roots without disturbing the plants. Treatments included removal of 2 mm of root tip at various times ranging from 24 hours before to 10 days after inoculation, or roots left intact. In each experiment, all roots were inoculated at the same time with equal numbers of freshly hatched second-stage juveniles of Heterodera glycines. No differences in nematode development were detected in plants with root tips removed after inoculation compared to the control. When tips were removed at or before inoculation, fewer juveniles entered roots and relatively fewer nematodes developed. Penetration levels and development correlated with root tip removal such that progressively fewer nematodes entered roots and relatively greater numbers of nematodes remained undeveloped as the time interval between root tip removal and inoculation was increased.

Fitness Components and Selection of Biotypes of Heterodera glycines

Survival of biotypes of Heterodera glycines was studied in microplots and in the field. The field population was subjected to various cropping sequences. Viability of eggs overwintered in microplots was determined each spring by percentage hatch, percentage of hatched eggs penetrating roots, and numbers of females developing on Peking and PI 88788 soybeans. Eggs from the field were collected in the spring and fall and assayed for ability to develop on Peking and PI 88788. Hatch of isolates overwintered in the microplots averaged 13% in May 1989 and 19% in 1990. No differences in hatch were detected among the isolates in 1989. Numbers of juveniles penetrating susceptible roots averaged less than 20% of the hatched eggs each year. An isolate of a biotype parasitic on susceptible soybeans and the resistant soybean PI 88788 penetrated roots more successfully than other biotypes. A second isolate from North Carolina, parasitic on susceptible soybeans, PI 88788, and the resistant soybean Peking experienced selection against development on Peking during two winters. Only 17 % of the expected numbers of females developed on Peking from this isolate. In the microplot experiment, parasitism of PI 88788 and Peking had a selective disadvantage (selection coefficient) of s = 0.29 and 0.62 over all isolates, respectively. In the field experiment, the relative numbers of cysts on Peking and PI 88788 increased between the spring and fall on soybean, then decreased over the winter and under corn. Selection coefficients against parasitism of PI 88788 and Peking averaged 0,19 and 0.3 in the field population. In neither experiment did juveniles lose their ability to parasitize susceptible soybeans.

Breeding plants for resistance to nematodes

Plant breeders and nematologists have developed improved cultivars of important crop species with resistance to plant-parasitic nematodes. The effectiveness of these breeding efforts has depended on the availability of efficient screening procedures, identification of adequate sources of durable resistance, nature of the nematode feeding habit, and knowledge of the inheritance of resistance. These factors determine to a large degree the breeding method and potential success of the research. Systematic searches for nematode resistance have identified resistant germplasm lines within crop species or from related species. When the resistance gene(s) is from related species, incongruity barriers or sterility of the resulting hybrids often must be overcome. In these situations, backcrossing is usually necessary to incorporate the resistance gene(s) and recover the desirable commercial traits of the crop species. If the resistance gene(s) is present within the crop species, the choice of breeding method depends on the inheritance of the resistance, type of screening procedure, and other important breeding objectives for the species. In the future, plant molecular biologists and geneticists will make available novel sources of nematode resistance through incorporation of transgenes from other genera. These efforts will likely require conventional breeding strategies before commercial utilization of an improved resistant cultivar.

Problems and Strategies Associated with Long-term Use of Nematode Resistant Cultivars

Plant-parasitic nematodes are obligate parasites, and planting cultivars that are highly resistant to these organisms places extensive selection pressure on the target species and affects nontarget nematodes as well. Problems encountered with long-term planting of cultivars resistant to nematodes include shifts in nematode races or species and the occurrence of multiple species of nematodes within the same field. These problems can be alleviated to some extent when crop management is used to lessen the selection pressure for change on the nematode populations. Race shifts within populations and possibly shifts between nematode species can be delayed by rotating susceptible cultivars and nonhost crops with resistant cultivars. Nematicides in conjunction with resistant cultivars may be used to limit damage by multiple species of nematodes. Some cultivars have resistance to multiple species of nematodes, but greatly increased research effort is needed in this area. More intensive plant breeding effort will be required to make nematode resistant cultivars competitive in quality and yield with more productive, susceptible cultivars.

A Technique for Evaluating Heterodera glycines Development in Susceptible and Resistant Soybean

A technique was developed to evaluate Heterodera glycines development in susceptible and resistant soybean. Roots of 3-day-old soybean were exposed to infective juveniles of H. glyci.nes in sand for 8 hours followed by washing and transfer to hydroponic culture. The cotyledons and apical meristem were removed and plants were maintained under constant light, which resulted in a dwarfed plant system. After 15 or 20 days at 27 C, nematodes were rated for development. Emerged males were sieved from the culture water and females were counted directly from the roots. Nematodes remaining in the roots were rated for development after staining and clearing the tissues. The proportion of nematodes at each stage of development and the frequency of completed molts for each stage were calculated from these data. This technique showed that resistance to H. glycines was stage related and did not affect males and females equally in all resistant hosts. The resistance of plant introduction PI 209332 primarily affected development of third and fourth-stage juveniles; 'Pickett' mainly affected second and third-stage juveniles, whereas PI 89772 affected all stages. Male development was markedly affected in PI 89772 and 'Pickett' but not in PI 209332.

An Alternative Method for Evaluating Soybean Tolerance to Heterodera glycines in Field Plots

Alternate planting dates and periodic destruction of the previous year's soybean crop as well as 1-year bare fallow were used to establish a range of population densities ofHeterodera glycines for the subsequent year. Soybean cultivar Coker 156 (susceptible, moderately tolerant) was compared to cultivars Essex (susceptible, intolerant) and Bedford (resistant) to evaluate tolerance at different H. glycines population densities established through the previous year's treatments. Yield of Coker 156 was consistently intermediate between yields of Bedford and Essex in 1986 and 1987. Yield of Essex was negatively correlated (P = 0.05) with preplant egg numbers of H. glycines in 1987, whereas yield of Bedford and Coker 156 were not related to nematode density. Reproduction of H. glycines was greater (P = 0.05) on the moderately tolerant Coker 156 than on either of the other cultivars.

Separation and Characterization of Heterodera glycines Acetylcholinesterase Molecular Forms

The composition and biochemical properties of acetylcholinesterases isolated from Heterodera glycines were determined. Heterodera glycines contains three separable AChE molecular forms that can be grouped into two classes corresponding to classes A and C found in some other nematode species. The apparent lack of class B AChE is unusual and may have significant behavioral ramifications. The class C enzyme isolated from H. glycines is similar to that from Meloidogyne arenaria and M. incognita but is somewhat more sensitive to AChE inhibitors such as eserine. Heterodera glycines possesses a larger percentage of its total acetylcholinesterase as class C than other nematodes thus far examined.

Soybean Cultivars Resistant and Susceptible to Heterodera glycines

Additional tests of 178 cultivars and lines of soybean against soybean cyst nematode are reported. Cultivars are available with some level of resistance to races 1, 2, 3, 4, 5, 6, 9, and 14, but in some cases the resistance level is moderate. No cultivar or line (except possibly PI 437654) has resistance to all of the races tested. Thirteen cultivars and lines, however, were resistant or moderately resistant to race 1, 14 to race 2, 87 to race 3, 2 to race 4, 3 to race 5, 42 to race 6, 18 to race 9, and 36 to race 14. Some have resistance to more than one race.