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Published online 11 April 2006
Published in Agron J 98:569-578 (2006)
DOI: 10.2134/agronj2005.0185
© 2006 American Society of Agronomy
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Production Papers

Rotation Crop Evaluation for Management of the Soybean Cyst Nematode in Minnesota

D. R. Millera, S. Y. Chena,*, P. M. Porterb, G. A. Johnsona, D. L. Wyseb, S. R. Stetinac, L. D. Klossnerc and G. A. Nelsond

a Univ. of Minnesota Southern Research and Outreach Center, 35838 120th St., Waseca, MN 59093
b Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108
c Univ. of Minnesota Southwest Research and Outreach Center, Lamberton, MN 56152
d Univ. of Minnesota West Central Research and Outreach Center, Morris, MN 56267; S.R. Stetina, current address: USDA-ARS-MSA Crop Genetics and Production Research Unit, Stoneville, MS 38776

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

Received for publication June 20, 2005.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Crop rotation is an effective tactic for soybean cyst nematode (SCN) management. In the North Central region of the USA, corn is almost exclusively used as a nonhost rotation crop with soybean. This study was conducted to determine the effectiveness of crops common to or having potential use in the North Central region as rotation crops for managing SCN. Sixteen potential rotation crops and SCN-resistant and susceptible soybeans were grown along with six fallow controls in three commercial field sites near Waseca, Lamberton, and Morris, MN, in 2001, and SCN-susceptible soybean was grown on all plots in 2002. Nematode populations at planting, midseason, and harvest were measured both years; soybean yield was measured in 2002. There was large variability in SCN populations and soybean yields at the three sites. Nevertheless, significant treatment effects were detected at all sites. While all of the rotation crops lowered SCN populations compared with SCN-susceptible soybean, there were only subtle differences among the individual rotation crops and among different groups of the crops. Leguminous nonhosts or poor hosts were best in reducing SCN population density. Corn, the most common rotation crop in Minnesota, was among the least effective in reducing nematode populations. There was an undetectable yield benefit from SCN management, although differences in yield were observed among the rotation crop treatments—probably due to agronomic factors. The data suggest that a single year of rotation of soybean with any of these crops before planting a susceptible soybean may not be sufficient in managing SCN.

Abbreviations: PCF, population change factor • Pi01, Pm01, Pf01, Pi02, Pm02, and Pf02, soybean cyst nematode egg population density (eggs per 100 cm3) at planting, midseason, and harvest in 2001, and at planting, midseason, and harvest in 2002, respectively • SCN, soybean cyst nematode


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE soybean cyst nematode, Heterodera glycines Ichinohe, was first detected in Minnesota in 1978 (MacDonald et al., 1980). Since then, the SCN has been detected in most (55) counties in southern and central Minnesota where soybean [Glycine max (L.) Merr.] is grown. The nematode has become a major yield-limiting factor in the state (Chen et al., 2001a). Management of the nematode has been dependent on planting resistant cultivars and the use of crop rotations (Schmitt, 1991; Niblack and Chen, 2004).

A number of studies have reported on the effect of rotation crops on SCN populations and soybean yields (Ross, 1962; Weaver et al., 1988; Edwards et al., 1988; Rodriguez-Kabana et al., 1991; Weaver et al., 1993; Koenning et al., 1993; Hershman and Bachi, 1995; Koenning et al., 1995; Howard et al., 1998; Long and Todd, 2001; Chen et al., 2001c; Noel and Wax, 2003). In these studies, however, only one or a few nonhost (mainly corn [Zea mays L.], wheat [Triticum aestivum L.], and sorghum [Sorghum bicolor (L.) Moench] or poor-host crops were compared with soybean. In general, SCN population densities following a nonhost or poor host were lower than following soybean. The effectiveness of crop rotation depends on the host status of crop species, the number of years of rotation crops, and geographical location. For example, in North Carolina, 1 to 2 yr of a nonhost in a rotation was generally sufficient to lower SCN population density to below damaging levels (Schmitt, 1991; Koenning et al., 1993). In contrast, 5 yr of nonhost and SCN-resistant soybean may be needed in Minnesota to reduce the SCN population density to a low level where a susceptible cultivar can be grown without significant yield loss (Chen et al., 2001c). Although most nonhost species tested in fields had similar influence on mortality (Niblack and Chen, 2004), variations in effects of crop species on the SCN have been reported from greenhouse (Riga et al., 2001) and field (Rodriguez-Kabana et al., 1991) studies. Annual ryegrass (Lolium multiflorum Lam.) was more effective than other nonhosts in reducing infectivity of soybean by SCN (Riga et al., 2001). In a field study, corn appeared to be more effective than sorghum in lowering SCN second-stage juvenile population densities at the end of the following soybean season (Rodriguez-Kabana et al., 1991).

