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.     

Impact of Cotton Production Systems on Management of Hoplolaimus columbus

The effectiveness of selected cultural practices in managing the Columbia lance nematode, Hoplolaimus columbus, on cotton was evaluated in experiments in growers'' infested fields. The effects of planting date, cotton cultivar, treatment with the growth regulator mepiquat chloride, and destruction of cotton-root systems after harvest on cotton-lint yield and population densities of H. columbus were studied. The yield of cotton cultivar Deltapine 50 was negatively related (P = 0.054) to initial population density of H. columbus whereas the yield of Deltapine 90 was not affected by preplant density of this nematode, indicating tolerance in Deltapine 90. Reproduction of this nematode did not differ on the two cultivars. Planting date and treatment with the growth regulator mepiquat chloride did not influence cotton yield in a consistent manner. Application of mepiquat chloride suppressed (P ≤ 0.05) numbers of Columbia lance nematode, although there was an interaction (P ≤ 0.05) with cultivar and year. Early vs. late destruction of cotton-root systems did not impact population densities of this nematode either year, and had no impact on the subsequent cotton crop. The nematicide fenamiphos increased (P ≤ 0.03) cotton yield when H. columbus numbers exceeded the damage threshold.     

Enhanced Hatching of Globodera tabacum solanacearum Juveniles by Root Exudates of Flue-cured Tobacco

Stimulation of hatching of a tobacco cyst nematode (Globodera tabacum solanacearum) by root exudates from resistant NC 567 and susceptible K 326 cultivars of flue-cured tobacco, Nicotiana tabacum, was investigated. Root exudates were collected by soaking seedlings in deionized water for 2 hours at 22 °C in the dark. Fifteen mature and uniformly sized cysts were exposed at 15, 20, or 25 °C to undiluted root exudate, root exudate diluted 1:1 or 1:3 with deionized water, or deionized water alone. Hatched juveniles were counted and removed at weekly intervals during 42 and 53 days of exposure in experiments conducted in 1994 and 1995, respectively. Root exudates from both susceptible cultivar K 326 and resistant cultivar NC 567 stimulated more hatching than deionized water at 25 °C in 1994, and at all three tested temperatures in 1995. In 1994, dilution of root exudates 1:3 reduced stimulation of hatching at 25 °C compared to undiluted exudate. Hatching at 25 °C was similarly stimulated by exposure to undiluted root exudate and exudate diluted 1:1. In 1995, both dilutions reduced stimulation of hatching by root exudates at all the temperatures.     

Effects of Tobacco Cyst Nematode on Growth of Flue-cured Tobacco

The effects of infection by tobacco cyst nematode (Globodera tabacum solanacearum) on growth of flue-cured tobacco cultivars NC 567 (resistant) and K 326 (susceptible) were evaluated in the field in 1993 and 1994. Infection by G. t. solanacearum suppressed number of leaves, plant height, and fresh weight of leaves and feeder roots. Correlations between weekly egg densities of G. t. solanacearum collected from soil and host growth during 11 weeks after transplanting (WAT) were often inconsistent between cultivars and years. However, consistent correlations were obtained between root weight and egg densities collected 9 WAT, as well as between leaf weight from susceptible K 326 and nematode egg densities 6 WAT. Leaf and feeder root weights were significantly correlated with the area under the curve for all nematodes per gram of feeder root for K 326 in 1993 and for both cultivars in 1994. Reduction in feeder root weight by G. t. solanacearum was similar for the resistant and susceptible cultivars. Reduction in fresh leaf weight by G. t. solanacearum was twice as great (P ≤ 0.07) for K 326 as for NC 567 in 1994. Incorporating nematode resistance into germplasm possessing improved yield and quality traits should produce cultivars more acceptable to growers.     

