© 2008 Plant Management Network.
Effect of Soybean Cultivars Moderately Resistant to Soybean Cyst Nematode on SCN Populations and Yield
Eric A. Adee, Principal Research Specialist, and Martin L. Johnson, Research Specialist, Northwestern Illinois Agricultural Research and Demonstration Center, 321 210th Avenue, Monmouth 61462; and Terry L. Niblack, Professor, Department of Crop Sciences, University of Illinois, 1102 S. Goodwin Ave., Urbana 61801
Adee, E. A., Johnson, M. L., and Niblack, T. L. 2008. Effect of soybean cultivars moderately resistant to soybean cyst nematode on SCN populations and yield. Online. Plant Health Progress doi:10.1094/PHP-2008-0618-03-RS.
Spread of soybean cyst nematode Heterodera glycines (SCN) to much of the soybean (Glycine max) growing region in the Midwest has created a persistent and significant annual yield loss for soybean. Host resistance has been the primary means of reducing yield loss to SCN. It is not known how moderately resistant cultivars fit into the management of SCN. Moderately resistant cultivars can have high yield potential, but nematode reproduction is greater than on resistant cultivars. Moderate resistance is defined by a SCN female index (FI) of 10 to 29 in standardized tests, whereas cultivars with an FI < 10 are considered resistant. Two each of SCN-resistant, moderately resistant, and susceptible (FI > 60) cultivars were planted in the same plots for two soybean crops in annual rotation with corn. The SCN population was reduced 80 and 54% by resistant and moderately resistant cultivars, respectively, and increased 189% by the susceptible. Yields of the resistant and moderately resistant were 8.2 and 11.8 bu/acre better, respectively, than for the susceptible. All plots were planted to a susceptible cultivar in the final year of the study, and demonstrated there was a carry-over effect from previous cultivars. Following resistant and moderately resistant cultivars, yields of the susceptible were 6.6 and 4.3 bu/acre above following susceptible cultivars. This study showed that moderately resistant soybean cultivars can be an effective tool for improving profitability of soybean.
Soybean cyst nematode (SCN), Heterodera glycines, has spread to much of the soybean (Glycine max) growing region in the Midwest. A survey in Illinois found 83% of the fields infested with SCN (7). Yield losses up to 30% have been reported, which do not always correlate well with population densities of SCN, because losses are also influenced by other factors such as soil type, cultivar, and environment (3); however, reducing population densities of SCN has been a major component of managing SCN.
Host resistance has been one of the main tools utilized to reduce yield loss to SCN through increased yield and reduced SCN population density (2). Lengthening the time between the planting of SCN-susceptible hosts through crop rotation has been another successful tool for reducing the yield loss to SCN (5). However, in much of the northern soybean growing region of the United States, soybean is grown in annual rotation with corn, resulting in heavy reliance upon resistant cultivars to improve yields in the presence of SCN (2).
Many cultivars labeled as resistant do not perform equally, either in terms of yield or reducing SCN reproduction. Initially, lower yield potential, commonly know as yield drag, was associated with SCN resistance, but the yield potential of resistant cultivars has improved. As a result, many of the top cultivars in yield trials have been those labeled as SCN-resistant (4) Additionally, testing at the University of Illinois has shown that there is a high level of variability among cultivars labeled "resistant" in terms of nematode reproduction (9). The classical definition of SCN resistance (8) is based on SCN development, measured as a female index (FI) (6). The FI is the mean number of females produced on a soybean cultivar, divided by the mean number of females on a susceptible check cultivar, × 100, in a standardized test. A cultivar with an FI < 10 is SCN-resistant (R); cultivars with FI 10 to 29 are moderately resistant (MR); those with FI 30 to 60 are moderately susceptible (MS); and those with FI > 60 are susceptible (s). Yield trial results have shown some MR cultivars can have very good yield potential. The goals of this research were to: (i) compare the effects of a set of R, MR, and S soybean cultivars on the population dynamics of SCN in a naturally-infested field through two soybean crops in an annual rotation of corn/soybean; (ii) compare the yields of these cultivars; and (iii) determine if the use of resistant cultivars in previous years may have benefits for subsequent soybean crops through reduction in SCN population density.
