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.     

Tolerance to Hololaimus columbus in Glyphosate-Resistant,Transgenic Soybean Cultivars

Transgenic soybean cultivars, resistant to glyphosate herbicide in maturity groups V and VI, were evaluated for tolerance to the Columbia lance nematode, Hoplolaimus columbus, in field experiments conducted in 1998 and 1999. Treatment with 43 liter/ha of 1,3-dichloropropene was effective in suppressing H. columbus population densities in a split-plot design. Fumigation increased soybean yield, but a significant cultivar × fumigation interaction indicated variation in cultivar response to H. columbus. A tolerance index (yield of nontreated ÷ yield of treated × 100) was used to compare cultivar differences. Two cultivars in maturity group VI and one cultivar in maturity group V had a tolerance index greater than 90, indicating a high level of tolerance.     

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.     

Response of Soybean Cultivars to Nematicidal Treatments of Soil Infested with Meloidogyne incognita

A comparison of untreated and nematicide-treated soil for soybean production revealed that Meloidogyne incognita hastened crop maturity and reduced plant ht, seed wt, and yield. Reductions of yield varied from 32-90% depending on cultivar susceptibility. DBCP was more consistent in increasing crop performance than organo-phosphale or oxime carbamate nematicides. Greatest yield increases were produced by nematicidal treatment of soils planted to soybean cultivars with the lowest susceptibility.     

Meloidogyne incognita Resistance Characteristics in Tomato Genotypes Developed for Processing

Nine resistant processing tomato (Lycopersicon esculentum) cultivars and advanced lines were compared with four susceptible cultivars in 1,3-dichloropropene-fumigated and nontreated plots on Meloidogyne incognita-infested sites over 3 years. Yield of all resistant genotypes grown in nontreated and nematicide-treated plots did not differ and was greater than yield of susceptible genotypes. M. incognita initial soil population densities caused 39.3-56.5% significant (P = 0.05) yield suppressions of susceptible genotypes. Nematode injury to susceptible plants usually caused both fruit soluble solids content and pH to increase significantly (P = 0.05). Only trace nematode reproduction occurred on resistant genotypes in nontreated plots, whereas large population density increases occurred on susceptible genotypes. Slightly greater nematode reproduction occurred on resistant genotypes at the southern desert location, where soil temperature exceeded 30 C, than at other locations. At two locations resistant MOX 3076 supported greater reproduction than other resistant genotypes.     

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 cm³ 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.     

Efficacy of Organic Soil Amendments for Management of Heterodera glycines in Greenhouse Experiments

In a repeated greenhouse experiment, organic soil amendments were screened for effects on population density of soybean cyst nematode (SCN), Heterodera glycines, and soybean growth. Ten amendments at various rates were tested: fresh plant material of field pennycress, marigold, spring camelina, and Cuphea; condensed distiller’s solubles (CDS), ash of combusted CDS, ash of combusted turkey manure (TMA), marigold powder, canola meal, and pennycress seed powder. Soybeans were grown for 70 d in field soil with amendments and SCN eggs incorporated at planting. At 40 d after planting (DAP), many amendments reduced SCN egg population density, but some also reduced plant height. Cuphea plant at application rate of 2.9% (amendment:soil, w:w, same below), marigold plant at 2.9%, pennycress seed powder at 0.5%, canola meal at 1%, and CDS at 4.3% were effective against SCN with population reductions of 35.2%, 46.6%, 46.7%, 73.2%, and 73.3% compared with control, respectively. For Experiment 1 at 70 DAP, canola meal at 1% and pennycress seed powder at 0.5% reduced SCN population density 70% and 54%, respectively. CDS at 4.3%, ash of CDS at 0.2%, and TMA at 1% increased dry plant mass whereas CDS at 4.3% and pennycress seed powder at 0.1% reduced plant height. For Experiment 2 at 70 DAP, amendments did not affect SCN population nor plant growth. In summary, some amendments were effective for SCN management, but phytoxicity was a concern.     

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.     

Effects of Belonolaimus longicaudatus Management and Nitrogen Fertility on Turf Quality of Golf Course Fairways

Field experiments evaluated the effects of nematicide and fertility on performance of ‘Tifway 419’ bermudagrass parasitized by the sting nematode (Belonolaimus longicaudatus). Plot treatments were nontreated or nematicide (1,3-dichloropropene) treated combined with different nitrogen (N) fertilizer levels. Effects of treatments on numbers of B. longicaudatus and turf performance were compared. Nematicide consistently reduced numbers of B. longicaudatus, but fertilizer level had no effect on B. longicaudatus. Turf performance of nematicide-treated plots was improved compared with nontreated plots during both experiments. Increasing N fertilizer level improved turf performance in nematicide-treated plots in some cases, but had no effect on turf performance in nontreated plots in either experiment. Results suggest that increasing N fertilizer levels may not improve turf performance at sites infested with B. longicaudatus unless nematode management tactics are effective in reducing nematode densities.     

Effect of Soil Temperature and pH on Resistance of Soybean to Heterodera glycines

Soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is a major pest of soybean, Glycine max L. Merr. Soybean cultivars resistant to SCN are commonly grown in nematode-infested fields. The objective of this study was to examine the stability of SCN resistance in soybean genotypes at different soil temperatures and pH levels. Reactions of five SCN-resistant genotypes, Peking, Plant Introduction (PI) 88788, Custer, Bedford, and Forrest, to SCN races 3, 5, and 14 were studied at 20, 26, and 32 C, and at soil pH''s 5.5, 6.5, and 7.5. Soybean cultivar Essex was included as a susceptible check. Temperature, SCN race, soybean genotype, and their interactions significantly affected SCN reproduction. The effect of temperature on reproduction was quadratic with the three races producing significantly greater numbers of cysts at 26 C; however, reproduction on resistant genotypes remained at a low level. Higher numbers of females matured at the soil pH levels of 6.5 and 7.5 than at pH 5.5. Across the ranges of temperature and soil pH studied, resistance to SCN in the soybean genotypes remained stable.     

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.     

Shade Tobacco Yield Loss and Globodera tabacum tabacum Population Changes in Relation to Initial Nematode Density

Field microplot experiments were conducted from 1987 to 1992 to determine the relationship between fresh weight leaf yield of shade tobacco (Nicotiana tabacum) and initial density of Globodera tabacum tabacum (encysted J2 per cm³ soil). Initial nematode densities of 0.1 to 1,097 J2/cm³ soil were negatively correlated with leaf yield, total shoot weight, and normalized plant height 5 to 6 weeks after transplanting (r = -0.73, -0.73, and -0.52, respectively). Nonlinear damage functions were used to relate initial G. t. tabacum densities to the yield and shoot weight data. The model described leaf yield losses of < 5 % for initial nematode densities of less than 100 J2/cm³ soil. Densities above 100 J2 resulted in yields decreasing exponentially to a maximum yield loss of >40% at 500 to 1,000 J2/cm³ soil. A similar initial density tolerance threshold relationship was observed for total shoot weight. No threshold effect was evident for standardized plant height, which was a poor predictor of leaf yield. Globodera tabacum tabacum population increase over a growing season was described by a linear relation on a log/log plot (R² = 0.73).     

Maximizing the Potential of Cropping Systems for Nematode Management

Quantitative techniques were used to analyze and determine optimal potential profitability of 3-year rotations of cotton, Gossypium hirsutum cv. Coker 315, and soybean, Glycine max cv. Centennial, with increasing population densities of Hoplolaimus columbus. Data collected from naturally infested on-farm research plots were combined with economic information to construct a microcomputer spreadsheet analysis of the cropping system. Nonlinear mathematical functions were fitted to field data to represent damage functions and population dynamic curves. Maximum yield losses due to H. columbus were estimated to be 20% on cotton and 42% on soybean. Maximum at-harvest population densities were calculated to be 182/100 cm³ soil for cotton and 149/100 cm³ soil for soybean. Projected net incomes ranged from a $17.74/ha net loss for the soybean-cotton-soybean sequence to a net profit of $46.80/ha for the cotton-soybean-cotton sequence. The relative profitability of various rotations changed as nematode densities increased, indicating economic thresholds for recommending alternative crop sequences. The utility and power of quantitative optimization was demonstrated for comparisons of rotations under different economic assumptions and with other management alternatives.     

