Influence of Selected Cultural Practices on Winter Survival of Pratylenchus brachyurus and Subsequent Effects on Soybean Yield

Planting date of soybean, Glycine max, influenced winter survival of Pratylenchus brachyurus in microplots at two locations in North Carolina. Delayed planting resulted in a linear decrease (P = 0.05) in the numbers of P. brachyurus at soybean harvest. Effects of planting date on nematode numbers persisted over winter, indicating that survival in the absence of a host is density independent. Compared with winter fallow, winter wheat, Triticum aestivum, reduced winter survival of P. brachyurus. Subsequent soybean yields were suppressed by the overwintering population of this nematode at one location but not at another.     

Damage and Reproductive Potentials of Pratylenchus brachyurus and P. penetrans on soybean

Damage and reproductive potentials of Pratylenchus brachyurus and P. penetrans on soybean, Glycine max, cvs. Essex, Forrest, and Lee 68, were determined in microplot tests. Cultivar Essex was generally tolerant to P. brachyurus. Yield of Forrest was suppressed linearly with increasing P_i''s in the sandy soil (r = -0.92) and loamy sand soil (r = -0.99). Low to moderate P_i''s in the sandy clay loam gave an increase in yields as compared to plants without nematodes. Yield was not affected by this nematode in muck. Lee 68 was very sensitive to P. penetrans in microplots. Yield vs. P_i was fitted by a quadratic model (r = 0.82) with yield decreasing sharply as P_i''s were increased. The reproduction of both species decreased with increases in P_i. Lee 68 was a good host for P. penetrans, whereas Essex and Forrest were fair to poor hosts for P. brachyurus.     

Effects and Dynamics of a Nematode Community on Soybean

The relationships between densities of all members of a plant-parasitic nematode community and yield of ''Davis'' soybean and between final and preplant population levels were examined in small plots on sandy soils in north-central Florida. Plant-parasitic nematodes present in the community included Belonolaimus longicaudatus, Criconemella sphaerocephala, Meloidogyne incognita, Paratrichodorus minor, Pratylenchus brachyurus, and Xiphinema sp. Plant growth, including stand count, soybean yield (kg/ha), and size of young plants, was occasionally inversely correlated (P ≤ 0.05) with densities of B. longicaudatus or P. brachyurus, but not with densities of other species or with a range of soil variables. The nature of this relationship varied with season, with more severe stand losses noted during 1987 than in 1988. Final population densities (Pf) of most nematode species showed significant (P ≤ 0.05) linear relationships to densities measured at planting or earlier (Pi). These relationships were stronger (higher r²) with the ectoparasite B. longicaudatus than with the endoparasites M. incognita and P. brachyurus. Criconemella sphaerocephala declined under soybean cultivation, reaching levels near zero after two seasons. A quadratic model showed an improvement (P ≤ 0.05) over the linear model in describing the relationship between Pf and Pi measured at planting for B. longicaudatus, and gave a better indication of the leveling off of Pf at high values of Pi.     

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.     

Selection Against Heterodera glycines Males by Soybean Lines with Genes for Resistance

Soybeans with genes for resistance select against Heterodera glycines with the corresponding genes for avirulence. There may be a differential effect of sex with some specific gene interactions, which would influence the magnitude of gene frequency changes. No effect on H. glycines males was detected with one selected nematode population and the resistant soybean line PI88788. The selective effect of PI89772 against male nematodes was greater with two inbred nematode populations than with one selected (on PI88788) population, presumably due to differences in H. glycines gene frequencies. ''Peking'' also had few males with the one inbred nematode population, whereas Forrest and ''Pickett 71'' had intermediate numbers. Apparently Forrest and Pickett 71 did not get all the Peking genes for resistance that affect male as well as female nematode development. Other H. glycines-soybean genes stop only females, since there were few or no cysts, except on the susceptible soybean Williams. The number of males'' phenotype will help identify specific genes in both organisms.     

Population Dynamics of Plant Nematodes in Cultivated Soil: Effect of Summer Cover Crops in Old Agricultural Land

In a 6-year cover crop sequence study, nematode population densities varied with different cover crops. Millet favored rapid increase of Belonolaimus longicaudatus and supported relatively large numbers of Pratylenchus brachyurus. Beggarweed and ''Coastal'' bermudagrass favored rapid increase of B. Iongicaudatus and to a lesser extent P. brachyurus and Trichodorus christiei. Hairy indigo and Crotalaria supported relatively large numbers of P. brachyurus but suppressed B. longicaudatus. Hairy indigo also supported increases of T. christiei and Xiphinema americanum. Marigold did not favor development of any parasitic nematode species present. Tomato transplant yield was inversely related to nematode population, particularly to B. Iongicaudatus. Largest yields were obtained from plots with smallest numbers of B. longicaudatus and smallest yields were from plots with largest numbers of B. longicaudatus.     

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.     

