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Crop sequence effects on soybean cyst nematode and soybean and corn yields.

THE SOYBEAN CYST NEMATODE is destructive pest 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However, additional strategies are necessary to reduce SCN population density and minimize soybean yield loss (S. Chen, unpublished).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.sup.-3] soil from thousands by a factor of 10, and 5 yr of corn lowered number of eggs in 100 [cm.sup.-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

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.sup.-1] sand, 460 g [kg.sup.-1] silt, 320 g [kg.sup.-1] clay, 99 g [kg.sup.-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.sup.-1] sand, 420 g [kg.sup.-1] silt, 350 g [kg.sup.-1] clay, 71 g [kg.sup.-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.

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.sup.-1] for corn following soybeans and 196 kg N [ha.sup.-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.sup.-1] and linuron [beta]-(3,4-dichlorophenyl)-1-methoxy-1-methylurea; Lorox, du Pont, Wilmington, DE) at 1.12 kg a.i. [ha.sup.-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.sup.-1] and bentazon [beta]-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide; Basagran, BASF) at 0.82 kg a.i. [ha.sup.-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 [degrees] C before being processed. Each soil sample was thoroughly mixed. Cysts were extracted with a hand-decanting method. A subsample of 100 [cm.sup.3] 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-[micro]m-aperture sieve "nested" on a 250-[micro]m-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-[micro]m-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-[micro]m-aperture sieve nested on a 38-[micro]m-aperture sieve. Eggs were caught on the 38-[micro]m-aperture sieve and collected into a 50-mL tube and stored at 4 [degrees] 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.sup.-1] moisture, and corn yield was standardized at 155 g [kg.sup.-1].

Data Analysis

For plots in which the egg density at sowing was more than 500 eggs 100 [cm.sup.-3] soil, the SCN population change (Pr/Pi = egg density at harvest/egg density at sowing) was computed. Nematode egg counts were transformed with [log.sub.10] (x + 1) and Pr/Pi with [log.sub.10](x) to improve homogeneity of variances before analyses of variance (ANOVA). Average Pr/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 Pr/Pi against Pi with the linear model In(Pf/Pi) = In(a) + bin(Pi) derived from the equation Pf/Pi = a[pi.sup.b] 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(Pf/Pi) = ln(a) + bln(Pi) when Pi = Pf then E = Pi = [a.sup.1/-b]. The relationships between soybean yield and Pi were determined by regression with the equation Y = [ae.sup.bpi] or In(Y) = ln(a) + 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 SCN Egg Density

Waseca

In 1996, SCN population density increased from 7803 to 14690 eggs 100 [cm.sup.-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.sup.-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.sup.-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.sup.-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.sup.-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.sup.-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.sup.-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.sup.-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.sup.-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.sup.-3] soil in corn (Table 1).

In 1997, the highest Pf (14 210 eggs 100 [cm.sup.-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.sup.-3] soil, whereas susceptible soybean following corn (CS) increased egg density from 6474 to 9031 eggs 100 [cm.sup.-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.sup.-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.sup.-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.sup.-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.sup.-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.sup.-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).

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.sup.-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.sup.-3] soil in 1996, 1998, and 1999, respectively (Table 6).

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.sup.-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.sup.-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.sup.-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.sup.-1] higher yield than susceptible soybean in 1996 (Table 1). In 1997, resistant soybean following corn produced the highest yield (1808 kg [ha.sup.-1]), and susceptible soybean in monoculture produced the lowest yield, only 386 kg [ha.sup.-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.sup.-1] and was lower than other treatments except soybean rotated with 2-yr corn (SCCS), which produced 1186 kg [ha.sup.-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).

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.sup.-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.sup.-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.sup.-1] higher than yield of corn following corn (--CC) (Table 4).

DISCUSSION

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.sup.-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.sup.-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.sup.-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.sup.-3] soil (Table 6). One season of susceptible soybean could increase egg density from hundreds 100 [cm.sup.-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.sup.-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.
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.