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.     

Genotypic variability in physiological,biomass and yield response to drought stress?in pigeonpea

Three pigeonpea (Cajanus cajan L. Millsp.) genotypes- GT-1, AKP-1 and PRG-158 with varying crop duration, growth habit and flowering pattern were evaluated for variability in their response for drought stress. Drought stress was imposed at initiation of flowering and the observations on biomass and seed yield parameters were recorded at harvest. The magnitude of response of individual component to drought stress was found to be genotype specific. Drought stress significantly decreased photosynthetic rate (P_N), transpiration rate (Tr) and relative water content (RWC) in all the genotypes, however the magnitude of reduction differed with genotype. With drought stress, the reduction of P_N was highest in GT-1 while reduction in Tr was highest in PRG-158. The genotype AKP-1, accumulated significantly higher concentrations of osmotic solutes especially proline under water deficit stress, this facilitated it to maintain higher relative water content (RWC) and lower malondialdehyde (MDA) content as compared to other genotypes. Drought stress also impacted biomass production and their partitioning to vegetative and reproductive components at harvest. There was significant variability between the genotypes for seed yield under drought stress while it was non-significant under well-watered condition. Drought stress enhanced flower drop and decreased flower to pod conversion resulting in reduced pod number and seed number in PRG-158 and GT-1. The genotype AKP-1 recorded superior performance for seed yield under stress environment due to its ability in maintaining pod and seed number as well as improved test weight (100 seed weight). Under drought stress, significant positive association of seed yield with proline, seed number, pod number and test weight clearly indicating their role in drought tolerance.     

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.     

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.     

Interactions Between Meloidogyne incognita and Pratylenchus brachyurus on Soybean

Interactions among Meloidogyne incognita, Pratylenchus brachyurus, and soybean genotype on plant growth and nematode reproduction were studied in a greenhouse. Coker 317 (susceptible to both nematodes) and Gordon (resistant to M. incognita, susceptible to P. brachyurus) were inoculated with increasing initial population densities (Pi) of both nematodes individually and combined. M. incognita and P. brachyurus individually usually suppressed shoot growth of both cultivars, but only root growth on Coker 317 was influenced by a M. incognita × P. brachyurus interaction. Reproduction of both nematodes, although dependent on Pi, was mutually suppressed on Coker 317. P. brachyurus reproduced better on Gordon than on Coker 317 but did not affect resistance to M. incognita. Root systems of Coker 317 were split and inoculated with M. incognita or P. brachyurus or both to determine the nature of the interaction. M. incognita suppressed reproduction of P. brachyurus either when coinhabiting a half-root system or infecting opposing half-root systems; however, P. brachyurus affected M. incognita only if both nematodes infected the same half-root system.     

Response of Resistant Soybean Plant Introductions to Meloidogyne arenaria Races 1 and 2

Resistant plant introductions, PI 230977 and PI 200538, and partially resistant Jackson and susceptible CNS were evaluated for seed yield in response to races 1 and 2 of Meloidogyne arenaria. Initial soil population densities (Pi) of the nematode were 0, 31, 125, and 500 eggs/100 cm³ soil. At the highest Pi, yield suppressions of CNS, Jackson, PI 230977, and PI 200538 were 55, 28, 31, and 29%, and 99, 86, 66, and 58% for races 1 and 2 compared with uninfested controls. Numbers of second-stage juveniles (J2) present in roots 14 days after planting increased as Pi increased, but did not differ between the two races. At the highest Pi, fewer race 1 (40-57%) and race 2 (53-68%) J2 were present in roots of the plant introductions than in roots of Jackson. Soil population densities of race 1 J2 at 135 days after planting were 83-89% lower on the resistant genotypes than on CNS. These numbers did not differ for race 2. Reproductive factors were considerably higher for race 2 compared to race 1 for all genotype by Pi combinations, except for CNS at the highest Pi.