Field Comparison of Moderately Resistant Cultivars
The experimental design was a randomized complete block with six replications, planted at the University of Illinois Northwestern Illinois Agricultural Research and Demonstration Center (NWRC) at Monmouth, Illinois. Two fields were used, designated F4 and E1. The soil types were Muscatune silt loam (2% sand, 73% silt, 25% clay, 4 to 6% organic matter) and Sable silty clay loam (2% sand, 67% silt, 31% clay, 4 to 6% organic matter). Six soybean cultivars, two each labeled susceptible (S), moderately resistant (MR), and resistant (R) to SCN, were planted individually in 50-ft-long by 20-ft-wide plots, with 8 rows of 30-inch spacing. The cultivars planted in 2001 and 2002 were 9306 (S), 93B82 (S), 93B11 (MR), 93B66 (MR), 92B91 (R), and 93B67 (R). Due to seed availability issues in 2003 and 2004 the following cultivars were replaced, 9306 replaced with 92B84 (S), 93B11 replaced with 93B15 (MR), and 93B66 replaced with 93B86 (MR). Planting dates for the study were 16 May for 2001 and 2003, and 10 May for 2002 and 2004. All cultivars were provided by Pioneer Hi-Bred International, and ranged in maturity from 2.8 to 3.8. Resistance labeling by Pioneer was as follows: "R" if their isolate of SCN ‘Race 3’ produced FI of 8 to 15, and "MR" for FI of 16 to 32 (Jeff Thompson, personal communication).
Each field was annually rotated between corn and soybean crops, chisel plowed in the fall and field cultivated in the spring prior to planting. The plots were measured to ensure the same type of cultivar was planted in the same location as in the previous soybean cropping year. The cropping timeline for the study is shown in Table 1.
Table 1. Cropping timeline for SCN study in two fields at NWRC.
Field F4 in 2005 and field E1 in 2006 were planted 5 May 2005 and 10 May 2006, respectively, to the S cultivar Pioneer 92M91 in order to determine if the effects of the previous cultivar choices could still be detected. Yields were taken from the sites of individual plots corresponding to the respective cultivars planted in the two previous soybean crops.
In 2001, field F4 had moderate SCN pressure of 890 SCN eggs/100 cm³ soil (stderr = 226) at planting. Field E1 in 2002 had an initial SCN population of 5888 SCN eggs/100 cm³ soil (stderr = 837) at planting. Population densities of SCN were determined for each plot at planting and soybean maturity, except in the final year when samples were collected at planting only. Soil samples were a composite of eight 1 inch -diameter cores taken to a depth of ca. 8 inches from the middle 40 ft of the center two rows of each plot. The SCN population density was quantified as number of eggs per 100 cm³ of soil by elutriation as described previously (1,10).
Grain yield and moisture measurements were taken from combine harvest of the center two rows of each plot. Plots were harvested 12 October 2001, 8 October 2002, 7 October 2003, 5 October 2004, 10 October 2005, and 20 September 2006. Yields were corrected to 13% grain moisture. Data were analyzed with SAS (SAS Institute Inc., Cary, NC) procedures, including general linear models (GLM) and regression (REG).
MR cultivars reduced the SCN population densities in both fields similarly to the R cultivars, even though the initial population density was moderately high in one field and relatively low in the other (Tables 2 and 3). The average reduction in SCN population after two years with the moderately resistant cultivars was slightly less than with the resistant cultivars, 2092 and 2923 eggs/100 cm³ soil, respectively, which were not significantly different. This reduction contrasts with a significant increase (average of 4660 eggs) in SCN population after two years of planting SCN susceptible cultivars (Table 2 and 3). There was an interaction between year and resistance (Pr > F = 0.001) for SCN population that was the result of higher number of eggs being produced on the S cultivars for certain years.
