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Crop Science 41:1843-1849 (2001)
© 2001 Crop Science Society of America

CROP ECOLOGY, MANAGEMENT & QUALITY

Crop Sequence Effects on Soybean Cyst Nematode and Soybean and Corn Yields

Senyu Chen*,a, Paul M. Porterb, Curtis D. Reesec and Ward C. Stienstrad

a Univ. of Minnesota Southern Research and Outreach Center, 35838 120th Street, Waseca, MN 59093
b Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108
c Dep. of Plant Science, South Dakota State Univ., Brookings, SD 57007
d Dep. of Plant Pathology, Univ. of Minnesota, St. Paul, MN 55108

* Corresponding author (chenx099{at}umn.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is a destructive pest of soybean, Glycine max (L.) Merr. Field research was conducted at Waseca and Lamberton, MN, from 1996 through 1999 to evaluate the effect of crop sequence on SCN population density and on crop yields. Cropping sequence treatments were (i) monocultures of corn (Zea mays L.), SCN-resistant soybean, and SCN-susceptible soybean; (ii) susceptible soybean rotated with 1, 2, or 3 yr of corn; (iii) resistant soybean rotated annually with corn; and (iv) rotation of corn–resistant soybean–corn–susceptible soybean. Egg density was determined at sowing and harvest, and crop yields were determined each year. In general, yields of resistant soybean were higher than susceptible soybean. Resistant soybean in annual rotation with corn produced the highest yield, and susceptible soybean in monoculture produced the lowest yield among all treatments. A longer period of corn in rotation resulted in higher yield of subsequent susceptible soybean in most instances. Yields of corn following corn were lower than following soybean. Egg density at the start of the study was 6994 and 14 000 eggs 100 cm-3 soil at Waseca and Lamberton, respectively. Average Pf/Pi (egg density at harvest/egg density at sowing) was 0.59 (0.23–0.86) for corn, 0.49 (0.21–0.73) for resistant soybean, and 3.3 (0.74–9.91) for susceptible soybean for all treatments at the two sites across the 4-yr period. After 3 yr of corn, egg density decreased to 889 and 3695 eggs 100 cm-3 soil at Waseca and Lamberton, respectively. Annual rotation of resistant soybean and corn resulted in the lowest SCN population density and produced the highest yield of both crops.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE SOYBEAN CYST NEMATODE is a destructive pest of soybean. Yield loss to SCN in 1994 was estimated to be $620 million worldwide and $450 million in the USA (Wrather et al., 1997). A recent survey indicated SCN reduced soybean production by about $1500 million in 1996 and in 1997 in the USA (Wrather and Stienstra, 1999). In Minnesota, SCN was first detected in Faribault County in 1978 (MacDonald et al., 1980). Since then, SCN has been detected in at least 53 counties and continues to spread throughout soybean-growing areas in Minnesota.

Growing resistant cultivars is a major SCN management strategy. A number of resistant cultivars in Maturity Groups I and II have been developed for the use in the northern soybean-growing areas of the USA. However, additional strategies are necessary to reduce SCN population density and minimize soybean yield loss (S. Chen, unpublished).

Nematicides have been used to control different nematodes including SCN (Schmitt et al., 1983; Noel, 1987; Sasser and Uzzell, 1991; Smith et al., 1991). Nematicides, however, are not acceptable for control of SCN because they are not cost effective and potentially have negative environmental impacts.

Crop rotation is an effective strategy for SCN management, especially in the southern USA. A number of studies have demonstrated that growing nonhost crops reduced SCN population densities and increased soybean yield (Young and Hartwig, 1992; Koenning et al., 1993; Trevathan and Robbins, 1995; Weaver et al., 1995; Young, 1998; Howard et al., 1998). Two or more years of a nonhost crop resulted in low or undetectable SCN densities and acceptable soybean yields (Francl and Dropkin, 1986; Schmitt, 1991). Koenning et al. (1993) demonstrated that 1 yr of a nonhost crop was sufficient to control SCN with no additional benefit beyond 2 yr of growing a nonhost crop. Soybean cyst nematode survival rate is higher in northern region than in southern regions of the USA (Riggs et al., 2001). Consequently, a longer rotation may be needed for effective management of SCN in the northern soybean-growing regions of the USA.