Effect of Planting Date,Alachlor, and Fenamiphos on Heterodera glycines Development

The effects of alachlor (2.25 kg a.i./ha) and fenamiphos (2.25 kg a.i./ha) on the penetration and development of Heterodera glycines were examined on Glycine max cultivars Deltapine 105 planted 29 April, 29 May, and 29 June 1986 and Deltapine 105 and Centennial planted 15 May, 15 June, and 15 July 1987. Penetration was lowest on the third planting of soybeans and on fenamiphos-treated plants. Development from second-stage juveniles to adult females required 270 (1986) and 260 (1987) DD20/32 on roots from the first planting control and alachlor treatments. Fenamiphos, alone or with alachlor, retarded development in Deltapine 105 (1986) and in Centennial (1987). Males matured in roots from the second planting in 190 (1986) and 180 (1987) DD20/32 regardless of treatment or cultivar. No development occurred in roots from the third planting until 400 DD20/32 in 1986, but in 1987 development was similar to that in roots from the second planting. Nematode development was similar in alachlor-treated and control roots regardless of planting date. Fenamiphos restricted nematode penetration on most planting dates and slowed development. Simultaneous applications of alachlor and fenamiphos usually also inhibited development.     

Development of Selected Tobacco Cyst Nematode Isolates on Resistant and Susceptible Cultivars of Flue-cured Tobacco

Tobacco cyst nematode (Globodera tabacum solanacearum) isolates were collected from 11 locations in Virginia, 3 in North Carolina, and 1 in Maryland. Isolates from each location were maintained and increased on flue-cured tobacco, Nicotiana tabacum cv K326. Plants of flue-cured tobacco cultivars K326 (susceptible) and NC567 (resistant) were each inoculated with 6,000 G. t. solanacearum eggs/plant. Tests were conducted over one (6 weeks) or two (14 weeks) generations of the nematode. Shoot and root weights and the number of nematodes present within a 1-g subsample of feeder roots were recorded at completion of the tests. Nematode counts were categorized by nematode life stage (vermiform, swollen, pyriform, and adult). Although significant differences in nematode development were detected among isolates, differences were not consistent across experiments. Results indicate similar virulence among G. t. solanacearum isolates on resistant and susceptible flue-cured tobacco cultivars. Therefore, plant breeders may effectively use a single G. t. solanacearum isolate when screening tobacco germplasm for resistance.     

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.     

Effect of Initial Nematode Density on Managing Globodera rostochiensis with Resistant Cultivars and Nonhosts

Cropping systems in which resistant potato cultivars were grown at different frequencies in rotation with susceptible cultivars and a nonhost (oats) were evaluated at four initial nematode population densities (Pi) for their ability to maintain Globodera rostochiensis at a target level of <0.2 egg/cm³ of soil. At a Pi of 0.1 to 1 egg/cm³ of soil, cropping systems with 2 successive years of a resistant cultivar every 3 years of potato production reduced and maintained G. rostochiensis at <0.2 egg/cm³ of soil. At a Pi of 1 to 4 eggs/cm³ of soil, 2 successive years of a resistant cultivar followed by 1 year of oats for every 4 years of production were necessary to reduce and maintain G. rostochiensis populations at <0.2 egg/cm³ of soil. At a Pi greater than 4 eggs/cm³ of soil, 2 successive years of a resistant cultivar plus 1 year of oats reduced G. rostochiensis densities to <0.2 egg/cm³ of soil, but the population increased above that density after cropping 1 year to a susceptible cultivar. The numbers of cysts and eggs per cyst in the final population (Pf) of G. rostochiensis were influenced by initial density and the frequency of growing a susceptible cultivar in a cropping system. The lowest number of cysts and eggs per cyst in the final G. rostochiensis population occurred with a cropping system consisting of 2 successive years of a resistant cultivar followed by oats with a susceptible cultivar grown the fourth year of production.     

Effects of Initial Nematode Density on Population Dynamics of Globodera rostochiensis on Resistant and Susceptible Potatoes

The influence of resistant and susceptible potato cultivars on Globodera rostochiensis population density changes was studied at different nematode inoculum levels (Pi) in the greenhouse and field. Soil in which one susceptible and two resistant cultivars were grown and fallow soil in pots was infested with cysts to result in densities of 0.04-75 eggs/cm³ soil. A resistant cultivar was grown in an infested field with Pi of 0.7-16.7 eggs/cm³ soil. Pi was positively correlated with decline of soil population densities due to hatch where resistant potatoes were grown in the greenhouse and in the field but not in fallow soil. However, Pi was not correlated with in vitro hatch of G. rostochiensis cysts in water or potato root diffusate. Under continuous culture o f a resistant cultivar, viable eggs per cyst declined 60-90% per plant growth cycle (4 weeks) and the number of cysts containing viable eggs had decreased by 77% after five cycles. The rate of G. rostochiensis reproduction on both resistant and susceptible cultivars was negatively correlated with Pi. These data were used to predict the effect of resistant and susceptible potato cultivars on G. rostochiensis soil population dynamics.     

