Effect of Within-field Variation in Soil Texture on Heterodera glycines and Soybean Yield

The influence of soil texture on Soybean yield in the presence of Heterodera glycines was investigated by comparing yields of susceptible cultivars with a resistant cultivar for 2 years. Soybean yield was negatively correlated with increasing sand content (P = 0.05). Yields of susceptible cultivars were suppressed with increasing sand content. Final nematode population densities were lowest in plots with greatest sand content. Soybean infection by SCN, as determined by the number of cysts 30 days after planting, was not consistently related to soil texture over 2 years. Initial nematode population density was positively related to soybean yield the first year and negatively related to soybean yield the second, probably a result of greater yield suppression by H. glycines in plots with greater sand content.     

Interactions of Vesicular-Arbuscular Mycorrhizal Fungi,Phosphorus, and Heterodera glycines on Soybean

Effects of vesicular-arbuscular mycorrhizal (VAM) fungi and soil phosphorus (P) fertility on parasitism of soybean cultivars Bragg and Wright by soybean cyst nematode (SCN) were investigated in field micropiot and greenhouse experiments. VAM fungi increased height of both cultivars and yield of Wright in microplot studies in 1986 and 1987. Conversely, yield of mycorrhizal and nonmycorrhizal plants of both cultivars was suppressed by SCN. Soil population densities of SCN were unaffected by VAM fungi in 1986 but were greater in microplots infested with VAM fungi than in control microplots in 1987. Growth of Wright soybean was stimulated by VAM fungi and suppressed by SCN in greenhouse experiments. The effect of VAM fungi on SCN varied with time. Numbers of SCN in roots and soil were decreased by VAM fungi by as much as 73% at the highest SCN inoculum level through 49 days after planting. Later, however, SCN numbers were usually comparable on mycorrhizal and nonmycorrhizal plants. Soil P fertility generally had no effect on SCN. Results of a split-root experiment indicated that VAM fungal suppression of SCN was not systemic.     

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.     

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.     

Effects of Soil Type on the Damage Potential of Meloidogyne incognita on Soybean

Effects of soil type on the reproduction and damage potential of Meloidogyne incognita on soybean, Glycine max (L.) Merr., were determined at five locations in North Carolina, including one site where plots with six soil types were established. M. incognita reproduced readily on a susceptible soybean cultivar in most soil types, with somewhat limited reproduction in muck soils. The relationship between initial population densities and yield varied among soil types and nematode populations. Yield losses were greatest in sandy and muck soil types, with less nematode damage occurring in the clay soil types. A North Carolina and a Georgia population of M. incognita differed greatly in their ability to reproduce on soybean and suppress growth. The North Carolina population had a moderate effect on yield in 1981 and only a slight effect in 1982. In contrast, a Georgia population severely limited soybean growth and yield at lower initial population densities in 1983, Initial population densities of the nematodes and physical and chemical edaphic factors accounted for much of the variation of soybean yield and nematode reproduction.     

Response of Soybean to Heterodera glycines Races 1 and 2 in Different Soil Types

Experiments were conducted for 3 years at four locations and 1 year with six soil types at a common location in North Carolina to determine damage and control-cost functions for Heterodera glycines races 1 and 2 on soybean. In the experiments on native loamy sand and sandy soils, tolerance limits for initial population densities were 0 or very low, whereas in a muck, the tolerance limit was 315 eggs/500 cm³ soil. The aggressive race 2 was more damaging than race 1 in Lakeland sand and Norfolk loamy sand. The crop response was not different between races in the Appling sandy clay loam and Belhaven muck. Soybean yield responses to H. glycines were linear in six soil types in microplots at a common site. The amount of damage varied among these soil types, with lowest yields in the muck because of severe drought stress in this soil. An exponential function adequately described soybean yield response relative to nematode control with increasing rates of aldicarb in Norfolk loamy sand. Treatment with aldicarb in the Lakeland sand decreased the effective egg population of H. glycines but had only a minor effect in the muck.     