In a previous study, the effect of long-term corn-soybean rotation on SCN population was investigated in Minnesota (Porter et al., 2001). The crop sequences in that study included 5-yr corn rotated with 5-yr SCN-susceptible soybean, corn-soybean rotation, and monoculture of each crop. Two years of corn generally lowered the number of SCN eggs 100 cm-3 soil from thousands by a factor of 10, and 5 yr of corn lowered number of eggs in 100 cm-3 of soil from the thousands by a factor of 100. In that study, however, there was no soybean crop following 2- to 4-yr corn. In 1996, we initiated an experiment at two sites to determine the effectiveness of rotation involving SCN-resistant and SCN-susceptible soybean with 1 to 3 yr of corn for SCN management in southern Minnesota. In this paper, we report results of the first 4-yr rotation cycle.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Experiment Establishment and Maintenance
The experiments were initiated at two field sites located in southwest (Lamberton) and south central (Waseca) Minnesota in 1996. The Waseca site had been in an annual corn-soybean rotation prior to 1996 and with susceptible soybean in 1995. At Lamberton site, susceptible soybean had been grown for 4 yr prior to the experiment. The soil at the Waseca site was a Webster clay loam (fine-loamy, mixed, mesic Typic Endoaquoll) with 220 g kg-1 sand, 460 g kg-1 silt, 320 g kg-1 clay, 99 g kg-1 organic matter, and 7.8 pH. The soil at the Lamberton site was a Revere clay loam (fine-loamy, mesic Typic Calciaquoll) with 230 g kg-1 sand, 420 g kg-1 silt, 350 g kg-1 clay, 71 g kg-1 organic matter, and 7.7 pH. The SCN population at the Waseca site was classified as race 3 according to the race scheme with the four differential soybean cultivars and lines (Riggs and Schmitt, 1988). The Lamberton population was classified as race 1, but the female index of the population on PI 88788 (Riggs and Schmitt, 1988) was still low (15%).

The experiment consisted of 18 treatments arranged in a randomized complete block design with four replicates. At both sites, the 18 treatments were (i) continuous SCN-susceptible soybean (S); (ii) continuous SCN-resistant soybean (R); (iii) continuous corn (C); (iv) and (v) resistant soybean rotated annually with corn (RC and CR); (vi) and (vii) susceptible soybean rotated with 1-yr corn (SC and CS); (viii–x) susceptible soybean rotated with 2-yr corn (SCC, CSC, CCS); (xi–xiv) susceptible soybean rotated with 3-yr corn (SCCC, CSCC, CCSC, CCCS); and (xv–xviii) a 4-yr rotation of susceptible soybean–corn–resistant soybean–corn (SCRC, CRCS, RCSC, and CSCR) (Tables 1–4). The resistant soybean was ‘Freeborn’ (resistance derived from PI 88788) (Orf and Young, 1997), the susceptible soybean was ‘Sturdy’, and corn hybrids were ‘Pioneer 3730’ during 1996 through 1998 and ‘Dekalb 493sr’ in 1999.


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  		Table 1. Population densities of Heterodera glycines at sowing (Pi) and harvest (Pf) and crop yields as influenced by crop at two sites in Minnesota in 1996.

 

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  		Table 4. Population densities of Heterodera glycines at sowing (Pi) and harvest (Pf) and crop yields as influenced by crop sequences at two sites in Minnesota in 1999.

 

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  		Table 2. Population densities of Heterodera glycines at sowing (Pi) and harvest (Pf) and crop yields as influenced by crop sequences at two sites in Minnesota in 1997.