Table 2. Effect of SCN resistance for 2 years on SCN population with corn/soybean rotation.*
* Data are the mean of two cultivars in each category.
Table 3. Effect of SCN resistance for 2 years on SCN population with corn/soybean rotation.*
* Data are the mean of two cultivars in each category.
In field F4, with a relatively low population of SCN (890 eggs) at the start of the study, the resistant cultivars reduced the population 80%, almost below the action threshold of 150 eggs (Table 2). The MR cultivar reduced the population 30%, whereas the susceptible cultivars increased the SCN population over six times higher to a moderately high level after only one year, demonstrating how rapidly SCN can increase if not managed.
Results were similar in field E1, with the moderately high SCN population at the beginning of the study (5888 eggs) (Table 3). The main difference in these results from those from field F4 was in the amount by which the R and MR cultivars reduced the population. The MR and R cultivars reduced the SCN population 4000 to 5000 eggs from the initial population over two cropping years, or 65 and 85%, respectively. Sixty percent of the reduction in SCN numbers happened in the first year with both R and MR cultivars, with the R cultivars causing further reduction the second year. As in field F4, the SCN population in field E1 increased greatly with susceptible cultivars causing a 300% increase in SCN population the first year; with SCN numbers increasing the second year to 170% higher than the initial egg counts.
These results show that cultivars labeled R and MR can have very similar effects on the SCN population densities. Other subsets of cultivars from these respective groups could be less similar in the degree they reduce SCN population because of the FI of values assigned to each category. The criteria used to determine if a cultivar is classified R or MR varies among seed companies, which could include different FI values or relative yields in the presence of SCN. Even with this standardized method of classifying varietal resistance to SCN, there are still ranges for each group (6,8). Therefore, it should not always be assumed that the SCN population will be reduced the same amount by resistant and moderately resistant cultivars, as was the case in this study.
Moderately resistant cultivars consistently had the highest yields throughout in this study (Table 4), averaging 27% greater than the yields of the susceptible cultivars. The resistant cultivars averaged 19% greater yields than the susceptible cultivars. The resistant cultivars tended to yield less than the moderately resistant cultivars each year, although the difference was significant only in 2004. There was no interaction between year and resistance for yield (Pr > F = 0.38).
Table 4. Effect of SCN resistance on soybean yield in corn/soybean rotation.*
* Data are the mean of two cultivars in each category.
This study shows that soybean cultivars with moderate resistance to SCN can be effective management tools for reducing SCN populations, much like cultivars classified as resistant to SCN. While the R cultivars reduce the SCN population more than MR cultivars, these differences were not significant in this study. Placing the MR cultivars in most fields with SCN would be acceptable, although it would be better to use R cultivars if the SCN populations are very high (i.e., 7000 to 8000 eggs or more per 100 cm³). In such fields, lowering the SCN population quickly is recommended, especially because our data show that the yields of all cultivars are affected by SCN (Table 5).
Table 5. Regression model parameters between spring SCN population and yield, 2001-2004.
Moderately resistant cultivars clearly have the potential to significantly increase productivity of soybeans in a SCN infested field. The increased yield potential seen with the MR cultivars over the R cultivars in this study suggests that they would be a viable tool in managing SCN. Previously, there has been reluctance by growers to plant SCN resistant cultivars because of the reported yield drag. However, our data show that improvements in soybean genetics have increased the yields of cultivars with moderate to high levels of resistance to result in a yield benefit over high-yielding S cultivars even when the SCN pressure is relatively low.
The yield comparison of MR and R cultivars in this study is between a very small number of cultivars that are no longer sold by Pioneer Hi-Bred International. However, comparing the top 20 yielding cultivars in University of Illinois Region 2 Roundup Resistant cultivar trial in 2006, 16 were resistant to HG 0, 2 moderately resistant and 2 susceptible in Maturity Group 2 (4,9) based on Schmitt and Shannon’s differentiation of soybean cultivars. Similarly, in the same trial with Maturity Group 3 cultivars, 12 of the 20 cultivars with the highest yields were resistant, 7 moderately resistant, and 1 susceptible (labeled resistant). This indicates that the genetics have improved yield potential of SCN resistant soybean cultivars currently available, and that there is not the yield drag once associated with all cultivars having resistance to SCN.
Bioassay of ‘Carry-over’ Effect from Previous Cultivars
The yield data from the S cultivar planted at the end of the study following two years of soybeans that were resistant, moderately resistant or susceptible to SCN showed there was a definite benefit of planting cultivars with some level of SCN resistance that carries over to the next soybean crop (Table 6). The yield of the S cultivar in the bioassay was 18% and 11% greater following the R and MR cultivars, respectively, than following the susceptible cultivars. This is primarily due to the reduced population density of SCN at planting (Fig. 1, Table 6). The population density of SCN following two years with susceptible cultivars was 3.8 and 9.7 times greater than following two years with MR and R cultivars, respectively.
Table 6. Effect of prior planting of SCN resistant cultivars for two crops on yield and height of a susceptible soybean cultivar in corn/soybean rotation.*
* Data are mean for cultivar Pioneer 92M91 planted in field F4 in 2005 and field E1 in 2006.
The data from the bioassay with the SCN susceptible cultivar (Fig. 1) showed a negative relationship between the spring SCN egg counts and yield when the population was less than10,000 eggs. This linear relationship is described by Y = 46.85 - 0.00259X (Pr > F = 0.0001, R² = 0.36). This means there was 2.6 bu/acre loss of yield for every increase of 1000 SCN eggs. Previous data from NWRC has shown the yield loss per 1000 SCN eggs to range from 0.8 (Table 5) to 6.3 bu/acre (not shown). This range of yield loss can be related to the virulence of the nematode and the environment (3) the first 60 days after planting, when SCN are infecting the soybeans. In both years of the bioassay, the rainfall totals during the growing seasons were very similar. As a result, in the combined analysis of the two years of the bioassay, year was not a significant factor (Pr > F = 0.23), nor was there an interaction of year with level of SCN resistance (Pr > F = 0.29).
As the SCN population exceeds 10,000 eggs the relationship between population and yield flattens out. The yield loss to SCN often levels off at the higher populations, and even ultra-high levels of SCN will not kill soybeans (T. L. Niblack, personal communication). However, it is imperative to reduce the population of SCN as quickly and as far as possible to be able grow soybeans profitably and to help avoid the development of SCN populations virulent against current sources of resistance to SCN.
Almost all of the Maturity Group 2 and 3 cultivars having SCN resistance are from the same source, PI 88788, which creates concerns over cultivars losing resistance after repeated cropping and selection of parasitic forms in the SCN population. Therefore, it will be important to follow current recommendations to not plant the same SCN resistant cultivars in the same field for consecutive soybean crops. Even with the same source of resistance, the difference in the package of resistance genes between cultivars will help avoid the same type of intense selection pressure present when the same resistant variety is planted in the same field. It was not a goal of this study to determine if there were shifts in populations of SCN; however, it was determined that the population in field E1 was HG Type 2.5.7 at the conclusion of the study (T. L. Niblack, personal communication).
Incorporating Moderately Resistant Cultivars into SCN Management
This study shows the benefit of incorporating cultivars with some degree of resistance to SCN to manage SCN, which includes those classified as moderately resistant. Utilizing results from university cultivar trials in combination with SCN resistance can reduce losses of yield to SCN and increase profit. Furthermore, the benefit of SCN resistance carried over into the subsequent planting of SCN-susceptible cultivars because of the reduction in SCN populations. The improvements in genetics and information sources have given growers the tools necessary to make good decisions to limit yield losses to SCN.
We thank Shelly Adee for her work on this paper. This study was funded in part by Pioneer Hi-Bred International.
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