Factors Affecting Population Trends of Plant-Parasitic Nematodes on Rangeland Grasses

The effects of environmental conditions on population trends of plant-parasitic nematodes were studied in experimental plots of five wheatgrasses in the western Utah desert. In a 3-year (1984-86) field study, soil water and temperature affected the population trends of the ectoparasites, Tylenchorhynchus acutoides and Xiphinema americanum, and the migratory endoparasite, Pratylenchus neglectus, on Fairway crested wheatgrass, Agropyron cristatum; ''Hycrest'' crested wheatgrass, A. cristatum X A. desertorura; ''Rosana'' western wheatgrass, Pascopyrum smithii; ''Oahe'' intermediate wheatgrass, Thinopyrum intermedium; and RS-1 hybrid (Elytrigia repens X Pseudoroegneria spicata). The largest soil populations of these nematode species were collected in 1984 under good plant-growth conditions. A reduction in nematode populations occurred in 1985 and 1986, possibly because of low soil-water conditions. There was a positive relationship between high soil water and maximum population densities of T. acutoides in the spring and fall of 1984, and between low soil water and minimum population densities of the nematode in 1985 and 1986. Pratylenchus neglectus populations were affected by soil water, although to a lesser degree than the ectoparasitic nematodes. Population densities of the three nematode species were significantly lower in the drier years of 1985 and 1986 than in 1984. Nematode populations were greater at the lower soil depths in the fall than in the spring or summer.     

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.     

Quantifying Potential Tolerance of Selected Cotton Cultivars to Belonolaimus longicaudatus

Glyphosate-tolerant cotton cultivars were evaluated for tolerance to Belonolaimus longicaudatus in field experiments conducted from 2004 to 2005. Field trials were arranged in a split-plot design that included treatment with four levels of 1, 3-dichloropropene (0.0, 13.9, 27.8, and 41.7 1 a.i./ha) to establish a range of population densities of B. longicaudatus. Six cotton cultivars (early-to-mid maturity: DP444BG/RR SG501BR, ST5242BR; mid-to late maturity: DP451B/RR, ST5599BR, DP655BRR) were planted as whole plots. Fumigation was effective in suppressing B. longicaudatus population densities at mid-season, but not at cotton harvest, and increased cotton lint yield. The cultivar × fumigation interaction for cotton lint yield was not significant for the six cultivars evaluated, indicating that tolerance did not occur in this nematode-host combination. Early-to-mid maturity cultivars yielded significantly more than mid-to-late maturity cultivars in both years. Small but significant differences in nematode final population density were observed between cultivars that may be related to relative maturity.     

Evaluation of NemX,a New Cultivar of Cotton with High Resistance to Meloidogyne incognita

The level of resistance to root-knot nematode, Meloidogyne incognita, in NemX, a new cultivar of the Acala-type upland cotton, was evaluated in relation to four resistant breeding lines (N6072, N8577, N901, and N903) and four susceptible cultivars (Maxxa, SJ2, Royale, and Prema). In growth pouch tests, an average of only 4 nematode egg masses was produced on roots of NemX or the resistant lines, compared to a significantly higher average of 21 on the susceptible cultivars. In pot tests, the nematode reproduction factor (RF = Pf/Pi) in NemX and the resistant lines averaged 0.7, compared to a significantly higher average of 10 on the susceptible cultivars. Root galling in NemX or other resistant cotton averaged 15%, compared to 74% on the susceptible cultivars, in either pot or field tests. In plots with low levels of nematode infestation (Pi ≤ 150 second-stage juveniles [J2]/500 g soil), lint yield of NemX averaged 1,370 kg/ha and was less than the yield of susceptible Maxxa (1,450 k g /h a ). However, in plots with medium or high levels of nematode infestation (Pi = 151-300 or >300 J2/500 g soil, respectively), yields of NemX decreased only slightly and averaged 1,300 or 1,050 kg/ha, respectively, whereas yields of Maxxa were severely reduced to 590 or 503 kg/ha, respectively. Fusarium wih symptoms were observed on both NemX and Maxxa, and percent occurrence increased with increasing preplant nematode density. In plots with the highest nematode densities, 22% of NemX and 65% of Maxxa plants were wilted. NemX was highly effective against five M. incognita isolates and moderately effective against a sixth isolate that had been exposed to resistant cotton over several seasons. These results showed that NemX is as resistant to M. incognita as the four breeding lines, and much more resistant than the tested susceptible cultivars of cotton.     