 

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  		Table 3. Population densities of Heterodera glycines at sowing (Pi) and harvest (Pf) and crop yields as influenced by crop sequences at two sites in Minnesota in 1998.

 
Soybean and corn were sown in May each year at both sites. Plot size was 7.6 m long and 9.2 m wide (12 rows). A conventional tillage regime of fall chisel plowing and spring field cultivation prior to sowing was used. Fertilizers were applied according to the University of Minnesota Soil Testing Service recommendations. Each year, 168 kg N ha-1 for corn following soybeans and 196 kg N ha-1 for corn following corn were applied. No N fertilizer was applied in soybean plots. During 1996 through 1998, weeds were controlled by means of a preemergence application of alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl) acetanilide; Lasso, Monsanto, St. Louis, MO] at 1.37 kg a.i. ha-1 and linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea; Lorox, du Pont, Wilmington, DE) at 1.12 kg a.i. ha-1. In 1999, weeds were controlled with a post-emergence application of sethoxydim [(+/-) 2 [1(ethoxyimino) butyl]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-1-one; Post Plus, BASF, Ludwigshafen/Rhein, Germany) at 0.34 kg a.i. ha-1 and bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide; Basagran, BASF) at 0.82 kg a.i. ha-1. In addition, plots were cultivated or hand-weeded as needed.

Data Collection
A soil sample composed of 20 cores was taken from the two central rows of each plot near the plant root zone to a 20-cm depth with a 2.5-cm-diam soil probe at sowing and harvest. The soil samples were stored at -20° C before being processed. Each soil sample was thoroughly mixed. Cysts were extracted with a hand-decanting method. A subsample of 100 cm3 of soil was placed in a 1-L beaker containing 500-mL of water, soaked for at least 30 min and stirred with a motorized stirrer at about 3000 rpm for 3 to 5 min to break soil aggregates. The soil suspension was washed into a 2-L bucket. After a few seconds, the soil suspension was poured through an 850-µm-aperture sieve "nested" on a 250-µm-aperture sieve. The bucket was filled with a strong jet of water again and the suspension was poured on the sieves. This procedure was repeated at least three times for each soil sample. Cysts with debris and soil particles on the 250-µm-aperture sieve were collected, and the cysts were separated from the soil particles and debris with centrifugation in 76% (w/v) sucrose solution at 1500 g. Eggs were released from the cysts by breaking the cysts in a 40-mL glass tissue grinder (Fisher Scientific, Pittsburgh, PA). The egg suspension was poured through a 70-µm-aperture sieve nested on a 38-µm-aperture sieve. Eggs were caught on the 38-µm-aperture sieve and collected into a 50-mL tube and stored at 4°C before being counted within 2 wk. If the egg samples could not be counted within 2 wk, they were stored in a freezer until counted.

Yield of corn and soybean was measured from a 6.7-m length of the two central rows with a small plot combine. The soybean yield was standardized at 130 g kg-1 moisture, and corn yield was standardized at 155 g kg-1.

Data Analysis
For plots in which the egg density at sowing was more than 500 eggs 100 cm-3 soil, the SCN population change (Pf/Pi = egg density at harvest/egg density at sowing) was computed. Nematode egg counts were transformed with log10(x + 1) and Pf/Pi with log10(x) to improve homogeneity of variances before analyses of variance (ANOVA). Average Pf/Pi for each crop were back-transformed. Yields were not transformed for the ANOVA. Least significant difference (LSD, P = 0.05) was used to compare means. Regressions of Pf/Pi against Pi with the linear model ln = ln + bln derived from the equation = aPib were performed to determine relationships between the population change rate during the growing season and the initial nematode density, and to determine an equilibrium density (Ferris, 1985). Predicated equilibrium (E = carrying capacity) was determined by solving the equation ln = ln + bln when Pi = Pf then E = Pi = a1/-b. The relationships between soybean yield and Pi were determined by regression with the equation Y = aebPi or ln = ln + bPi (where Y is soybean yield, Pi is egg density at sowing, a is maximum yield, e is base of the natural logarithm, and b is rate parameter) modified from Appel and Lewis (1984) without determining minimum yield.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
SCN Egg Density
Waseca
In 1996, SCN population density increased from 7803 to 14690 eggs 100 cm-3 soil after one season of susceptible soybean (Table 1). In contrast, corn and resistant soybean reduced SCN population density from 7013 to 2257 and from 5583 to 1425 eggs 100 cm-3 soil, respectively.