The mechanisms through which rotation crops affect SCN populations are not fully understood. Some nonhost and poor-host crops may be effective in lowering nematode population densities by producing root exudates or decomposition products toxic to the nematodes. For example, Brassica spp., such as cabbage, rapeseed, and mustard, produce chemicals as they decompose that are toxic to nematodes (Ellenby, 1945; Mojtahedi et al., 1993; Donkin et al., 1995). Also, phenolic acids from some cereal crops such as wheat can be involved in reducing SCN population densities (Hershman and Bachi, 1995; Blum, 1996). A poor-host crop such as pea (Pisum sativum L.) may stimulate SCN to hatch, but the nematodes may not be able to reproduce well (Sortland and MacDonald, 1987; Schmitt and Riggs, 1991). Therefore, growing a poor-host crop may reduce SCN population density (Chen et al., 2001b). Riga et al. (2001) looked at the potential of plant residues and plant root exudates to protect soybean from SCN and found that incorporation of residues from a number of plant species into the soil reduced nematode population densities compared with incorporation of soybean residues alone.

In southern Minnesota, corn is almost exclusively used as the nonhost crop in rotation with soybean. The SCN egg densities were reduced 20 to 80% during a year when corn was grown (Chen et al., 2001c). The overwinter survival rate of SCN is high in the northern regions of the USA (Riggs et al., 2001), however, and consequently more frequent use of nonhost crops is necessary compared with the southern USA. Increasing the number of years of corn in a rotation sequence to reduce SCN is not advisable due to the yield penalty associated with corn following corn (Crookston et al., 1991; Porter et al., 1997; Porter et al., 2001; Chen et al., 2001c). Therefore, a need exists to find alternative, economically acceptable nonhost crops for use in rotation with soybean for long-term effective management of the nematode. Field crops commonly produced in Minnesota that were classified as nonhost or poor-host crops for the SCN include alfalfa (Medicago sativa L.), barley (Hordeum vulgare L.), canola (Brassica napus L.), corn, sorghum, oat (Avena sativa L.), pea, potato (Solanum tuberosum L.), rye (Secale cereale L.), red clover (Trifolium pretense L.), sugarbeet (Beta vulgaris L.), sunflower (Helianthus annuus L.), and wheat (Riggs, 1992; Riggs and Hamblen, 1962, 1966). Our objective was to evaluate crops common to Minnesota for their potential use as rotation crops with soybean in the management of the SCN.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Field Sites
This research was conducted on three commercial farms in south-central (Waseca), southwest (Lamberton), and west-central (Morris) Minnesota in 2001 and 2002. At each location, a field was selected and planted with SCN-susceptible soybean in 2000. In the spring of 2001, the Waseca, Lamberton, and Morris fields had natural SCN infestations of 4 120, 20 700, and 26 300 SCN eggs per 100 cm3 of soil, respectively (Tables 1, 2, and 3). The soil at the Waseca site is Nicolett clay loam (fine-loamy, mixed, superactive, mesic Aquic Hapludoll) and Canisteo clay loam (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquoll). The soil at the Lamberton site is a Canisteo clay loam, and the soil at the Morris site is a Hamerly clay loam (fine-loamy, mixed, superactive, frigid Aeric Calciaquoll).