Nematicide Efficacy,Root Growth,and Fruit Yield in Drip-irrigated Pineapple Parasitized by Rotylenchulus reniformis

A 3-year field trial near Kunia, Oahu, Hawaii, was conducted to evaluate four nematicide treatments for efficacy against Rotylenchulus reniformis in drip-irrigated pineapple (Ananas comosus L. (Merr.)). The treatments were (A) preplant fumigation with 1,3-dichloropropene (1,3-D) (336 liter/ ha) and postplant drip application of fenamiphos (3.4 kg/ha) with restricted irrigation, (B) preplant 1,3-D only, weekly irrigation, (C) 1,3-D fenamiphos, weekly irrigation, and (D) postplant fenamiphos only, weekly irrigation. Fenamiphos was applied at 3-month intervals for 1 year after planting in three treatments. Although nematode populations increased in all treatments 1 year after planting, no differences in fruit yield were detected among treatments in the first (plant crop) harvest 19 months after planting. In the second (ratoon) crop (33 months after planting) significant yield differences, larger fruit size, and greater root biomass were obtained in the dual nematicide treatments. Root biomass increased continuously throughout the crop cycle, was greatest near the drip line, and showed a shallow depth distribution (30-40 cm). Rotylenchulus reniformis populations and fenamiphos concentrations were negatively correlated in soil profiles taken 13 months after planting. In the absence of postplant fenamiphos applications, nematode numbers were positively correlated with root biomass.     

Identification of Sources of Resistance to Four Species of Root-knot Nematodes in Tobacco

Resistance to the southern root-knot nematode, Meloidogyne incognita races 1 and 3, has been identified, incorporated, and deployed into commercial cultivars of tobacco, Nicotiana tabacum. Cultivars with resistance to other economically important root-knot nematode species attacking tobacco, M. arenaria, M. hapla, M. javanica, and other host-specific races of M. incognita, are not available in the United States. Twenty-eight tobacco genotypes of diverse origin and two standard cultivars, NC 2326 (susceptible) and Speight G 28 (resistant to M. incognita races 1 and 3), were screened for resistance to eight root-knot nematode populations of North Carolina origin. Based on root gall indices at 8 to 12 weeks after inoculation, all genotypes except NC 2326 and Okinawa were resistant to M. arenaria race 1, and races 1 and 3 of M. incognita. Except for slight root galling, genotypes resistant to M. arenaria race 1 responded similarly to races 1 and 3 of M. incognita. All genotypes except NC 2326, Okinawa, and Speight G 28 showed resistance to M. javanica. Okinawa, while supporting lower reproduction of M. javanica than NC 2326, was rated as moderately susceptible. Tobacco breeding lines 81-R-617A, 81-RL- 2K, SA 1213, SA 1214, SA 1223, and SA 1224 were resistant to M. arenaria race 2, and thus may be used as sources of resistance to this pathogen. No resistance to M. hapla and only moderate resistance to races 2 and 4 of M. incognita were found in any of the tobacco genotypes. Under natural field infestations of M. arenaria race 2, nematode development on resistant tobacco breeding lines 81-RL-2K, SA 1214, and SA 1215 was similar to a susceptible cultivar with some nematicide treatments; however, quantity and quality of yield were inferior compared to K 326 plus nematicides.     