In 1997, Pf in susceptible soybean (CS or SS) was not significantly different from the sequence of susceptible soybean in 1996 and corn in 1997 (SC), and these rotations supported higher Pf than sequences without susceptible soybean in 1996 and 1997 (CC, RC, CR, and RR) (Table 2).

In 1998, Pf in susceptible soybean (—S = CCS, RCS, SCS, and SSS) ranged from 13 259 to 19 823 eggs 100 cm-3 soil, and were higher than any other sequence. There was no significant difference among Pf in susceptible soybean (—S) (Table 3). The sequences in which susceptible soybean was grown either in 1997 or 1996 (CSC, SCC, and SCR) often resulted in higher egg density than the sequence without susceptible soybean (CCC, CRC, RCR, and RRR). The sequence in which resistant soybean rotated annually with corn (CRC and RCR) resulted in the lowest egg density, with values less than 300 eggs 100 cm-3 soil (Table 3).

In 1999, the Pf in susceptible soybean (—S = SSSS, CSCS, SCCS, CCCS, and CRCS) ranged from 8206 to 20 913 eggs 100 cm-3 soil, and they were not significantly different regardless of the different crops in the previous 3 yr (Table 4). Egg densities after one season of corn following susceptible soybean (—SC = SCSC, CCSC, and RCSC) were still high (6575–11 988 eggs 100 cm-3 soil) and not significantly different from the sequences with susceptible soybean in 1999 (—S) (Table 4). Two-year corn (CSCC) resulted in a Pf of 856 eggs 100 cm-3 soil, which was lower than sequences with 1-yr corn (—SC) or susceptible soybean in 1999 (—S). The SCN population density after 3-yr corn (SCCC) was 1938 eggs 100 cm-3 soil and not significantly different from that after 2-yr corn (CSCC). The lowest Pf was observed in corn monoculture (CCCC) or corn annually rotated with resistant soybean (RCRC and CRCR). The Pf in monoculture of resistant soybean (RRRR) was higher than in resistant soybean annually rotated with corn (CRCR and RCRC).

Lamberton
Overall, the nematode egg density was higher at the Lamberton site than at the Waseca site. In 1996, SCN population density increased from 14 206 to 30 306 eggs 100 cm-3 soil in susceptible soybean but decreased from 18 458 to 11 011 in resistant soybean and from 12 623 to 10 768 eggs 100 cm-3 soil in corn (Table 1).

In 1997, the highest Pf (14 210 eggs 100 cm-3 soil) was observed in the sequence with susceptible soybean in 1996 and corn in 1997 (SC) (Table 2). Susceptible soybean in monoculture (SS) reduced nematode density from 20 780 to 3800 eggs 100 cm-3 soil, whereas susceptible soybean following corn (CS) increased egg density from 6474 to 9031 eggs 100 cm-3 soil. The reduction of egg density in susceptible soybean may have been caused by early-season flooding, which severely damaged soybean growth. The resistant cultivars (RR and CR) also reduced SCN egg density.