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  		Table 1. Population density of the soybean cyst nematode Heterodera glycines at planting, midseason, and harvest in 2001 (Pi01, Pm01, and Pf01, respectively) and 2002 (Pi02, Pm02, and Pf02, respectively) in response to rotation crops in Minnesota–Waseca site.{dagger}

 

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  		Table 2. Population density of the soybean cyst nematode Heterodera glycines at planting, midseason, and harvest in 2001 (Pi01, Pm01, and Pf01, respectively) and 2002 (Pi02, Pm02, and Pf02, respectively) in response to rotation crops in Minnesota–Lamberton site.{dagger}

 

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  		Table 3. Population density of the soybean cyst nematode Heterodera glycines at planting, midseason, and harvest in 2001 (Pi01, Pm01, and Pf01, respectively) and 2002 (Pi02, Pm02, and Pf02, respectively) in response to rotation crops in Minnesota–Morris site.{dagger}

 
Experimental Design
The experiment consisted of 24 treatments in a completely randomized block design with six replicates. The 24 treatments were combinations of crops and fallow with appropriate herbicides (Table 1). The experimental unit was a 4.57 by 3.05 m plot. The 16 crops commonly produced in Minnesota or having potential use in the state were selected as rotation crops for this study: barley, flax (Linum usitatissimum L.), oat, sorghum, wheat, buckwheat (Fagopyrum sagittatum Gilib), canola, corn, rye, sugarbeet, potato, sunflower, alfalfa, hairy vetch (Vicia villosa Roth), red clover, and pea. The controls included an SCN-resistant soybean cultivar (Pioneer 9234), an SCN-susceptible soybean cultivar (Parker), fallow with each herbicide commonly used for these crops (fallow with Buctril [bromoxynil (3,5-dibromo-4-hydroxybenzonitrile)], fallow with Liberty [glufosinate (2-amino-4-(hydroxymethylphosphinyl)butanoic acid)], fallow with Prowl [pendimethalin (N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine)], and fallow with Pursuit [imazethapyr (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid)]), fallow with hand weeding, and fallow without weed control.

Plot Establishment and Maintenance
The rotation crops were planted in mid-May 2001 except for rye, which was planted in the fall following corn harvest. Barley, flax, oat, alfalfa, hairy vetch, red clover, rye, and pea were all planted with 25-cm row spacing, while sorghum, corn, sugarbeet, potato, sunflower, and soybean were planted with 76-cm row spacing. The pre-emergence herbicide Prowl (pendimethalin, 1.388 kg a.i. ha–1 [N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine]) was applied before planting, and Buctril (0.280 kg a.i. ha–1), Liberty (0.410 kg a.i. ha–1), or Pursuit (0.070 kg a.i. ha–1) postemergence herbicides were applied 5 wk after planting to the appropriate crops. The corn (Pioneer 37H26 LL), sugarbeet (2012 LL), and canola (InVigor 2573) cultivars were Liberty tolerant. No postemergence herbicide was used on potato and sunflower plots. The corn, sunflower, sugarbeet, potato, and soybean crops were harvested. Residue from all crops was mowed in October and incorporated into the soil with a rototiller at the Waseca and Lamberton sites and with a moldboard plow at the Morris site. In early May 2002, rye in the corn–rye treatment was rototilled to incorporate the residue, and all treatments were field cultivated before planting. In mid-May, the SCN-susceptible soybean ‘Pioneer 92B36’ was planted at Waseca and Lamberton and ‘Asgrow 1602’ was planted at Morris in all plots at 76-cm row spacing. Both cultivars were resistant to Roundup (glyphosate [N-(phosphonomethyl) glycine]), so Roundup at 0.683 kg a.i. ha–1 was applied in mid-July to control weeds.

Nematode Population and Yield Measurements
Nematode egg densities were determined at planting (Pi), at midseason (Pm, 2 mo after planting), and at harvest (Pf) both years. In 2001, a composite soil sample consisting of 20 cores was taken with a 2.5-cm-diam. soil probe to a 20-cm depth across the central area of approximately 3.5 by 1.5 m of each plot. In 2002, the soil samples were taken from near the soybean root zone of the two central rows of each plot. The soil samples were stored in a cool room (4°C) before being processed. Each soil sample was thoroughly mixed and cysts were extracted from a subsample of 100 cm3 of soil with a semiautomatic elutriator (Byrd et al., 1976) and separated from soil particles and debris with centrifugation in a 63% (w/v) sucrose solution. Eggs were released from the cysts mechanically (Faghihi and Ferris, 2000) and collected in a 50-mL tube. The number of eggs was counted in 0.5 to 2.0 mL of the egg suspension, depending on the number of eggs, and the total number of eggs in 100 cm3 of soil was derived. The nematode population density was expressed as number of eggs per 100 cm3 of soil. To determine nematode population change during the crop season, population change factors (PCF) were computed. The PCF at midseason 2001, at harvest 2001, and at planting 2002 were determined by dividing the egg densities from Pm01 (at midseason in 2001), Pf01 (at harvest in 2001), and Pi02 (at planting in 2002), respectively, by the egg densities from Pi01(at planting in 2001). The PCF at harvest of soybean in 2002 was determined by dividing the egg densities from Pf02 (at harvest in 2002) by the egg densities from Pi02.

Soybean yields were measured in 2002 from a 4.57-m length of the two central rows with a small plot combine. The soybean yield was standardized at 130 g kg–1 moisture.

Data Analysis
The data were initially analyzed using SAS repeated measures ANOVA with whole plots at the three locations, blocks within locations, and treatments within blocks. The repeated measure was conducted on log10(x + 1)-transformed nematode population densities and PCFs. There was no transformation for yield data. All sampling date effects, date x location, date x treatment, and date x location x treatment interactions were highly significant (P < 0.0001), so we continued the analysis of each date and location separately, treating the study as a randomized complete block design. At Lamberton, severe Fe-deficiency chlorosis affected late season growth across two blocks and consequently these two blocks were removed from the data set (Pm02, Pf02, PCF at harvest, and soybean yield in 2002) before analysis. Means of individual treatments were compared using Tukey's Studentized Range (HSD) test at {alpha} = 0.05. To determine differences among groups of crop treatments, the data were averaged in four groups: (i) monocots (barley, oat, sorghum, wheat, corn, and corn–rye); (ii) nonleguminous dicots (flax, buckwheat, canola, sugarbeet, potato, and sunflower); (iii) leguminous nonhosts or poor hosts (alfalfa, red clover, and pea); and (iv) fallows with herbicide treatments, and contrasts were performed. Soybean and hairy vetch were hosts of the nematode and were excluded from any of the groups. Regressions of PCF at 2002 harvest against Pi02 with the linear model ln(PCF) = ln(a) + b ln(Pi02) derived from the equation PCF = a(Pi02)b (Ferris, 1985) were performed to determine relationships between PCF during the soybean-growing season and the initial nematode density, and to determine an equilibrium density on susceptible soybean. In this analysis, because there was no significant interaction between Pi02 and location, the data of the three sites were pooled together.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Soybean Cyst Nematode Population Densities
The SCN population densities at Waseca, Lamberton, and Morris are presented in Tables 1, 2, and 3, respectively. At Waseca, Pf01 and Pi02 in the rotation crop treatments except for flax, corn, corn–rye, and hairy vetch were significantly lower than that following susceptible soybean (Table 1). The Pm02 in the hairy vetch treatment was lower than in the sorghum and wheat treatments. No differences were observed among the fallows with different weed-control treatments. Contrast analysis showed that Pm01 and Pf01 were lower in leguminous nonhosts and poor hosts than monocots, and Pf01 was also lower in leguminous nonhosts and poor hosts than nonleguminous dicots. Nonleguminous dicots resulted in lower Pm01 than monocots (Table 1); however, the Pi01 were lower in plots of nonleguminous dicots and the plots of leguminous nonhosts and poor hosts than the monocots and fallow plots, suggesting that the difference in Pm01 and Pf01 could be due to experimental error. At 2002 harvest, the average egg density with fallow with herbicides was higher than with monocots and leguminous dicots.

At Lamberton, significant differences for SCN population densities were detected at midseason 2001 and harvest 2002 (Table 2). The Pm01 in plots with resistant soybean was lower than in plots with susceptible soybean, potato, and flax. The Pf02 was higher in the wheat treatment than in the canola, corn, corn–rye, sugarbeet, and pea treatments. No significant differences were detected among other individual crop treatments or between the groups of treatments (Table 2).

At Morris, significant differences between individual treatments were detected at harvest 2001, at planting 2002, and at harvest 2002 (Table 3). Susceptible soybean had a higher Pf01 than resistant soybean, barley, corn, and red clover, and a higher Pi02 than resistant soybean and pea. The Pi02 in the resistant soybean was lower than in flax, oat, wheat, corn–rye, sugarbeet, and red clover. At harvest 2002, the SCN population did not differ from those in susceptible plots, although the alfalfa treatment resulted in higher egg population density than flax, buckwheat, or canola. In fallow plots, the Pf02 was higher in the no-weeding treatment than in treatments with either Buctril or Pursuit. By group, fallow and leguminous nonhosts and poor hosts resulted in lower Pm01 and Pi02 than monocots (Table 3). The treatment with leguminous nonhosts and poor hosts resulted in higher (P < 0.05 or 0.001) Pf02 than any other group of crops or fallow (Table 3). Monocots also resulted in higher (P < 0.01) Pf02 than nonleguminous dicots and fallow.

Population Change Factor
At Waseca, the PCF during and after the 2001 rotation crop season of the susceptible soybean treatment was higher than that of other crops, except buckwheat, on at least one of the three sampling occasions (2001 midseason, 2001 harvest, and 2002 at planting; Table 4). The PCF was 2.56 at planting in 2002 following susceptible soybean, indicating a population increase, and the PCF following the other crops except buckwheat was <1, indicating a decrease in the egg population density. The resistant soybean resulted in the lowest PCF at planting in 2002 (Table 4). Fallow treatments resulted in lower PCF than monocots and nonleguminous dicots (Table 4). When susceptible soybean was grown in 2002 in all plots, the PCF was mainly influenced by the egg population density at planting (Pi02). Susceptible soybean in 2001 resulted in a lower PCF at harvest in 2002 than barley, wheat, potato, sunflower, pea, and resistant soybean (Table 4). The PCF with hairy vetch was also lower than with resistant soybean or potato treatments (Table 4).


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  		Table 4. Population change factor of Heterodera glycines in response to rotation crops in Minnesota–Waseca site.{dagger}

 
At Lamberton, there was no difference in PCF during and after the rotation crop season between treatments in most cases; however, the PCF with resistant soybean was lower than with susceptible soybean, flax, oat, wheat, buckwheat, corn, corn–rye, sugarbeet, potato, sunflower, hairy vetch, or red clover at midseason 2001 (Table 5). By group, the PCF at harvest in 2001 was lower in leguminous nonhosts and poor hosts than in nonleguminous dicots (Table 5). No difference in PCF at harvest in 2002 was observed between individual treatments or between groups of treatments at Lamberton.


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  		Table 5. Population change factor of Heterodera glycines in response to rotation crops in Minnesota–Lamberton site.{dagger}

 
At Morris, the PCF was higher in susceptible soybean than in sorghum, canola, corn, corn–rye, sugarbeet, potato, alfalfa, hairy vetch, red clover, pea, and resistant soybean at harvest in 2001 and at planting in 2002. The resistant soybean resulted in a lower PCF than most rotation crops at planting in 2002 (Table 6). The PCF for leguminous nonhosts and poor hosts was lower than for monocots, nonleguminous dicots, or fallow. No differences were observed among groups at harvest in 2001. Susceptible soybean in 2001 resulted in a lower PCF at harvest 2002 than corn, sunflower, alfalfa, red clover, or resistant soybean (Table 5). The PCF with flax was also lower than the resistant soybean or alfalfa treatments (Table 5). The PCF for leguminous nonhosts and poor hosts was higher than for monocots, nonleguminous dicots, or fallow.


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  		Table 6. Population change factor of Heterodera glycines in response to rotation crops in Minnesota–Morris site.{dagger}

 
Pooling the data of all three sites together, the PCF following soybean was negatively related with the Pi02 (R2 = 0.85, P < 0.0001; Fig. 1 ). There was no difference in the PCF–Pi02 relationship among the three sites. Based on this model (Fig. 1), the predicted equilibrium population density at the three sites at harvest in 2002 was 11490 eggs per 100 cm3 of soil. At Waseca, the average Pi02 was 2048 (range 125–9125) eggs per 100 cm3 of soil, and therefore there was an increase in population density. In contrast, the Pi02 at Lamberton and Morris (Tables 2 and 3) were far above the predicted equilibrium population density, and subsequently the population decreased during the soybean-growing season (PCF generally <1).


Figure 1
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  		Fig. 1. Relationship between the population change factor (PCF) during the soybean growing season in 2002 and initial egg population density in 2002 (Pi02) at Waseca, Lamberton, and Morris, MN (PCF = egg population density at harvest in 2002/Pi02).

 
Soybean Yield
The response of soybean yield to the treatments varied between sites (Table 7). Yields were highest at Waseca, but no difference was detected except for the corn–rye treatment, which had a lower yield than most other treatments at this site (Table 7). At Lamberton, the soybean yields following corn or corn–rye were lowest; they were significantly lower than the yield following potato and sunflower treatments. By group, fallow treatments resulted in the highest yield followed by nonleguminous dicots, monocots, and leguminous nonhosts and poor hosts (Table 7). Iron-deficiency chlorosis in the field was a major factor influencing soybean yield, which was negatively correlated with the Fe-deficiency chlorosis rating (r = –0.74, P < 0.0001). Complete yield loss occurred in 47 out of the 144 plots at this site. At Morris, heavy rainfall in June affected early season plant growth and consequently reduced yields. The hairy vetch and sunflower treatments produced higher yields than the canola, flax, or oat treatments (Table 7). By group, treatments of leguminous nonhosts and poor hosts and monocots resulted in higher (P < 0.05 or 0.01) yields than nonleguminous dicots and fallow (Table 7).


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  		Table 7. Soybean yield response to rotation crops in fields infested with Heterodera glycines in Minnesota.{dagger}

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In this study, we demonstrated that all rotation crops resulted in lower SCN egg population density, PCF, or both than susceptible soybean at least at one sampling occasion. This suggests that these crops can be used in rotation for SCN management in Minnesota. After 1 yr of any of these crops, however, the nematode population densities were still >1000 eggs per 100 cm3 of soil, which can cause significant yield loss to a susceptible soybean (Porter et al., 2001; Chen et al., 2001c). These results were similar to the results of previous studies with corn as the rotation crop, which did not reduce SCN egg population densities to below damaging levels in 1 yr (Porter et al., 2001; Chen et al., 2001c). In general, 5 yr of a corn–resistant soybean rotation were needed for effective SCN management. A similar rotation period may be needed with any of these crops, but further studies are necessary to develop a rotation scheme including any of the alternative crops for SCN management.

Subtle differences in SCN populations among the rotation crops were detected in this study. Leguminous nonhosts and poor hosts appeared to be the best crops for reducing the SCN population density, while monocots appeared to be the least effective. Similar results have been obtained in greenhouse studies (Vetter et al., 2005). Pea as a trap crop has been shown to reduce SCN population density compared with non-trap-crop treatments in the corn-growing season (Chen et al., 2001b). The leguminous nonhosts and poor hosts may release root exudates to stimulate the SCN to hatch, but the nematodes are not able to develop and reproduce well in these crops (Sortland and MacDonald, 1987; Schmitt and Riggs, 1991), resulting in a population decline. Pea is presently grown in many parts of southern Minnesota and is sometimes double-cropped with soybean. Although it may not be cost effective to use pea as a trap crop interseeded with corn for SCN management (Chen et al., 2001b), it may be a preferred crop for use in rotation with soybean and corn for SCN management. Alfalfa and red clover are perennial crops, and they can be used in rotation with soybean for SCN management where practical. These crops are currently being studied for their potential as cover crops in corn–soybean production systems. Their agronomic and economic potential in the production systems in Minnesota will be further evaluated.