Root Growth of Susceptible and Resistant Potato Cultivars and Population Dynamics of Globodera rostochiensis in the Field

Globodera rostochiensis population densities and potato root growth were measured in field plots of one susceptible and two resistant potato cultivars. Root growth and nematode densities were estimated from soil samples taken at three depths between plants within the rows, three depths 22.5 cm from the rows, and at two depths midway between rows (furrows). Four weeks after plant emergence (AE), nematode densities in the rows had declined 68% in plots of the susceptible cultivar and up to 75% in plots of both resistant cultivars. Significant decline in nematode densities in the furrows 4 weeks AE occurred only in plots of the susceptible cultivar. Total decline in nematode density in fallow soil was 50%, whereas in plots of the resistant cultivars, decline was more than 70% in the rows and more than 50% in the furrows. Nematode densities increased in the rows of the susceptible cultivar but declined in the furrows. We conclude that G. rostochiensis decline or increase is correlated with host resistance and the amount of roots present at any particular site.     

Reproductive and Damage Potential of Ditylenchus destructor on Six Peanut Cultivars

Commercial peanut cultivars were evaluated for host suitability and sensitivity to Ditylenchus destructor. All cultivars were susceptible. Approximately 94% of the final population were in the pods. Highest Pf occurred at harvest on early maturing cultivars. Damage occurred on four of six cuttivars at Pi = 100/3 liters of soil and all six cultivars at Pi = 1,000. Norden and Selmani were the most susceptible cultivars. Sellie was the most tolerant and highest yielding cultivar. This cultivar may be the most profitable one for growers.     

Soybean and Maize Cropping Models for the Management of Meloidogyne incognita in the Coastal Plain

Models are presented to describe the influence of rotations of Meloidogyne incognita-susceptible cultivars, resistant cultivars, and maize on postharvest abundance of M. incognita juveniles in the soil. Depending on initial densities of juveniles, monocultured regimes reached equilibrium densities after a few years of 287, 40, and 10 juveniles per 10 cm³ soil for susceptible soybean, resistant soybean, and maize, respectively. Yearly changes in the population density of juveniles due to rotation of these crops were simulated by iterative substitution of the model equations for each crop. A maximum density of 319 per 10 cm³ soil was reached following a susceptible cultivar in a susceptible-resistant soybean rotation. Soybean yield loss estimates are presented for monocultured regimes and for various rotations with maize.     

Impact of Herbicides on Heterodera glycines Susceptible and Resistant Soybean Cultivars

Several abiotic and biotic stresses can affect soybean in a growing season. Heterodera glycines, soybean cyst nematode, reduces yield of soybean more than any other pathogen in the United States. Field and greenhouse studies were conducted to determine whether preemergence and postemergence herbicides modified the reproduction of H. glycines, and to determine the effects of possible interactive stresses caused by herbicides and H. glycines on soybean growth and yield. Heterodera glycines reproduction factor (Rf) generally was less on resistant than susceptible cultivars, resulting in a yield advantage for resistant cultivars. The yield advantage of resistant cultivars was due to more pods per plant on resistant than susceptible cultivars. Pendimethalin reduced H. glycines Rf on the susceptible cultivars in 1998 at Champaign, Illinois, and in greenhouse studies reduced dry root weight of H. glycines-resistant and susceptible cultivars, therefore reducing Rf on the susceptible cultivars. The interactive stresses from acifluorfen or imazethapyr and H. glycines reduced the dry shoot weight of the resistant cultivar Jack in a greenhouse study. Herbicides did not affect resistant cultivars'' ability to suppress H. glycines Rf; therefore, growers planting resistant cultivars should make herbicide decisions based on weeds present and cultivar tolerance to the herbicide.     

Modeling of Yield Loss in Potato Early Dying Caused by Pratylenchus penetrans and Verticillium dahliae

Yield-loss models were developed for potato early dying, caused by an interaction between Verticillium dahliae and Pratylenchus penetrans. Yield data were collected over 5 years (1985-1989) from potato plants grown in microplots infested with V. dahliae and (or) P. penetrans. The model y = b₀ + (1 - b₀)/(1 + [VD/36.7]), where y was the relative yield (with uninfested controls = 1.0) and VD was the preplant density of V. dahliae microsclerotia per cm³ soil, was fitted to the data set. When P. penetrans = 0, b₀ = 0.55 (SE = 0.099), and when P. penetrans > 0, b₀ = 0.23 (SE = 0.035). This model assumed that yield loss was proportional to the concentration of preplant microsclerotia of V. dahliae, and only qualitatively related to presence or absence of P. penetrans. This study contrasts with previous reports that predict yield loss being proportional to preplant population densities of both P. penetrans and V. dahliae.