Because of the flooding effect in 1997 on the SCN population, interpreting data of the nematode population in the following years was complicated. Nevertheless, treatment effects on the SCN population were obvious in 1998 and 1999. In 1998, Pf in susceptible soybean (—S) ranged from 25 068 to 40 142 eggs 100 cm-3 soil and were generally higher than other sequences (Table 3). The Pf in resistant soybean (RCR, SCR, and RRR) ranged from 6573 to 9523 eggs 100 cm-3 soil and did not significantly differ from 1 or 2 yr of corn following susceptible soybean (11 200 for CSC and 15 816 eggs 100 cm-3 soil for SCC). The lowest Pf was observed in monoculture of corn (CCC) and the rotation of corn-resistant soybean-corn (CRC).

In 1999, the Pf in susceptible soybean (—S) were 15 250 to 33 350 eggs 100 cm-3 soil, which did not significantly differ from 1-yr corn following susceptible soybean (—SC) (Table 4). The Pf in 3-yr corn (SCCC) was 8219 eggs 100 cm-3 soil; it was significantly different from 1-yr corn following susceptible (—SC), but not from 2-yr corn (CSCC). The sequence of 2-yr corn (CSCC), however, resulted in final egg density lower than the sequence of 1-yr corn (—SC) or susceptible soybean in 1999 (—S). As at Waseca, the lowest egg density was observed in corn monoculture (CCCC) or corn annually rotated with resistant soybean (RCRC or CRCR). The Pf in monoculture of resistant soybean (RRRR) was numerically higher than resistant soybean annually rotated with corn (CRCR and RCRC), but the difference was not statistically significant at P = 0.05.

SCN Population Change during a Single Season
The Pf/Pi in corn ranged from 0.23 to 0.86, except the 1998 Lamberton site, in which Pf/Pi was 1.24 (Table 5). The Pf/Pi in resistant soybean was similar to corn, ranging from 0.21 to 0.73 (1.92 in 1988 at Lamberton). The Pf/Pi in susceptible soybean ranged from 1.76 to 9.91 and was higher than in corn or resistant soybean in all years at both sites except 1997 at the Lamberton site where soybean plants were damaged by flooding (Table 5).


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  		Table 5. Population changes of Pf/Pi (egg density at harvest/egg density at sowing) of Heterodera glycines in corn and soybean crops at two sites in Minnesota in 1996–1999.

 
The Pf/Pi in susceptible soybean was negatively related to Pi (except at 1997 Lamberton) (Table 6). At the Waseca site, the predicted population density at the equilibrium point (or carrying capacity) in susceptible soybean was 13 791, 4872, 30 628, and 17 558 eggs 100 cm-3 soil in 1996, 1997, 1998, and 1999, respectively. At the Lamberton site, the carrying capacity of SCN in susceptible soybean was 37 269, 34 838, and 22 779 eggs 100 cm-3 soil in 1996, 1998, and 1999, respectively (Table 6).


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  		Table 6. Relationship between reproduction factor and initial population density of Heterodera glycines in susceptible soybean.{dagger}

 
Crop Yield
Soybean Yield
Resistant soybean generally produced higher yield than susceptible soybean (Tables 1–4). This difference was especially obvious in 1996 when the egg density at sowing was high and similar among treatments (Table 1).

At the Waseca site, resistant soybean produced 386 and 474 kg ha-1 higher yield than susceptible soybean in 1996 and 1997, respectively (Tables 1 and 2). In 1998 resistant soybean in monoculture (RRR) or rotated annually with corn (RCR) produced higher yield than susceptible soybean in monoculture (SSS), or in annual rotation with corn (SCS) (Table 3). Yield of susceptible soybean following 2-yr corn (CCS) was higher than following 1-yr corn (SCS). In 1999, resistant soybean in annual rotation with corn (CRCR) produced the highest yield (3614 kg ha-1), although it was not significantly different from resistant soybean in other sequences (RRRR and CSCR) or from susceptible soybean in rotation with corn and resistant soybean (CRCS) (Table 4). Susceptible soybean in monoculture (SSSS) produced the lowest yield (1549 kg ha-1). No difference in yield of susceptible soybean was observed among rotations with 1-yr corn (CSCS), 2-yr corn (SCCS), and 3-yr corn (CCCS).