Assessing Heterodera glycines-Resistant and Susceptible Cultivar Yield Response

The soybean cyst nematode Heterodera glycines (SCN) is of major economic importance and widely distributed throughout soybean production regions of the United States where different maturity groups with the same sources of SCN resistance are grown. The objective of this study was to assess SCN-resistant and -susceptible soybean yield responses in infested soils across the north-central region. In 1994 and 1995, eight SCN-resistant and eight SCN-susceptible public soybean cultivars representing maturity groups (MG) I to IV were planted in 63 fields, either infested or noninfested, in 10 states in the north-central United States. Soil samples were taken to determine initial SCN population density and race, and soil classification. Data were grouped for analysis by adaptation based on MG zones. Soybean yields were 658 to 3,840 kg/ha across the sites. Soybean cyst nematode-resistant cultivars yielded better at SCN-infested sites but lost this superiority to susceptible soybean cultivars at noninfested sites. Interactions were observed among initial SCN population density, cultivar, and location. This study showed that no region-wide predictive equations could be developed for yield loss based on initial nematode populations in the soil and that yield loss due to SCN in our region was greatly confounded by other stress factors, which included temperature and moisture extremes.     

Tolerance to Heterodera glycines in Soybean

Fifty-four susceptible soybean, Glycine max, cultivars or plant introductions were evaluated for tolerance to H. glycines, the soybean cyst nematode (SCN). Seed yields of genotypes were compared in nematicide-treated (1,2-dibromo-3-chloropropane, 58 kg a.i./ha) and nontreated plots at two SCN-infested locations over 3 years. Distinct and consistent levels of tolerance to SCN were observed among soybean genotypes. PI 97100, an introduction from Korea, exhibited the highest level of tolerance with an average tolerance index ([yield in nontreated plot ÷ yield in nematicide-treated plot] × 100) of 96 over 2 years. Coker 156 and Wright had moderate levels of tolerance (range in index values 68 to 95) compared to the intolerant cuhivars Bragg and Coker 237 (range in index values 33 to 68). Most of the soybean genotypes evaluated were intolerant to SCN. The rankings of five genotypes for tolerance to SCN and Hoplolaimus columbus were similar. Tolerance for seed yield was more consistently correlated with tolerance for plant height (r = 0.55 to 0.64) than for seed weight (r = 0.23 to 0.65) among genotypes.     

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.     

Crop Yields and Nematode Population Densities in Triticale-Cotton and Triticale-Soybean Rotations

Triticale cv. Beagle 82, cotton cv. McNair 235, and soybean cv. Twiggs were arranged in three cropping sequences to determine the effects of fenamiphos and cropping sequence on nematode population densities and crop yields under conservation tillage for 4 years. The cropping sequences were triticale (T)-cotton (C)-T-C, T-soybean (S)-T-S, and T-C-T-S. Numbers of Meloidogyne incognita second-stage juveniles declined on trificale but increased on cotton and soybean each year. Root-gall indices of cotton and soybean ranged from 1.00 to 1.08 (1 to 5 scale: 1 = 0%, 2 = 1% to 25%, 3 = 26% to 50%, 4 = 51% to 75%, and 5 = 76% to 100% of roots galled) each year and were not affected by fenamiphos treatment or cropping sequence. Numbers of Pratylenchus brachyurus were maintained on trificale and generally increased more on soybean than on cotton. Population densities of Helicotylenchus dihystera were near or below detection levels in all plots during the first year and increased thereafter in untreated plots in the T-C-T-C and T-S-T-S sequences. Generally, yields of triticale in all cropping sequences declined over the years. Yields of cotton and soybean were not affected by fenamiphos at 6.7 kg a.i./ha. Cotton and soybean were grown successfully with little or no suppression in yields caused by nematodes in conservation tillage following triticale harvested for grain.     

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.     

Nematode Control Related to Fusarium Wilt in Soybean and Root Rot and Zinc Deficiency in Corn

Nematode and disease problems of irrigated, double-cropped soybean and corn, and zinc deficiency of corn were investigated. Ethylene dibromide, phenamiphos, and aldicarb were equally effective for controlling nematodes and increasing yields of corn planted minimum-till and soybean planted in a moldboard plow prepared seedbed. The residual effects on yields of nematicides applied to the preceeding crop occurred during 3 years for soybean and 1 year for corn. Fusarium wilt symptoms of soybean that developed during 2 years of the study were less severe in all nematicide-treated plots than in control plots. Typical zinc deficiency symptoms on 30-day-old corn plants were observed during 1 year of the study in certain plots. Symptoms were not evident on plants grown on plots treated with ethylene dibromide, and only occasional plants had symptoms on plots treated with phenamiphos and aldicarb. The amount of yield response directly related to nematode control could not be determined because of the apparent interaction of nematodes on the expression of Fusarium wilt of soybean. Our study strongly indicates that the expression of Fusarium wilt of soybean and zinc deficiency in corn are influenced by nematodes and that nematicides will reduce their severity.     

Nodulation of Soybeans as Affected by Half-root Infection with Heterodera glycines

A split-root technique was applied to soybean, Glycine max (L.) Merr. cv. Lee 68, to characterize the nature of the nodulation suppression by race 1 of the soybean cyst nematode (SCN), Heterodera glycines. Root-halves of each split-root plant were inoculated with Rhizobium japonicum, and one root-half only was inoculated with various numbers of SCN eggs. Nodulation (indicated by nodule number, nodule weights, and ratio of nodule weight to root weight) and nitrogen-fixing capacity (indicated by rate of acetylene reduction) were systemically and variously suppressed on both root-halves of the split-root plant 5 weeks after half-root inoculation with 12,500 SCN eggs. Inoculation with 500 eggs caused this suppression only on the SCN-infected (+NE) root-half; nodulation on the companion uninfected (-NE) root-half was stimulated slightly. The +NE root-halves inoculated with 5,000 eggs were excised at 2-week intervals; nodulation on the remaining -NE root-halves was not different from that of the noninoculated control when measured 6 weeks after the SCN inoculation. Thus, the systemic suppression of nodulation was reversible upon the removal of the SCN. Similarly, application of various levels of KNO₃ to the -NE root-halves of the split-root plant did not alleviate the suppressed nodulation on the companion +NE root-halves, even though plant growth was much improved at certain levels of nitrogen (125 μg N/g soil). This indicated that the localized suppression of nodulation by SCN was caused by factors in addition to poor plant growth.     

Damage Functions and Population Changes of Hoplolaimus columbus on Cotton and Soybean

Damage functions and reproductive curves were determined for Hoplolaimus columbus on cotton cv. Deltapine 90 and soybean cv. Gordon over 2 years in field plots in Georgia. Maximum potential yield suppressions of 18% on cotton and 48% on soybean were predicted with respect to increasing Pi. Similar functions indicated yield suppressions of 38% on cotton and 30% on soybean with respect to increasing midseason nematode densities (Pm). Maximum Pf predicted by reproductive curves were 123 and 474/100 cm³ soil on cotton and soybean, respectively. Thresholds at which 10% yield suppression would occur were lower on soybean (Pi of 4) than on cotton (Pi of 70/100 cm³ soil). The economic threshold for a control measure costing $72/ha was a Pi of 60/100 cm³ soil on cotton, assuming a price for cotton lint of $1.44/kg ($0.60/lb), whereas a similar treatment would not be economically feasible on soybean at any Pi with an assumed price of $0.04/kg ($5.50/bu) soybean seed. Damage functions and reproductive curves as determined in this study offer potentially useful tools for analyzing cropping systems and providing decision tools for nematode management.     

Rotylenchulus reniformis Management in Cotton with Crop Rotation

One-year crop rotations with corn or highly resistant soybean were evaluated at four locations for their effect on Rotylenchulus reniformis population levels and yield of a subsequent cotton crop. Four nematicide (aldicarb) regimes were included at two of the locations, and rotation with reniform-susceptible soybean was included at the other two locations. One-year rotations to corn or resistant soybean resulted in lower R. reniformis population levels (P ≤ 0.05) than those found in cotton at three test sites. However, the effect of rotation on nematode populations was undetectable by mid-season when cotton was grown the following year. Cotton yield following a one-year rotation to resistant soybean increased at all test locations compared to continuous cotton, and yield following corn increased at three locations. The optimum application rate for aldicarb in this study was 0.84 kg a.i./ha in furrow. Side-dress applications of aldicarb resulted in yield increases that were insufficient to cover the cost of application in 3 of the 4 years.     

Control of the Soybean Cyst Nematode by Crop Rotation in Combination with a Nematicide