Influence of Temperature on Population Development of Eight Species of Pratylenchus on Soybean

In a soil temperature study, population increase on ''Clark 63'' soybeatt was most rapid at 30 C in Pratylenchus alleni, P. brachyurus, P. cofleae, P. neglectus, P. scribneri, and P. zeae and at 25 C in P. penetrans and P. vulnus. The last two were the only species that reproduced at 15 C. Populations of all species increased over the range of 20-30 C, except those of P. neglectus at 20 C and P. coffeae, which was not tested below 25 C. Only P. brachyurus, P. neglectus, P. scribneri and P. zeae reproduced at 35 C. At their optimum temperatures, P. scribneri exhibited the greatest population increase, 1248-fold, and P. penetrans the least, 32-fold. This is the first report of soybean as a host for P. vulnus.     

Glomus fasciculatum,a Weak Pathogen of Heterodera glycines

The occurrence ofchlamydospores of Glomus fasciculatum (Gf) within cysts of the soybean cyst nematode, Heterodera glycines, and the effects of vesicular-arbuscular mycorrhizae on nematode population dynamics and soybean (Glycine max) plant growth were investigated. Chlamydospores occupied 1-24% of cysts recovered from field soil samples. Hyphae of Missouri isolate Gfl penetrated the female nematode cuticle shortly after she ruptured the root epidermis. Convoluted hyphae filled infected eggs, and sporogenesis occurred within infected eggs. G. microcarpum, G. mosseae, and two isolates of Gf were inoculated with H. glycines on plants of ''Essex'' soybeans. Each of the two Gf isolates infected about 1% of the nematode eggs in experimental pot cuhures. The Gfl isolate decreased the number of first-generation adult females 26%, compared with the nonmycorrhizal control. The total numbers of first-generation plus second-generation adult females were similar for both Gf isolates and 29-41% greater than the nonmycorrhizal control. Soybean plants with Gf and H. glycines produced more biomass than did nonmycorrhizal plants with nematodes, but only Gfl delayed leaf senescence.     

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.     

Population Dynamics of Plant-parasitic Nematodes on Cover Crops of Corn and Sorghum

Buildup of plant-parasitic nematode populations on corn (Zea mays), soybean (Glycine max), and sorghum (Sorghum bicolor) were compared in 1991 and 1992. Final population densities (Pf) of Meloidogyne incognita were lower following sorghum than after soybean in both seasons, and Pf after sorghum was lower than Pf after corn in 1992. In both seasons, Pf differed among the sorghum cultivars used. No differences in Pf on corn, sorghum, and soybean were observed for Criconemella spp. (a mixture of C. sphaerocephala and C. ornata) or Paratrichodorus minor in either season. Pf levels of Pratylenchus spp. (a mixture of P. brachyurus and P. scribneri) were greatest after corn in 1992, but no differences with crop treatments were observed in 1991. When data from field tests conducted with corn and sorghum during the past four seasons were pooled, negative linear relationships between ln(Pf/Pi) and ln(Pi) were observed for Criconemella spp. and P. minor on each crop, and for M. incognita on corn (Pi = initial population density). Although ln(Pf/Pi) and ln(Pi) were not related for M. incognita with pooled sorghum data, separate relationships were derived for various sorghum cultivars. Regression equations from pooled data were used to obtain estimates of equilibrium density and maximum reproductive rate, and these estimates were used to construct models expressing nematode Pf across a range of initial densities. Many of these models were robust, encompassing a range of sites, season, crop cultivars, and planting dates. Quadratic models derived from pooled field data provided an alternative method for expressing Pf as a function of Pi.     

Influence of Concomitant Pratylenchus brachyurus and Meloidogyne spp. on Root Penetration and Population Dynamics

Populations of Pratylenchus brachyurus on cotton were increased significantly in the presence of either Meloidogyne incognita or M. arenaria.This occurred with either simultaneous inoculation or prior invasion by M. incognita. P. brachyurus penetrated cotton roots previously invaded by, or simultaneously inoculated with, M. incognita, as well as, or better than, in the absence of M. incognita. Prior invasion by M. incognita, however, suppressed P. brachyurus populations on tomato, while it had no effect on alfalfa and tobacco. Populations of M. incognita on cotton were generally inhibited by the presence of P. brachyurus. Simultaneous inoculation with, or previous invasion by, P. brachyurus also inhibited root penetration by M. incognita. These findings emphasize the importance of host susceptibility in the study of concomitant nematode populations.     

Endoparasitic Nematodes in Maize Roots in the Western Transvaal as Related to Soil Texture and Rainfall

Eight endoparasitic nematode species were recovered from 170 maize root samples in western Transvaal, Republic of South Africa. Pratylenchus zeae had the highest average population density (17,454/5 g roots), followed by P. neglectus (5,827/5 g roots), P. penetrans (5,617/5 g roots), P. brachyurus (3,060/5 g roots), Meloidogyne incognita plus M. javanica (301 juveniles/5 g roots), P. crenatus (130/5 g roots), and Rotylenchutus parvus (64/5 g roots). The 17 reasonably homogeneous farming areas (RHFA) surveyed could be ranked on the basis of the incidence of the prevalent nematode species. A positive relationship was found between the incidence of P. brachyurus and R. parvus and long-term average annual rainfall. The incidence of P. penetrans and the Meloidogyne spp. was positively related to a combination of sand percentage and long-term average annual rainfall.     

Effects and Dynamics of a Nematode Community on Maize

Relationships between nematode density and yield and between final and preplant population levels were examined in small maize plots on sandy soils in north-central Florida. Plant-parasitic nematodes present in the community included Belonolaimus longicaudatus, Criconemella sphaerocephala, Meloidogyne incognita, Paratrichodorus minor, Pratylenchus brachyurus, and a Xiphinema sp. Plant growth--including stand count, grain yield, stalk weight, and size of young plants--often was inversely correlated (P ≤ 0.05) with densities of B. longicaudatus and occasionally with P. brachyurus, but not with densities of other species or with a range of soil variables. More severe losses in grain yields from B. longicaudatus occurred in 1987 than in 1988, although mean preplant nematode densities in February were similar in both years (4.4 vs. 3.9/100 cm³ soil). Final population densities of most nematode species were linearly related (P ≤ 0.05) to densities measured at planting or earlier. These relationships were stronger (higher r²) with the ectoparasites B. longicaudatus and C. sphaerocephala than with the endoparasites M. incognita and P. brachyurus. No significant correlations were found between population densities of different nematode species.     

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.     

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.     

Impact of Planting 'Bell', a Soybean Cultivar Resistant to Heterodera glycines, in Wisconsin

Although the soybean cyst nematode (SCN), Heterodera glycines, has been known to exist in Wisconsin for at least 14 years, relatively few growers sample for SCN or use host resistance as a means to manage this nematode. The benefit of planting the SCN-resistant cultivar Bell on a sandy soil in Wisconsin was evaluated in 1992 and 1993. A range of SCN population densities was achieved by planting 11 crops with varying degrees of susceptibility for 1 or 2 years before the evaluation. Averaged over nematode population densities, yield of ''Bell'' was 30 to 43% greater than that of the susceptible cultivars, ''Corsoy 79'' and ''BSR 101''. Counts of cysts collected the fall preceding soybean were more predictive of yield than counts taken at planting. Yields of all three cultivars were negatively related (P < 0.001) to cyst populations. Fewer (P < 0.01) eggs were produced on ''Bell'' than on the susceptible cultivars. The annual (fall to fall) change in cyst population densities was dependent on initial nematode density for all cultivars in 1992 and for the susceptible cultivars in 1993. Yield reductions induced by the SCN under the conditions of this study indicate that planting a SCN-resistant cultivar in Wisconsin can be beneficial if any cysts are detected.     

Population Development of Meloidogyne incognita on Soybean Defoliated by Pseudoplusia includens

Greenhouse studies examined population densities of Meloidogyne incognita race 4 on soybean (Glycine max ''Davis'') defoliated by larvae of soybean looper (Pseudoplusia indudens (Walker)). Plants were defoliated over a 2-week period beginning 5 weeks after seedlings were transplanted. Four groups of plants were infested with nematodes (5,000 eggs/pot) at 2-week intervals to allow harvesting of plants at 0, 2, 4, and 6 weeks postdefoliation (WPD). Plants in each group were harvested 4 weeks after nematode infestation. Root and nodule weights of defoliated plants were suppressed at 0 WPD, but differences were not detectable at 2, 4, and 6 WPD. Population densities of M. incognita were similar on defoliated and control plants at 0 WPD but were greater on defoliated plants at 4 and 6 WPD. Percentage hatching of eggs produced on the latter plants also was higher. Effects of insect-induced defoliation on development of M. incognita remained detectable even after soybean plant growth apparently returned to normal.     

Interaction of Fensulfothion and Phorate with Preemergence Herbicides on Soybean Parasitic Nematodes

The herbicides alachlor, linuron, vernolate, and metribuzin were applied to plots treated with the nematicide fensulfothion or the insecticide phorate and planted to soybean in two locations in North Carolina. In 1976 treatment with fensulfothion + alachlor or vernolate, phorate + alachlor or metribuzin resulted in greater nematode population densities than no treatment, or treatment with fensulfothion alone, or phorate alone. In 1977 fensulfothion and phorate alone and in combination with the preemergence herbicides effectively controlled Tylenchorhynchus cIaytoni. Late season population resurgence of Heterodera glycines occurred in fensulfothion + alachlor treated plots. Correlation coefficients for H. glycines vs. yield were -0.48 (P = 0.05) and -0.46 (P = 0.05) for 30 and 68 d after planting, respectively.     

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.