At the Lamberton site, resistant soybean produced 503 kg ha-1 higher yield than susceptible soybean in 1996 (Table 1). In 1997, resistant soybean following corn produced the highest yield (1808 kg ha-1), and susceptible soybean in monoculture produced the lowest yield, only 386 kg ha-1 (Table 2). One-year corn increased the soybean yield for both resistant and susceptible cultivars in the following year (CR vs. RR; CS vs. SS). In 1998, susceptible soybean in monoculture (SSS) produced lower yield than any other treatment (Table 3). Yield of resistant soybean in monoculture (RRR) was lower than the yield of resistant soybean in rotation with corn and susceptible soybean (SCR) and yield of susceptible soybean following 2-yr corn (CCS). In 1999, susceptible soybean in monoculture (SSSS) produced only 959 kg ha-1 and was lower than other treatments except soybean rotated with 2-yr corn (SCCS), which produced 1186 kg ha-1 (Table 4). No significant difference in soybean yield was observed among other treatments.

Yield of susceptible soybean was negatively related with the egg density at sowing. Statistical significance (P < 0.05) of the relationship, however, was observed only in 1998 and 1999 at Waseca, and 1996 and 1999 at Lamberton (Table 7). Yield of resistant soybean was also reduced with increasing egg density at sowing in 1998 and 1999 at Waseca, and in 1997 at Lamberton (Table 7).


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  		Table 7. Relationship between soybean yield and egg densities of Heterodera glycines at sowing.{dagger}

 
Corn Yield
Response of corn yield to the crop sequence varied between the two sites and among the years. At the Waseca site, no significant difference in corn yield was observed among the crop sequences in 1997 (Table 2). In 1998, corn annually rotated with resistant soybean (CRC) produced higher yield than corn following corn (SCC and CCC) (Table 3). Corn following susceptible soybean (CSC) produced 1537 kg ha-1 higher yield than corn in monoculture. In 1999, corn following resistant soybean (SCRC and RCRC) produced higher yield than other treatments (Table 4). The third year of corn (SCCC) produced the lowest yield.

At the Lamberton site, corn following soybean (RC and SC) produced 1674 kg ha-1 higher yield than corn following corn (CC) in 1997 (Table 2). No difference in corn yield was observed among the treatments in 1998 (Table 3). In 1999, corn yield following soybean (—RC and —SC) was 1341 kg ha-1 higher than yield of corn following corn (—CC) (Table 4).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In this study, we demonstrated that annual rotation of susceptible soybean with corn was not an effective method in managing SCN. If the egg density after harvest of susceptible soybean is 20 000 eggs 100 cm-3 soil, it may take 5 yr of corn to reduce SCN density to a level that does not cause significant damage to a susceptible soybean. This result contrasts with a report from the southern USA in which 2 yr of nonhost crop reduced SCN egg density to a barely detectable level (Koenning et al., 1993), probably because of higher mortality of the nematode in southern regions (Riggs et al., 2001). Corn is one of the predominant crops in southern Minnesota. Replacing corn with other more effective nonhost crops on a large scale for managing SCN in the region is not practical for agronomic and market concerns. Furthermore, increasing the number of year of corn in a rotation sequence to reduce SCN is also unacceptable for two major reasons: soybean is a predominant crop because of its good market value, and there is a yield penalty for corn following corn (Crookston et al., 1991; Porter et al., 1997).

Egg density decreased with increasing number of corn years within a rotation sequence, and soybean yield was negatively related to egg density. Therefore, we would expect a susceptible soybean to produce higher yield in the sequences following 2 yr (SCCS) or 3 yr (CCCS) than 1 yr (-SCS) of corn. However, soybean yield benefit by growing more years of corn in the preceding year was observed in some, but not all, years (Tables 1–4), indicating other factors were involved in the rotation effect on soybean yield.