An experiment to evaluate the control of soybean cyst nematodes compared 1-year, 2-year, and 3-year nonhost rotations with continuous soybeans (Glycine max) in 0.2-ha plots. In a second 1-year rotation, the plots were planted to soybean or corn (Zea mays) after fumigation in the spring with a split application of 1,3-dichloropropene (748.2 liters/ha). The effects of the nematicide were apparent the first year. Soybean yield was 1,482 kg/ha compared to 233 kg/ha in the untreated plots. In the second year, the highest yielding plants (2,035 kg/ha) were those following 1 year of corn that had been treated the previous year; plants in untreated plots yielded 288 kg/ha. Average yield of soybean following 1 year of corn was 957 kg/ha compared to 288 kg/ha for continuous soybean. In the third year, the effects of the nematicide were still evident. Soybean plants in plots treated the first year, followed by corn, then soybean, yielded 1,044 kg/ha compared to 761 kg/ ha for soybean following 1 year of corn and 991 kg/ha for soybean following 2 years of corn. Plots planted to soybean for 3 consecutive years yielded 337 kg/ha. Nematicidal effects were no longer evident during the fourth year. Yields were most improved by the greatest number of years in the nonhost crop; highest yields in descending order were from plants following 3 years of corn, 2 years of corn, and 1 year of corn. Plots planted to soybean for 4 consecutive years yielded 130 kg/ha. Highly significant negative correlations occurred each year between initial nematode population densities and seed yield.     

Population Density and Spatial Pattern of Heterodera glycines in Relation to Soybean Phenology

Population dynamics of Heterodera glycines (SCN) were influenced by initial nematode population density in soil, soybean root growth pattern, soil type, and environmental conditions in two field experiments. Low initial populations (Pi) of SCN increased more rapidly during the growing season than high Pi and resulted in greater numbers of nematodes at harvest. Egg and juvenile (J2) populations increased within 2-6 weeks after planting when early-season soil temperatures were 20 C and above and were delayed by soil temperatures of 17 C or below in May and early June. Frequencies of occurrence and number of nematodes decreased with increasing depth and distance from center of the soybean row. Spatial pattern of SCN paralleled that of soybean roots. Higher clay content in the subsoil 30-45 cm deep in one field restricted soil penetration by roots, indirectly influencing vertical distribution of SCN. Shoot dry weight was a good indicator of the effect of SCN on seed yield. Root dry weight was poorly correlated with soybean growth and yield. The relationship of yield (seed weight) to Pi was best described by a quadratic equation at one site, but did not fit any regression model tested at the second site.     

Effect of Planting Date on Population Densities of Hoplolaimus columbus and Yield of Soybean

During the 1991 and 1992 soybean growing seasons, field plots were established in South Carolina to study the effect of planting date on at-planting nematode densities and subsequent yield losses caused by Hoplolaimus columbus. The susceptible and intolerant soybean cv. Braxton was planted on five dates from to May to 28 June in 1991 and from 12 May to 28 June in 1992. Nematodes were recovered from soil samples collected before nematicide treatment with 1,3-D (Pi), at 6 weeks after planting (Pm), and at harvest (Pf). Initial nematode population densities did not differ among the five dates of planting in either year. The increase in numbers of nematodes from planting to 6 weeks after planting (Pm/Pi) and from planting to harvest (Pf/Pi) were not different among the five planting dates in either year. Root samples also were collected at 6 weeks after planting and at harvest, but planting date did not affect the number of nematodes extracted from roots on any sample date in either year. Altering planting dates between early May and late June was not effective in preventing yield suppression due to H. columbus.     

Tolerance of Soybean to Heterodera glycines

Seven soybeans were selected from 200 entries evaluated for tolerance to soybean cyst nematode (SCN), Heterodera glycines. Tolerance to SCN was measured by comparing the seed yield from aldicarb-treated vs. nontreated plots. A yield response index (YRI) was calculated for each entry: YRI = (seed yield from nontreated plot/seed yield from treated plot) × 100. The soybean entries Coker 156, PI 97100, and S79-8059 exhibited high tolerance (YRI) to SCN when compared to Essex even though they became heavily infected with SCN. Tolerance in soybeans to SCN may be useful in pest management programs designed to stabilize soybean yield.     

Root-knot Nematode Management and Yield of Soybean as Affected by Winter Cover Crops,Tillage Systems,and Nematicides

Management of Meloidogyne incognita on soybean as affected by winter small grain crops or fallow, two tillage systems, and nematicides was studied. Numbers of M. incognita did not differ in plots planted to wheat and rye. Yields of soybean planted after these crops also did not differ. Numbers of M. incognita were greater in fallow than in rye plots, but soybean yield was not affected by the two treatments. Soybean yields were greater in subsoil-plant than in moldboard plowed plots. Ethylene dibromide reduced nematode population densities more consistently than aldicarb and phenamiphos. Also, ethylene dibromide increased yields the most and phenamiphos the least. There was a positive correlation (P = 0.001) of seed size (weight of 100 seeds) with yield (r = 0.79), indicating that factors affecting yield also affected seed size.