                                             Waseca

Crop        n ([double-          Pi            Pf         Yield
([dagger])  dagger])
                      -- eggs 100 [cm.sub.-3] soil --kg  [ha.sup.-1]

C           40           7 013a ([section])   2 257b       9 871
R           12                 5 583a         1 425b       2 841a
S           20                 7 803a        14 690a       2 455b

                                             Lamberton

Crop        n ([double-          Pi            Pf          Yield
([dagger])  dagger])
                      -- eggs 100 [cm.sup.-3] soil --kg  [ha.sup.-1]

C           40                12 623a        10 768b       9 340
R           12                18 458a        11 011b       1 729a
S           20                14 206a        30 306a       1 226b

([dagger]) C = Corn; R = SCN-resistant cultivar (Freeborn);
S = SCN-susceptible cultivar (Sturdy).

([double dagger]) n = number of observations (plots).

([section]) Means in columns followed by the same letter(s)
are not significant according to LSD (P = 0.05); comparison
with corn yield was not included.
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.

                                             Waseca

Crop
Sequence    n ([double-
([dagger])   dagger])            Pi            Pf        Yield

                    -- eggs 100 [cm.sup.-3] soil --  kg [ha.sup.-1]

CC              16       1 213b ([section])  1 345b      9 737a
RC               8               997b        1 013b      9 987a
SC              16             6 442a        5 624a     10 019a
CR               8             1 224b          355b      2 756y
RR               4               836b          734b      2 616y
CS              16               964b        4 983a      2 259z
SS               4            10 675a        5 581a      2 140z

                                             Lamberton

Crop
Sequence    n ([double-
([dagger])   dagger])            Pi            Pf           Yield

                       -- eggs 100 [cm.sup.-3] soil --  kg [ha.sup.-1]

CC              16             7 802b         4 365b       8 042b
RC               8             6 090b         5 392ab      9 454a
SC              16            17 779a        14 210a       9 978a
CR               8             9 498b         2 913b       1 808x
RR               4            12 687ab        5 741ab      1 265y
CS              16             6 474b         9 031ab      1 155y
SS               4            20 780a         3 800ab        386z

([dagger]) C = Corn; R = SCN-resistant cultivar (Freeborn);
S = SCN-susceptible cultivar (Sturdy). The last letter represents
the crop in the year when the data were collected.

([double dagger]) n = number of observations (plots).

([section]) Means in columns followed by the same letter(s)
are not significant according to LSD (P = 0.05).
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.

                             Waseca

Crop
Sequence
([dagger])  n ([dagger])        Pi             Pf            Yield

                       -- eggs 100 [cm.sup.-3] soil --  kg [ha.sup.-1]

CCC              8       769cd ([section])     889c         7 743c
CRC              8              448cd          188e        10 587a
CSC             16            2 607ab        2 005b         9 281ab
SCC              8            3 020a         2 409b         8 703bc
RCR              4              309d           259de        2 950x
RRR              4              291cd          966cd        2 953x
SCR              4            3 566a         1 329bc        2 415xyz
CCS              8              618bcd      13 259a         2 494xy
RCS              4              784abc      15 475a         2 339xyz
SCS              4            2 703ab       19 823a         1 780z
SSS              4            4 954a        16 850a         2 103yz

                                            Lamberto

Crop
Sequence
([dagger])  n ([dagger])        Pi             Pf         Yield

                       -- eggs 100 [cm.sup.-3] soil --  kg [ha.sup.-1]

CCC              8            2 553d         3 695d        11 409a
CRC              8            3 369cd        4 284d        12 083a
CSC             16            7 762cd        9 687cd       11 968a
SCC              8           15 158a         1 5816ab      11 961a
RCR              4            4 598bcd       6 573bcd       2 201wxy
RRR              4            5 675bcd       8 815bcd       1 699y
SCR              4            5 275cd        9 523bc        2 659w
CCS              8            6 581cd       38 462a         2 415wx
RCS              4            4 140cd       40 142a         2 005xy
SCS              4           13 124abc      37 938a         2 353wxy
SSS              4           11 863ab       25 068ab        1 154z

([dagger]) C = Corn; R = SCN-resistant cultivar (Freeborn); S =
SCN-susceptible cultivar (Sturdy). The last letter represents the crop
in the year when the data were collected.

([double dagger]) n = number of observations (plots).

([section]) Means in columns followed by the same letter(s) are not
significant according to LSD (P = 0.05).
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.

                         Waseca
Crop
Sequence    n ([double-
([dagger])   dagger])           Pi             Pf            Yield

                       -- eggs 100 [cm.sup.-3] soil --  kg [ha.sup.-1]

CCCC             4       622ef ([section])     413cd        9 087bc
RCRC             4              538f           225d        10 976ab
SCSC             4           14 439a         6 575ab        9 063bc
CCSC             8           14 407a         7 353a         8 828c
CSCC             8            2 859c           856cd        9 268bc
SCCC             4            3 540bc        1 938bc        7 920c
RCSC             4           11 998ab       11 988a         9 238bc
SCRC             4            1 930cde       1 184cd       11 635a
RRRR             4            1 098cde       1 434c         3 224xy
CRCR             4              741def         531d         3 614x
CSCR             4            2 171cde         934cd        2 958xy
SSSS             4           21 075a        20 913a         1 549z
CSCS             4            1 983cde      11 581a         2 860y
SCCS             4            1 818cd       10 969ab        2 679y
CCCS             4            2 231cde      11 769a         2 885y
CRCS             4              353g         8 206ab        3 030xy

                                         Lamberton

Crop
Sequence    n ([double-
([dagger])   dagger])           Pi            Pf           Yield

                       -- eggs 100 [cm.sup.-3] soil --  kg [ha.sup.-1]

CCCC             4           5 790abc      1 513e         8 853bcd
RCRC             4           3 138bc       3 575d         9 560abc
SCSC             4          31 100ab      24 650ab       10 476a
CCSC             8          24 875a       19 194ab       10 480a
CSCC             8           9 000bc       6 091d         8 064d
SCCC             4           9 985abc      8 219cd        8 704cd
RCSC             4          17 900abc     22 733abc      10 345ab
SCRC             4          11 983abc      5 367abcd      9 723abc
RRRR             4           5 975abc      7 750abcd      2 209y
CRCR             4           3 722bc       2 600de        2 278y
CSCR             4          10 434bc       3 375d         2 581y
SSSS             4          23 550abc     26 350a           959z
CSCS             4           2 437d       22 050ab        2 094y
SCCS             4          19 138abc     15 250abc       1 186z
CCCS             4           3 813cd      16 767bcd       2 538y
CRCS             4           3 431cd      33 350ab        2 579y

([dagger]) C = Corn; R = SCN-resistant cultivar (Freeborn);
S = SCN-susceptible cultivar (Sturdy). The last letter represents
the crop in the year when the data were collected.

([double dagger]) n = number of observations (plots).

([section]) Means in columns followed by the same letter(s) are
not significant according to LSD (P = 0.05).
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.

                                   1996

Crop                 n ([dagger])           Pf/Pi
                                Waseca
Corn                      40       0.23b ([double dagger])
Resistant soybean         12                0.21b
Susceptible soybean       20                1.76a
                                Lamberton
Corn                      40                0.65b
Resistant soybean         12                0.64b
Susceptible soybean       20                2.36a

                       1997       1998       1999

Crop                  n  Pf/Pi   n  Pf/Pi   n  Pf/Pi
                                Waseca
Corn                 35  0.86b  30  0.66b  37  0.40b
Resistant soybean     9  0.33c   6  0.57b  11  0.49b
Susceptible soybean  15  2.26a  15  9.91a  16  3.09a
                                Lamberton
Corn                 32  0.75   30  1.24b  37  0.57b
Resistant soybean     9  0.45   10  1.92b  12  0.73b
Susceptible soybean  19  0.74   18  5.66a  17  2.92a

([dagger]) n = number of observations {plots).

([double dagger]) Means in columns followed by the same
letter(s) are not significant according to LSD (P = 0.05).
Table 6. Relationship between reproduction factor and initial
population density of Heterodera glycines in susceptible
soybean. ([dagger])

Year  df    a       b     E ([dagger])    r      P

                      -- Waseca --
1996  18  3 376   -0.851     13 971     -0.52  0.0188
1997  13  3 613   -0.965      4 872     -0.88  0.0000
1998  13  3 146   -0.780     30 628     -0.90  0.0000
1999  14    565   -0.648     17 558     -0.64  0.0000
                    -- Lamberton --
1996  18    523   -0.595     37 269     -0.58  0.0080
1997  17    --      --         --       -0.11  0.6468
1998  16  20 947  -0.951     34 838     -0.88  0.0000
1999  14  19 061  -0.982     22 779     -0.92  0.0000

([dagger]) The reproduction factor is expressed as Pf/Pi, where
Pf is the eggs 100 [cm.sup.-3] soil at harvest and Pi is the
eggs 100 [cm.sup.-3] soil at sowing. The relationship between
the Pf/Pi and Pi was determined by the linear model In (Pf/Pi) =
In(a) + bin(Pi) derived from an empirical model Pf/Pi = a[Pi.sup.b]
(Ferris, 1985).

([double dagger]) E means predictated equilibrium point (carrying
capacity), which was determined by solving the Pf/Pi = a[Pi.sup.b]
when Pf = Pi then E = Pi = [a.sup.1/-b].
Table 7. Relationship between soybean yield and egg densities
of Heterodera glycines at sowing.  ([dagger])

                              Waseca            Lamberton

Soybean Cultivar    Year    r    n     p       r    n     p

Susceptible Sturdy  1996  -0.23  20  0.3264  -0.58  20  0.0086
                    1997  -0.17  20  0.4772  -0.38  20  0.0972
                    1998  -0.47  20  0.0366  -0.31  20  0.1872
                    1999  -0.88  20  0.0000  -0.57  20  0.0101
Resistant Freeborn  1996  -0.56  12  0.0577  -0.06  12  0.8446
                    1997  -0.38  12  0.2268  -0.75  12  0.0052
                    1998  -0.59  12  0.0431  -0.41  12  0.1811
                    1999  -0.69  12  0.0123  -0.28  12  0.4143

([dagger]) The relationship between soybean yield and Pi was
determined by regression with the equation Y = [ae.sup.bpi]
or In(Y) = In(a) + bpi (Y is soybean yield, Pi is nematode 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.


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.

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Senyu Chen, * Paul M. Porter, Curtis D. Reese, and Ward C. Stienstra

Senyu Chen. Univ. of Minnesota Southern Research and Outreach Center, 35838 120th Street, Waseca, MN 59093; Paul M. Porter, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108; Curtis D. Reese, Dep. of Plant Science, South Dakota State Univ., Brookings, SD 57007: and Ward C. Stienstra, Dep. of Plant Pathology, Univ. of Minnesota, St. Paul, MN 55108. This research was supported by Minnesota Soybean Producers Check-off Funding through Minnesota Research and Promotion Council and Minnesota Agric. Exp. Stn. Received 8 Feb. 2001. * Corresponding author (chenx099@ umn.edu).
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Author:Chen, Senyu; Porter, Paul M.; Reese, Curtis D.; Stienstra, Ward C.
Publication:Crop Science
Article Type:Abstract
Geographic Code:1USA
Date:Nov 1, 2001
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