Fallow is rarely used in corn–soybean production in the region. We included fallow with different weed control treatments for the purpose of identifying any herbicide effect on SCN, which might confound the rotation crop effect when different herbicides were used in the different crops. Although the effect of herbicides on SCN population has been reported (Levene et al., 1998), no effect of herbicide on SCN population density was observed in this study except that the Pf02 was higher in the no-weeding treatment than in treatments with either Buctril or Pursuit. Thus, the rotation crop effect on SCN was unlikely to be due to herbicide treatment.

Yield response to the rotation crop was also limited. Soybean yield was not (at Lamberton and Morris) or weakly (r = –0.19, P = 0.02 at Waseca) correlated with SCN population density (data not shown), suggesting that there was little yield benefit from SCN management with the rotation crop for 1 yr. The response of yield to crop rotation may vary depending on environmental conditions. In a previous study, a 1-yr corn rotation resulted in higher yields than monoculture of soybean in SCN-infested fields at Waseca, but not at Lamberton (Chen et al., 2001c). The difference in yield among some crop treatments in this study was probably due in part to agronomic factors (Porter et al., 1997). At Waseca, the lower soybean yield in the corn–rye treatment was probably due to poor germination of the soybean in this treatment due to the rye residue effect. There were greater differences in soybean yields among crop treatments in Lamberton and Morris, but the trends appear to be opposite between the two sites. At Lamberton, yields following nonleguminous crops and fallow were higher than leguminous nonhosts and poor hosts or monocots; at Morris, the leguminous nonhosts and poor hosts and monocots resulted in higher yields than nonleguminous crops or fallow. The reason for the difference between the two sites is unclear. At Lamberton, however, these treatments may have affected the development of Fe-deficiency chlorosis; treatments with monocots apparently increased Fe-deficiency chlorosis compared with nonleguminous dicots, especially sugarbeet and canola (data not shown). Subsequently, the soybean yield following monocots was lower than following nonleguminous dicots.

Hairy vetch, a leguminous crop, supported the development of SCN females on the roots in the field and was probably a moderate host of SCN. This is probably why the PCF for hairy vetch was relatively high across the three sites. High PCF was also observed for buckwheat, but the reason is unclear.

Theoretically, the nematode population increases if PCF > 1. In this study, however, PCF was >1 for a number of the rotation crops at some sampling occasions, especially at the Lamberton site. This doesn't mean that the nematode population increased in these crops. The higher PCF than what we expected was due to experimental error in soil sampling and sample processing. At the Lamberton site, the average Pi01 was lower than Pm01 and Pf01. The reason for this is unclear.

In the 2002 soybean growing season, predicted equilibrium population density was similar among the three sites. Nematode equilibrium population density is a function of the size of the food source and the efficiency of a nematode population using that food source in producing offspring. Both the size of the food source and the efficiency of use are affected by many factors including cultivar and environment (Seinhorst, 1967; Li and Chen, 2005). The predicted equilibrium population density was the sum of the effects of factors that may have affected food source and the SCN's efficiency in using the food source at the three sites. These factors can be different among sites although the sum of the effects was similar. The equilibrium population density can also be different among years at the same site. The population densities at harvest in 2002 were lower than the population at planting in 2001 at Lamberton and Morris, suggesting that the environmental conditions were more favorable for SCN population development in the 2000 soybean growing season than the 2002 season.

In conclusion, there was large variability in the SCN populations and soybean yields at the three sites. Nevertheless, significant treatment effects were detected at all sites. While all of the rotation crops lowered SCN population compared with SCN-susceptible soybean, there were subtle differences among the individual rotation crops and among different groups of crops. Leguminous nonhosts and poor hosts were probably the best crops in reducing SCN population density. Corn, the most common rotation crop in Minnesota, was in the group that was the least effective in reducing the nematode population.


   ACKNOWLEDGMENTS
 
This research was supported by the Cooperative State Research, Education, and Extension Service, USDA, under Agreement no. 2002-34103-11990, and North Central Region Canola Research. We thank C. Johnson, W. Gottschalk, S. Liu, J. Ballman, S. Quiring, and R. Solyntjes for technical assistance, and S. Weisberg for assistance in statistical analysis.


   NOTES
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 MATERIALS AND METHODS
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 DISCUSSION
 REFERENCES
 
Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA and the Univ. of Minnesota.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
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 DISCUSSION
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