Growing resistant cultivars was the most effective means to lower SCN population density and increase soybean yield. Therefore, an annual rotation of corn with resistant soybean would be a good choice for SCN management before a susceptible soybean cultivar is grown. On the basis of the data of this study and a previous one (Chen et al., 2001a), if egg density is 20 000 eggs 100 cm-3 soil after growing a susceptible soybean, a 5-yr annual rotation of corn with resistant soybean cultivars is needed before the next susceptible soybean could be grown without significant yield loss. If the egg density is 5000 eggs 100 cm-3 soil, a 3-yr rotation of corn-resistant soybean-corn would be adequate to lower SCN density to where a susceptible soybean could be grown with limited or without yield loss. More years of a resistant cultivar may not further reduce the SCN density because the resistant cultivar supported a limited reproduction.

For an effective use of resistant cultivars in rotation, cultivars with different resistant sources or with different resistant genes should be included in the rotation. If a single resistant cultivar is used in the same field over years, genetic compositions of the SCN populations in the field may be changed (Young, 1984). There was a trend indicating that SCN reproduction potential increased in the resistant cultivar Freeborn. Further study, however, is needed to quantify reproduction potential of the SCN populations in soil where resistant soybean has been grown for several years.

Although resistant soybean produced higher yield than susceptible soybean, yield loss of both resistant and susceptible cultivars to SCN was evident in this study (Table 7). The Pearson correlation coefficients for the relationship between soybean yield and SCN was low (Table 7), indicating that other factors were involved in the variation of soybean yield. This phenomenon was also observed in a previous study (Chen et al., 2001b). The damage to soybean by SCN was more severe at Lamberton in 1997 when plots were flooded than other years without severe flooding. In the wet soil, the nematode penetration or infection may have increased root-rot diseases and caused more damage to soybean plants, a phenomenon that has been demonstrated in a previous greenhouse study (Adeniji et al., 1975). Consequently, there was a greater difference in soybean yield between high egg density and lower egg density (CR vs. RR; and SS vs. CS).

This study showed that SCN reproduction during the growing season was density dependent, a phenomenon that has been described previously (Ferris, 1985). Equilibrium of SCN egg density in soil grown with susceptible soybean was generally more than 10 000 eggs 100 cm-3 soil (Table 6). One season of susceptible soybean could increase egg density from hundreds 100 cm-3 soil to near the equilibrium density. Consequently, there was no difference in egg density at the end of season in susceptible soybean following 1 to 3 yr of corn. Crop rotation with corn or resistant soybean to lower SCN density resulted in higher yield of the resistant soybean and susceptible soybean in the following season, but it did not add any benefit to SCN management for future seasons of susceptible soybean. After one season of susceptible soybean, an extensive rotation with nonhost and resistant soybean was again needed before another crop of susceptible soybean can be grown in order to keep SCN population density low.

The equilibrium egg density is dependent on a number of factors, including environmental conditions, soybean growth, biological antagonists, and nematode fecundity. Further study is needed to determine key factors affecting SCN reproduction and survival to find ways to reduce the SCN population equilibrium and increase the efficiency of crop rotation in SCN management. It appeared that SCN egg density in corn-growing seasons declined faster in a previous study (Porter et al., 2001) than in this study. In the previous study, 2 yr of corn generally lowered egg density from thousands to below 200 eggs 100 cm-3 soil. The reason for the difference in SCN survival between the two studies is unclear. Whether fungal parasites of eggs was a major factor in lowering SCN egg density at the two sites in the previous study (Porter et al., 2001) as compared with the two sites in this study could not be determined or eliminated.


   ACKNOWLEDGMENTS
 
The authors thank S.R. Quiring, J. Jin, E.A. Senst, and J.G. Ballman for technical assistance and R. McSorley and R. Nyvall for critical review of the manuscript.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
This research was supported by Minnesota Soybean Producers Check-off Funding through Minnesota Research and Promotion Council and Minnesota Agric. Exp. Stn.

Received for publication February 8, 2001.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome