PORTER ET AL.: RESPONSE OF SOYBEAN CYST NEMATODE TO CORN SOYBEAN ROTATIONS 619

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1 PORTER ET AL.: RESPONSE OF SOYBEAN CYST NEMATODE TO CORN SOYBEAN ROTATIONS 619 Harris, W Competition among pasture plants: V. Effects of Sheaffer, C.C., D.L.Wyse, G.C. Marten, and P.H. Westra The frequency and height of cutting on competition between Agrostis potential of quackgrass for forage production. J. Prod. Agric. tenuis and Trifolium repens. N. Z. J. Agric. Res. 17: : Jung, G.A., L.L. Wilson, P.J. LeVan, R.E. Kocher, and R.F. Todd. Smith, D., R.J. Bula, and R.P. Walgenbach Forage management Herbage and beef production from ryegrass alfalfa and or- 5th ed. Kendall Hunt Publ. Co., Dubuque, IA. chardgrass alfalfa pastures. Agron. J. 74: Sprague, C.C., R.R. Robinson, and A.W. Clyde Pasture renova- Knight, W.E Productivity of crimson and arrowleaf clovers tion: I. Seedbed preparation, seedling establishment, and subsequent grown in a coastal bermudagrass sod. Agron. J. 62: yields. J. Am. Soc. Agron. 39: Paine, L.K., D.J. Undersander, and M.D. Casler Pasture growth, Steel, R.G.D., J.H. Torrie, and D.A. Dickey Principles and production, and quality under rotational and continuous grazing procedures of statistics. 2nd ed. McGraw-Hill Book Co., New management. J. Prod. Agric. 12: York, NY. SOYBEAN Population Response of Soybean Cyst Nematode to Long Term Corn Soybean Cropping Sequences in Minnesota Paul M. Porter, Senyu Y. Chen,* Curt D. Reese, and Lee D. Klossner ABSTRACT during the 1980s and 1990s. As soybean cropping frequency and acreage increased in central and northwest- Soybean cyst nematode [Heterodera glycines Ichinohe] (SCN) can reduce soybean [Glycine max (L.) Merr.] yields. Rotating soybean ern Minnesota, so too did the presence of SCN (Chen, with a nonhost crop usually reduces SCN populations. Cropping se- unpublished data, 2000). quence experiments initiated in the early 1980s at two Minnesota Crop rotation, cultural practices, resistant cultivars, locations were monitored in 1996, 1997, and 1998 for changes in SCN and nematicides have been employed to reduce soybean egg densities. Cropping sequences were: (i) 5-yr consecutive corn (Zea yield suppression caused by SCN (Koenning et al., 1993; mays L.) alternated with 5-yr consecutive soybean, (ii) continuous Riggs and Schuster, 1998; Schmitt, 1991; Young and monoculture of each crop, (iii) annual alternation of two cultivars Hartwig, 1992; Young, 1998a, 1998b). Rotation with within a continuous monoculture of each crop, and (iv) annual rotation nonhosts of SCN such as corn, wheat (Triticum aestivum of each crop. In 1989, SCN was detected in several of the plots at L.), or grain sorghum [Sorghum bicolor (L.) Moench] both locations. By 1996, all cropping sequences had detectable populahas been an effective management tactic. Early research tions of SCN eggs at both locations, regardless of whether the land had been planted to continuous corn since the early 1980s. Lowest on crop rotation by Ross (1962) in the southeastern densities of SCN eggs were typically found in cropping sequences that USA showed that two or more years in production of involved continuous corn and where corn had been planted for the a nonhost crop resulted in decreases of the nematode to last three or more years, whereas highest levels of SCN eggs were low or undetectable levels with acceptable subsequent found in cropping sequences that involved continuous soybean and yields of soybean. Schmitt (1991), however, stated where soybean had been planted for the last two or more years. These even after 3 yr of growing a nonhost, SCN population results suggest planting a nonhost to SCN for as long as 5 yr on density increased rapidly on soybean in a single season. infested land will not eliminate future problems with this pathogen. Based on research in the midwestern USA, Noel (1985) In addition, the results suggest that at least a portion of the crop stated that SCN would not be a problem if soybean was sequence effects on yield has a soil microbiological basis involving grown once every 4 or 5 yr. mechanisms that are specific to a location. In a field with no known history of soybean production, Noel and Edwards (1996) determined that it took S 5 to 6 yr in a continuous soybean (SS) monoculture for oybean cyst nematode (SCN) was first discovered the nematode to increase to yield-damaging densities, in the USA in North Carolina in 1954 (Winstead whereas rotation of a susceptible soybean with corn was et al., 1955). It was identified in several midwestern effective in slowing SCN population increases. Their states in the 1960s (Miller, 1985) and was first documented data indicated that in a 2 yr corn soybean rotation, in Minnesota in 1978 (MacDonald et al., 1980). The SCN spread from southern Minnesota northward Abbreviations: DAP, days after planting; P f, end-of-season egg population density of soybean cyst nematode; P i, early season egg population density of soybean cyst nematode; P m, midseason egg population P.M. Porter, Dep. of Agron. and Plant Genet., Univ. of Minnesota, density of soybean cyst nematode; SCN, soybean cyst nematode. In St. Paul, MN 55108; S.Y. Chen and C.D. Reese, Southern Res. and cropping sequences: C1, C2, C3, C4, and C5 are 1st, 2nd, 3rd, 4th, Outreach Cent., Waseca, MN 56093; and L.D. Klossner, Southwest and 5th yr corn following 5 yr of soybean; C C, annual alternation of Res. and Outreach Cent., Lamberton, MN Minnesota Agric. two cultivars within a continuous monoculture of corn; CC, continuous Exp. Stn. Journal Ser. Paper Received 10 Aug corn; C S, corn in annual rotation; S1, S2, S3, S4, and S5 are 1st, 2nd, *Corresponding author (chenx099@umn.edu). 3rd, 4th, and 5th yr soybean following 5 yr of corn; S C, soybean in annual rotation; S S, annual alternation of two cultivars within a Published in Agron. J. 93: (2001). continuous monoculture of soybean; SS, continuous soybean.

2 620 AGRONOMY JOURNAL, VOL. 93, MAY JUNE 2001 soybean yield loss would occur before nematode densimixed, Center near Waseca on a Nicollet clay loam (fine-loamy, ties were great enough to cause the diagnostic symptoms mesic Aquic Hapludoll) to evaluate the effect of corn of stunting and chlorosis. Despite precautions taken to soybean cropping sequences on crop production (Crookston prevent plot-to-plot spread of the nematode where SCN et al., 1991). At the time of establishment, the sites were not thought to be infected with SCN. By 1989, however, SCN was introduced into selected treatments, some plots in was detected in several of the plots at both locations (D. all treatments were contaminated with the nematode MacDonald, personal communication, 2000). after 5 yr (Noel and Edwards, 1996). The experimental design at each location consisted of 16 In the southeastern USA, delayed soybean planting cropping sequence treatments arranged in a randomized commay limit damage caused by SCN because of nematode plete block design with four replicates. The treatments included: attrition (Koenning et al., 1993; Schmitt, 1991). The (i) 5-yr consecutive corn alternated with 5-yr consecuattrition converse is that earlier planting dates, a trend observed tive soybean, arranged so that during each year, there occurred in Minnesota, may tend to favor increased damage of a 1st, 2nd, 3rd, 4th, and 5th yr of each crop (C1, C2, C3, soybean by SCN. In northern production regions of the C4, and C5 for corn and S1, S2, S3, S4, and S5 for soybean, USA, percent survival of SCN eggs and juveniles is respectively); (ii) continuous monoculture of each crop (SS for soybean and CC for corn); (iii) an annual alternation of greater than in southern regions (Noel, 1985; Riggs et two cultivars within a continuous monoculture of each crop al., 2001). In cooler soybean production areas, only three (S S for soybean and C C for corn); and (iv) an annual rotation to four nematode generations are produced per year, of each crop (S C for soybean and C S for corn) (Table 1). whereas in more temperate regions, six to seven genera- Because SCN was identified as a yield-reducing factor in the tions may occur (Noel, 1985). SS treatment at Lamberton in 1992, oat (Avena sativa L.) was Our objective was to evaluate the influence of estab- grown in that treatment from 1993 to 1995 instead of soybean. lished corn and soybean cropping sequences on SCN At both locations, the annual alternation of two cultivars populations at two Minnesota locations. Specifically, we within a continuous monoculture of each crop was eliminated determined SCN egg populations over the course of after 1994; therefore, during this study, all treatments were three growing seasons to monitor changes in the populaplanted to the same cultivar. At Lamberton, plots were 9.1 m (12 rows) wide by 10 m tions and related end-of-season SCN egg populations long. At Waseca, plots were 4.5 m (6 rows) wide by 18 m long. (P f ) to soybean yields in various cropping sequences. Row spacing was 76 cm at both locations. All plots were moldboard-plowed in the fall and field-cultivated before planting the following spring, except at Lamberton in 1996 when MATERIALS AND METHODS there was no fall plowing. Instead, the plot area was plowed Soybean cyst nematode populations were monitored for 3 and field-cultivated in the spring of yr starting in 1996 at two southern Minnesota locations where Details of soil fertility, fertilizers and pesticides used, and ongoing corn soybean cropping sequence experiments were yield responses have been published by Crookston et al. (1991) located. The experiments were established in 1981 at the and Porter et al. (1997). Recommended practices for optimum Southwest Research and Outreach Center near Lamberton production were followed. The same corn and soybean cultivars on a Webster clay loam (fine-loamy, mixed, mesic Typic Endoaquoll) were grown in all treatments at both locations. Corn and in 1982 at the Southern Research and Outreach hybrids included Pioneer Brand 3730 in 1996 and 1997 and Table 1. Corn (C) and soybean (S) cropping sequences at Lamberton and Waseca, MN. Crop sequence by year yr rotation 1 C C S S S S S C C C C C S1 S2 S3 2 C S S S S S C C C C C S S2 S3 S4 3 S S S S S C C C C C S S S3 S4 S5 4 S S S S C C C C C S S S S4 S5 C1 5 S S S C C C C C S S S S S5 C1 C2 6 S S C C C C C S S S S S C1 C2 C3 7 S C C C C C S S S S S C C2 C3 C4 8 C C C C C S S S S S C C C3 C4 C5 9 C C C C S S S S S C C C C4 C5 S1 10 C C C S S S S S C C C C C5 S1 S2 Continuous 11 C C C C C C C C C C C C CC CC CC 12 S S S S S S S S S S S S SS SS SS Continuous with alternating cultivars 13 C 2 C C 2 C C 2 C C 2 C C 2 C C 2 C C C C C C C 14 S 2 S S 2 S S 2 S S 2 S S 2 S S 2 S S S S S S S Corn soybean rotation 15 S C S C S C S C S C S C S C C S S C 16 C S C S C S C S C S C S C S S C C S C1, C2, C3, C4, and C5 are 1st, 2nd, 3rd, 4th, and 5th yr corn following 5 yr of soybean; C C, annual alternation of two cultivars within a continuous monoculture of corn; CC, continuous corn; C S, corn in annual rotation; S1, S2, S3, S4, and S5 are 1st, 2nd, 3rd, 4th, and 5th yr soybean following 5 yr of corn; S C, soybean in annual rotation; S S, annual alternation of two cultivars within a continuous monoculture of soybean; SS, continuous soybean. At Lamberton, for 3 yr, oat was grown in 1993 through 1995 in the SS cropping sequence because of the presence of yield reducing soybean cyst nematode (SCN) populations in that cropping sequence by After 1994, alternative cultivars (C and C 2 for corn; S and S 2 for soybean) were no longer planted in this cropping sequence involving continuous corn or soybean with alternating cultivars.

3 PORTER ET AL.: RESPONSE OF SOYBEAN CYST NEMATODE TO CORN SOYBEAN ROTATIONS 621 DeKalb 493RR in Soybean cultivars were Parker in nificant differences were performed using ANOVA (SAS 1996 and 1997 and Asgrow 2101RR in Planting dates Inst., 1992) on log 10 (x 1) transformed data because of were 20 May, 10 May, and 29 April at Lamberton and 29 May, variance heterogeneity. Treatment mean comparisons were 19 May, and 30 April at Waseca in 1996, 1997, and 1998, made for each sampling date when the F-test was significant respectively. Seeding rates were and viable at P 0.05 using Fisher s protected LSD test at the P 0.05 seeds ha 1 for corn and soybean, respectively. Soybean was level. The P f /P i (P i in 1997 and 1998 and P m in 1996) ratio for harvested with a plot combine from 8 m of four central rows each cropping sequence was determined each year by dividing at Lamberton and from 15 m of two central rows at Waseca. the average final population of four replicates by the average Soybean grain yields were adjusted to a moisture content of initial population of four replicates. Soybean grain yield data 130gkg 1. were analyzed using the general linear model (GLM) proce- Soil samples for nematode assays were collected on eight dure of SAS (Spector et al., 1985). At each location, regression dates over the 3-yr period to determine the initial (P i ), midsea- analysis was performed to determine the relationship between son (P m ), and final (P f ) population densities each year with P f and yield. the exception of 1996 when no P i samples were obtained. Twenty soil cores at Lamberton and 30 at Waseca, each 2.5 cm in diameter and taken to a depth of 20 cm within 10 cm of the soybean row, were composited from the harvest area of RESULTS AND DISCUSSION each plot. Soil samples were stored at 4 C until processed. The ANOVA of the log (x 1) transformed P f data The samples at Lamberton were taken on 24 July [65 d after showed significant year (P 0.014), location (P planting (DAP)] and 24 October (157 DAP) in 1996; 14 May 0.001), cropping sequence (P 0.001), year location (4 DAP), 19 August (101 DAP), and 20 October (163 DAP) (P 0.003), and location cropping sequence (P in 1997; and 5 May (6 DAP), 28 July (90 DAP), and 19 October 0.001) interactions. It did not show significant year (173 AP) in Samples at Waseca were taken on 22 July (54 DAP) and 11 November (166 DAP) in 1996; 3 June cropping sequence (P 0.526) or year location (15 DAP), 5 August (78 DAP), and 20 October (154 DAP) in cropping sequence (P 0.194) interactions. Because of 1997; and 4 May (4 DAP), 24 July (85 DAP), and 18 September the significant interactions, the SCN egg densities are (141 DAP) in presented each year for each location (Tables 2 and 3). The SCN egg density was determined from a subsample of In addition, the P f data are presented over years for 100 cm 3 of soil. Cysts were extracted from the soil by hand each location (Fig. 1). decanting and centrifugation in 76% (wt./vol.) sucrose solution By 1996, all cropping sequences had detectable densifor 5 min at 1500 g. Eggs were released from the cysts by ties of SCN eggs at both Lamberton and Waseca (Tables breaking the cysts in a 40-mL glass tissue grinder (Catalogue no D, Fisher, Pittsburgh, PA) and then collected in 2 and 3), regardless of whether the land had been a 50-mL tube. They were stored at 4 C before being counted planted to CC since the early 1980s. The presence of within 2 wk. If the egg samples could not be counted within low densities of SCN eggs on land where no soybean 2 wk, they were stored in a freezer until being counted. had been planted for more than 15 yr suggests that the Original SCN egg densities are presented, but tests for sig- SCN eggs were moved onto that land through agro- Table 2. Soybean cyst nematode (SCN) egg densities in the long-term rotation study at Lamberton, MN. Crop sequence July 24 Oct. 14 May 19 Aug. 20 Oct. 5 May 28 July 19 Oct. Eggs 100 cm 3 soil C1 C2 C3 166a c 155cd 129c e 19de 16ef 163de 3de 5d C2 C3 C4 50b d 10de 12ef 20de 44ef 10e 0e 20cd C3 C4 C5 19d 15de 12d f 9de 3f 13e 3de 86cd C4 C5 S1 19d 3e 8ef 7de 6f 159de 16de 159cd C5 S1 S2 13d 5de 21d f 47de 189de 147cd 1206ab 5856ab S1 S2 S3 56b d 600c e 328a c 1647ab a 2291a 4725a 9358a S2 S3 S4 119b d 110cd 50d f 1277ab 8 941ab 1875ab 4920a 9138a S3 S4 S5 1800ab 2775a 1100a 4096a a 4538a 4441a 5328ab S4 S5 C1 1131a 2775a 535ab 4736ab a 2200a 1193ab 829b S5 C1 C2 1413a 2885ab 818a 581bc 379cd 197bc 221c 60c CC CC CC 16d 30de 12d f 16e 9ef 13de 115d 26cd C C C C C C 63cd 10de 50b d 7de 3f 6e 8de 4d SS SS SS 66b d 520bc 83c e 1715ab 4 585ab 3041ab 3633a 7250ab S S S S S S 1096a 3915a 452ab 4198ab 9 094a 1891a 4343a 7380ab C S S C C S 31b d 33de 8f 83cd 1 559bc 222c 216c 164c S C C S S C 441b d 675a c 465ab 103de 100ef 125cd 1000bc 2690ab Mean CV, % Trt. effect (P f ) C1, C2, C3, C4, and C5 are 1st, 2nd, 3rd, 4th, and 5th yr corn following 5 yr of soybean; C C, annual alternation of two cultivars within a continuous monoculture of corn; CC, continuous corn; C S, corn in annual rotation; S1, S2, S3, S4, and S5 are 1st, 2nd, 3rd, 4th, and 5th yr soybean following 5 yr of corn; S C, soybean in annual rotation; S S, annual alternation of two cultivars within a continuous monoculture of soybean; SS, continuous soybean. Values in each column followed by the same letter are not significantly different at the 0.05 probability level. Data analysis to determine significance was made on log 10 (x 1) transformed values. Oat was grown in 1993 through 1995 in the SS cropping sequence because of the presence of yield-reducing SCN populations in that cropping sequence by 1992.

4 622 AGRONOMY JOURNAL, VOL. 93, MAY JUNE 2001 Table 3. Soybean cyst nematode (SCN) egg densities in the long-term rotation study at Waseca, MN. Crop sequence July 11 Nov. 3 June 5 Aug. 20 Oct. 4 May 24 July 18 Sept. Eggs 100 cm 3 soil C1 C2 C3 745ab 440c e 187d f 228c 447cd 191b f 228bc 34d f C2 C3 C4 366a d 195de 258b e 266bc 88de 150ef 241bc 22ef C3 C4 C5 114c e 100e 62f 16de 16ef 69c f 16de 26ef C4 C5 S1 60d f 5f 25g 25de 13ef 378d f 25de 3325ab C5 S1 S2 20fg 5f 3g 86d 2684a c 303a e 87cd 4288a S1 S2 S3 23fg 900b d 398c e 1327a c 4853a 1297ab 1380ab 3439ab S2 S3 S4 447b e 3025ab 726a e 2468a 5363ab 1116ab 2468a 4568a S3 S4 S5 1138a 3165ab 1133ab 1431a 3881ab 1972ab 2225a 4991a S4 S5 C1 551a d 3805a 430a e 1436ab 3100ab 1656a 2093a 323c e S5 C1 C2 717a c 2900ab 931a c 703a c 863a c 591a c 704ab 187cd CC CC CC 11g 25f 0g 47de 9f 72f 47de 6f C C C C C C 41e g 10f 6g 9e 22f 19g 10e 70d f SS SS SS 525a c 2235a c 1055a c 2563a 4341ab 1650a 2563a 3138ab S S S S S S 927ab 3487a 607a d 1942a 2838cd 1319ab 1481ab 2794ab C S S C C S 331a d 555cd 136ef 1284ab 4981a 1381ab 1216ab 448bc S C C S S C 385a d 3425ab 1378a 603a c 547bc 491a d 603ab 4275ab Mean CV, % Trt. effect (P f ) C1, C2, C3, C4, and C5 are 1st, 2nd, 3rd, 4th, and 5th yr corn following 5 yr of soybean; C C, annual alternation of two cultivars within a continuous monoculture of corn; CC, continuous corn; C S, corn in annual rotation; S1, S2, S3, S4, and S5 are 1st, 2nd, 3rd, 4th, and 5th yr soybean following 5 yr of corn; S C, soybean in annual rotation; S S, annual alternation of two cultivars within a continuous monoculture of soybean; SS, continuous soybean. Values in each column followed by the same letter are not significantly different at the 0.05 probability level. Data analysis to determine significance was made on log 10 (x 1) transformed values. nomic (e.g., tillage, planting, or harvest equipment) or (Tables 2 and 3). At Lamberton, over the 3-yr period climatic (e.g., wind or water erosion) means. The lowest and eight sampling dates, 18 of the 64 plots never had densities of SCN eggs were typically found in CC crop- more than 255 eggs 100 cm 3 soil: all four replicates of ping sequences and where corn had been planted for the the C2 C3 C4 and C C cropping sequences, three repli- last three or more years, whereas the highest densities of cates of the C3 C4 C5 and CC cropping sequences, two SCN eggs were found in SS cropping sequences and replicates of the C4 C4 S1 cropping sequence, and one where soybean had been planted for the last two or replicate of the C1 C2 C3 and C S cropping sequences. more years (Tables 2 and 3; Fig. 1). Six of the 64 plots never had fewer than 500 eggs 100 The high coefficient of variation for SCN egg densi- cm 3 soil: two replicates of the S3 S4 S5, S4 S5 C1, ties, which ranged from 18.5 to 58.4%, indicate a relatively and S S cropping sequences. At Waseca, over the 3-yr large amount of plot-to-plot variability, not un- period and eight sampling dates, 14 of the 64 plots never common in SCN research (Tables 2 and 3). Over the had more than 255 eggs 100 cm 3 soil: all four replicates eight sampling dates, which spanned 3 yr, SCN eggs of the C3 C4 C5, CC, and C C cropping sequences, and were detected in all 64 plots (16 cropping sequence two replicates of the C2 C3 C4 cropping sequence. treatments 4 replicates) at Lamberton and in 63 of Nine of the 64 plots never had fewer than 500 eggs plots at Waseca. The only plot where SCN eggs were cm 3 soil: all four replicates of the S3 S4 S5 cropping never detected was in the C C cropping sequence at Wa- sequence, two replicates of the S2 S3 S4 and S S cropping seca. For any one sampling date, however, SCN eggs sequences, and one replicate of the SS cropping were not detected in numerous plots. The number of sequence (individual plot data not shown). These data plots where SCN eggs were not detected ranged from suggest that, in spite of a relatively large amount of 13 to 20 at Lamberton and from 6 to 12 at Waseca for plot-to-plot variability, the changes in SCN egg densities the eight sampling dates. Soybean cyst nematode eggs due to crop sequence were consistent at both locations. were not detected on one or more sampling dates in 42 of 64 plots at Lamberton and in 26 of 64 plots at Waseca End-of-Season Population Density (individual plot data not shown). One possible reason and End-of-Season to Early Season for the difference between the Lamberton and Waseca Population Density Ratios site may be that the plot width was twice as wide at Lamberton, possibly resulting in more soil movement At both locations, P f of the S5, C1, and C5 cropping across plots at Waseca. The inability to detect any SCN sequences was in the thousands, hundreds, and tens of eggs on land that, within 1 yr, supported a detectable eggs 100 cm 3 soil, respectively, and more often than population of SCN eggs suggests a lack of sensitivity in not these populations were significantly different from the sampling and analysis protocol. each other (Tables 2 and 3; Fig. 1). The P f of S3, S4, While the Lamberton site had more plots in which and S5 were the same and among the largest P f of the no SCN eggs were detectable, the highest SCN egg densities cropping sequences evaluated for each of the 3 yr at in each of the 3 yr were found at Lamberton both Lamberton and Waseca. At Lamberton, the P f of

5 PORTER ET AL.: RESPONSE OF SOYBEAN CYST NEMATODE TO CORN SOYBEAN ROTATIONS 623 Fig. 1. Average end-of-season SCN egg population (P f ) for each cropping sequence over the 3-yr period ( ) at Lamberton and Waseca, MN. S1 was less than that of S3, S4, and S5 for all 3 yr, tent with research in Missouri where SCN densities were whereas at Waseca, the P f of S1 was less than that of at or near the limits of sampling detection after 3 yr of S3, S4, and S5 only in Except at Lamberton in a nonhost (Francl and Dropkin, 1986). 1996, the P f of S2 was not statistically different than that For all eight cropping sequences in the year that corn of S3, S4, and S5. was grown, the P f /P i ratio tended to be 1.0 in most The reason that the P f of S1 was lower than that of years at both Lamberton and Waseca (data not shown S3, S4, and S5 each year at Lamberton but not at Waseca but can be derived from inf. in Tables 2 and 3). In the is unclear, but perhaps it is related to the observation year that corn was grown in the cropping sequence, that the SCN populations were not as widespread at the the P f /P i ratio exceeded 3.0 only five times in the 48 Lamberton site. The SCN populations at Lamberton observations (2 locations 8 cropping sequences tended to increase faster in response to soybean in the 3 yr). For cropping sequences in the year that soybean cropping system but also to decrease faster in response was grown, the P f /P i ratio exceeded 3.0 a total of 35 to corn in the cropping system than at Waseca. At both times in the 48 observations. At both locations, the SCN locations, however, there was generally no difference egg densities increased more dramatically on soybean between the P f of C3, C4, and C5. Our results are consis- in 1997 compared with 1996 and End-of-season

6 624 AGRONOMY JOURNAL, VOL. 93, MAY JUNE 2001 Table 4. Soybean grain yield in the long-term rotation study at Lamberton and Waseca, MN. Crop sequence Lamberton Waseca Mg ha 1 C4 C5 S1 3.70a 3.85 C5 S1 S2 2.53a 3.30bc 4.35a 3.78 S1 S2 S3 3.65a 1.84bcd 2.81e b 3.92 S2 S3 S4 3.28cd 2.02bc 2.91de b 3.87 S3 S4 S5 3.18de 1.39de 3.07cde b 4.05 S4 S5 C1 3.01ef 1.16e b S5 C1 C2 2.95f 3.21 SS SS SS 3.55ab 1.76cd 3.14cd b 3.85 S S S S S S 2.93f 1.60cde 2.80e b 3.59 C S S C C S 2.31ab 3.48b S C C S S C 3.44bc 3.52ab Mean CV, % Trt. effect (P F ) (NS) (NS) LSD (0.05) (NS) (NS) C1, C2, C3, C4, and C5 are 1st, 2nd, 3rd, 4th, and 5th yr corn following 5 yr of soybean; C S, corn in annual rotation; S1, S2, S3, S4, and S5 are 1st, 2nd, 3rd, 4th, and 5th yr soybean following 5 yr of corn; S C, soybean in annual rotation; S S, annual alternation of two cultivars within a continuous monoculture of soybean; SS, continuous soybean. Values in each column followed by the same letter are not significantly different at the 0.05 probability level. NS, not significant at P SCN egg densities were generally much higher than P i cated that a relationship existed at Lamberton but not or P m in years when soybean was grown (Tables 2 and 3). at Waseca (Fig. 2). At Lamberton, where the P f was These results are consistent with other research showing more than 6000 eggs 100 cm 3 soil on 9 of 24 occasions, that the rapid increase in SCN egg densities in the presence yields decreased at higher SCN egg populations. At of a soybean host doesn t occur within 6 to 8 wk Waseca, where the P f was never more than 6000 eggs after planting (as would be suggested by the SCN life 100 cm 3 soil, a relationship did not exist between yield cycle), but instead occurs more than 100 DAP (Bonner and P f. Caution should be taken in interpreting the relationships and Schmitt, 1985; Wallace et al., 1995). between egg densities and soybean yields. First, yield is unlikely to be related to P f in all instances. Soybean Yields For example, in the presence of high P i and significant early season plant damage, P f can be lower than that The ANOVA of soybean yield showed significant on healthy plants in the presence of low P i. Perhaps year (P 0.023), location (P 0.001), cropping se- both P i and either P f or the P f /P i ratio should be considquence (P 0.001), year location (P 0.001), year ered when assessing damage relationships. Even if P f is cropping sequence (P 0.001), and location cropping a good predictor of damage, it doesn t necessarily exsequence (P 0.001) interactions. It did not show a plain the relationship. Second, differences in yield posignificant year location cropping sequence (P tential across environments (years and locations in this 0.689) interaction. Because of the significant interac- case) can mask damage relationships. tions, the yields are presented each year for each loca- The reason for the difference in SCN populations tion (Table 4). and the corresponding soybean yield between the two Soybean grain yield at Lamberton was affected by locations is unclear. Soil and climatic differences existed cropping sequence in all 3 yr (Table 4). First-year soy- between the two sites, and these factors could have bean yielded more than S2, S3, S4, and S5 each year contributed to the observed differences in SCN populaand across all 3 yr. Lowest soybean yields were obtained tion changes and yields. In addition, Chen and Reese from the SS, S S, S4, and S5 cropping sequences. Yields (1999) observed that a nematophagous fungus (Hirin 1997 were lower than in 1996 and 1998, in part due sutella rhossiliensis Minter and Bradly) parasitized high to poor weed control that year. The weed pressure in percentages (up to 50%) of SCN second-stage juveniles 1997 was greatest in the SS, S S, S4, and S5 cropping at the Waseca site in plots planted with more than 2 yr sequences and may have been due in part to poor plant of soybean. It is possible that at Waseca, the fungus canopy development (data not shown). limited the nematode population and influenced the Soybean grain yield at Waseca was affected by crop- soybean yields. The differing populations of SCN period ping sequence only in 1997 when the S1 cropping se- in response to crop sequence over the 3 yr and the quence yielded more than the other cropping sequences differing relationships between yield loss and SCN pop- (Table 4). Of the 3 yr studied, P f was highest in 1997 ulation at Lamberton and Waseca support the concept (Table 3). Soybean grain yields at Lamberton and Wa- that a portion of the crop sequence effects on yield have seca for the cropping sequences averaged across 1996 a soil microbiological basis involving mechanisms that through 1998 were consistent with longer term soybean are specific to a location. yield data reported by Porter et al. (1997). Because SCN survival during the winter months in Regression analysis of average soybean grain yield vs. northern USA production areas is typically greater than P f for the eight cropping sequences over the 3 yr indi- in southern USA production areas (Noel, 1985; Riggs

7 PORTER ET AL.: RESPONSE OF SOYBEAN CYST NEMATODE TO CORN SOYBEAN ROTATIONS 625 Fig. 2. Regression analysis of average soybean grain yield vs. end-of-season SCN egg population (P f ) for the eight cropping sequences over the 3 yr (1996, 1997, and 1998) at Lamberton and Waseca, MN. S1, S2, S3, S4, and S5 are 1st, 2nd, 3rd, 4th, and 5th yr soybean following 5 yr of corn; SS, continuous soybean; S C, soybean in annual rotation; S S, annual alternation of two cultivars within a continuous monoculture of soybean. tored in 1996, 1997, and 1998 for SCN. The SCN was thought not to be present at either location at the start of the studies. By 1996, all cropping sequences had detectable densities of SCN eggs at both locations, regard- less of whether the land had been planted to CC since the early 1980s. The lowest densities of SCN eggs were typically found in CC cropping sequences and where corn had been planted for the last three or more years. The highest densities of SCN eggs were found in SS cropping sequences and where soybean had been planted for the last two or more years. At both locations, the P f of S5, C1, and C5 cropping sequences were in the et al., 2001), our results may have been different had the research been conducted in the southern production areas. In northern regions, after the SCN population has increased to a high density, it may take longer to lower the SCN population to a density that doesn t cause subsequent soybean yield loss. SUMMARY AND CONCLUSIONS Field experiments initiated in the early 1980s at Lam- berton and Waseca, MN to evaluate the influence of various corn soybean cropping sequences were moni-

8 626 AGRONOMY JOURNAL, VOL. 93, MAY JUNE 2001 thousands, hundreds, and tens of eggs 100 cm 3 soil, Miller, L.I Economic importance of cyst nematodes in North America. p In F. Lamberti and C.E. Taylor (ed.) Cyst respectively. Regression analysis of average soybean nematodes. NATO Advanced Study Inst. Ser. A: Life Sci. Vol. grain yield vs. P f for the eight cropping sequences over 121. Plenum Press, New York. the 3 yr indicated a relationship at Lamberton where Noel, G.R The soybean cyst nematode. p In F. Lamberti the SCN population was greater for certain cropping and C.E. Taylor (ed.) Cyst nematodes. NATO Advanced Study Inst. Ser. A: Life Sci. Vol Plenum Press, New York. sequences. These results suggest that planting a nonhost Noel, G.R., and D.I. Edwards Population development of Hetof SCN for as long as 5 yr on infested land would reduce erodera glycines and soybean yield in soybean maize rotations the SCN egg population, but that population could re- following introduction into a noninfested field. J. Nematol. 28: bound within 2 yr if soybean was grown Porter, P.M., J.G. Lauer, W.E. Lueschen, J.H. Ford, T.R. Hoverstad, E.S. Oplinger, and R.K. Crookston Environment affects the ACKNOWLEDGMENTS corn and soybean rotation effect. Agron. J. 89: Contributions to this work by Steve Quiring, Laura Zelin- Riggs, R.D., T.L. Niblack, R.A. Kinloch, A.E. MacGuidwin, A. Mauromoustaskos, and L. Rakes Overwinter population dynamics ski, Betsy Sentz, Jun Jin, and Jeff Ballman are sincerely appreciated. of Heterodera glycines. J. Nematol. (in press). Support for this research came in part from the Riggs, R.D., and R.P. Schuster Management. p In S.B. Minnesota Soybean Research and Promotion Council. Sharma (ed.) The cyst nematodes. Kluwer Academic Publ., Boston. Ross, J.P Crop rotation effects on the soybean cyst nematode population and soybean yields. Phytopathology 52: REFERENCES SAS Institute SAS/STAT guide for personal computers. Version SAS Inst., Cary, NC. Bonner, M.J., and D.P. Schmitt Population dynamics of Hetero- Schmitt, D.P Management of Heterodera glycines by cropping dera glycines life stages on soybean. J. Nematol. 17: and cultural practices. J. Nematol. 23: Chen, S.Y., and C.D. Reese Parasitism of the nematode Hetero- Spector, P.C., H.J. Goodnight, J.P. Sall, and W.S. Sarle The dera glycines by the fungus Hirsutella rhossiliensis as influenced GLM procedure. p In SAS user s guide: Statistics. SAS by crop sequence. J. Nematol. 31: Inst., Cary, NC. Crookston, R.K., J.E. Kurle, P.J. Copeland, J.H. Ford, and W.E. Wallace, M.K., J.H. Orf, and W.C. Stienstra Field population Lueschen Rotational cropping sequence affects yield of corn dynamics of soybean cyst nematode on resistant and susceptible and soybean. Agron. J. 83: soybeans and their blends. Crop Sci. 35: Francl, L.J., and V.H. Dropkin Heterodera glycines population Winstead, N.N., C.B. Skotland, and J.N. Sasser Soybean-cyst dynamics and relation of initial population to soybean yield. Plant nematode in North Carolina. Plant Dis. Rep. 39:9 11. Dis. 70: Young, L.D. 1998a. Influence of soybean cropping sequences on seed Koenning, S.R., D.P. Schmitt, and K.R. Barker Effects of crop- yield and female index of the soybean cyst nematode. Plant Dis. ping systems on population density of Heterodera glycines and 82: soybean yield. Plant Dis. 77: Young, L.D. 1998b. Managing soybean resistance to Heterodera gly- MacDonald, D.H., G.R. Noel, and W.E. Lueschen Soybean cines. J. Nematol. 30: cyst nematode, Heterodera glycines, in Minnesota. Plant Dis. 64: Young, L.D., and E.E. Hartwig Cropping sequence effects on soybean and Heterodera glycines. Plant Dis. 76:78 81.