Crop Science Journal of Natural Resources and Life Sciences Education
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Published online 8 September 2006
Published in Crop Sci 46:2081-2083 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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PLANT GENETIC RESOURCES-NOTES

Using Independent Culling to Screen Plant Introductions for Combined Resistance to Soybean Cyst Nematode and Sudden Death Syndrome

J. R. Gelina,*, P. R. Arellib and G. A. Rojas-Cifuentesa

a Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105
b USDA-ARS, Mid South Area, Jackson, TN 38301

* Corresponding author (Jean.Gelin{at}ndsu.nodak.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Two infectious diseases that cause most yield losses in soybean [Glycine max (L.) Merr.] are soybean cyst nematode (SCN), caused by Heterodera glycines Ichinohe, and sudden death syndrome (SDS), caused by Fusarium solani (Mart.) Sacc. f. sp. glycines (Fsg). Because SCN and SDS have a synergistic effect on yield when they occur jointly in the field, breeders are attempting to develop varieties with dual resistance to these two diseases. Using independent culling as a selection strategy, we screened a set of 31 new soybean plant introductions (PI) that were field evaluated in 1995 at two locations in Southern Illinois. We identified 11 elite PIs that were resistant to SCN race 3, had yellow seed coat, a relatively good field response to SDS, and a moderate seed yield. These superior genotypes can be used as potential parents in soybean breeding programs.

Abbreviations: DI, disease incidence • DS, disease severity • DX, disease index • HG, Heterodera glycines • MG, maturity group • PI, plant introductions • FI, female index, SCN, soybean cyst nematode • SDS, sudden death syndrome • SQRTDX, square root of DX


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
DISEASE RESISTANCE is one of the most important goals of the genetic improvement of soybean. Two diseases that cause the most yield losses in soybean are Soybean Cyst Nematode (SCN), caused by Heterodera glycines Ichinohe (Rao-Arelli et al., 1992) and Sudden Death Syndrome (SDS), caused by Fusarium solani f. sp. glycines (Fsg) (Roy, 1997). Damages are generally more severe when these two diseases occur together in the field. From a purely economic standpoint, SCN is currently the most widespread and devastating disease both in the United States and throughout the world. According to Wrather et al. (2003) of the Plant Management Network, SCN caused a yield loss estimated at $783.8 million in the US alone during the 2002 season.

To date, 16 races of SCN have been identified with various degrees of virulence (Riggs and Schmitt, 1988). This makes it possible for resistance to be both race and cultivar specific. A major step needed for breeding SCN-resistant cultivars was the development of a classification scheme that would separate the major genetic groups of nematodes. Initially it was proposed based on comparative development of females on four differentials. This was expanded to include 16 different races which also failed to address the variability in the nematode population and resistance characterization of soybean. The HG Type test immediately supersedes the race test for describing nematode populations (Niblack et al., 2002). The test is very similar to that used for race determination, but conducted according to a standardized set of rules and set of indicator lines (Plant Introductions, PI) and Female Index is calculated. For example, a nematode population that produces FI of 10 or more on ‘Peking,’ PI88788, and PI89772 is an HG Type 1.2.6. A population that produces no FI of 10 or more on all indicator lines (seven currently) is HG Type zero (0) or Race 3 (Rao-Arelli, unpublished data, 2002).

The association between SCN resistance loci and yield depression has been established (Kopisch-Obuch et al., 2005), and a possible genetic linkage between the genes responsible for the plant resistance or susceptibility to SCN and SDS has also been reported (Chang et al., 1995). Although SCN is not required for SDS development in the field, soybean infection by F. solani in the presence of SCN-3 results in SDS symptoms that were more severe and developed earlier than when SCN-3 is not present (Lawrence et al., 1988). Therefore, breeding for separate resistance may not be a successful strategy because of the synergistic effect of the two diseases in the field.

Historically, soybean breeders have used different breeding strategies to develop disease resistant cultivars, ranging from various selection indices (Byth et al., 1969) to the more recent attempts of marker-assisted selection (Concibido et al., 1996; Hnetkovsky et al., 1996; Chang et al., 1997; Prabhu et al., 1999). The microsatellite marker Satt038, mapped to linkage group G, is known to be tightly linked (approximately 2.4 cM) to the major SCN resistance gene rhgI identified in soybean accessions (Akkaya et al., 1995; Mudge et al., 1996; Yuan et al., 2002). In addition, Prabhu et al. (1999) found that Satt038 alone could also be used successfully to identify recombinant inbred lines resistant to F. solani infection. It is therefore possible to use that marker, in combination with other markers with similar effects, to identify genotypes with dual resistance to both SCN and SDS.

Breeders concerned with SCN and SDS use various selection methods to evaluate and incorporate soybean PIs into their programs to develop improved cultivars with, preferably, dual resistance to both diseases. Generally, soybean PIs resistant to SCN have poor agronomic performance, a relatively high susceptibility to SDS in the field, and are characterized by a black seed coat (Gibson et al., 1994; Gelin, 1996). In addition, there is a genetic linkage between known SCN resistance genes and quantitative trait loci (QTL) that confer yield depression in resistant cultivars (Kopisch-Obuch et al., 2005). The association of SCN resistance with these traits is undesirable as it complicates the incorporation of these genotypes into breeding programs due to linkage drag.

One selection method that can identify elite soybean PI lines for breeding purposes is independent culling (Hallauer and Miranda Fo, 1988). With this selection strategy, important agronomic traits are identified first, and genotypes are selected for each trait one after the other in a stepwise manner. Depending on the correlation among traits, the number of selected genotypes normally decreases after selection is made for each individual trait. Our objective in this research was to analyze new soybean accessions and identify superior lines using independent culling, based on SCN resistance, seed coat color, SDS response, and seed yield.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Soybean PI lines were first evaluated for their reaction to isolates of H. glycines Ichinohe, using a standard protocol according to Rao-Arelli (1994). Seeds were planted in the greenhouse and seedlings were transplanted into pots containing infested soils. After 30 d, cysts were washed from the roots and counted under microscope. Response differentials were measured, and each PI genotype was classified according to its response to a specific SCN race.

In 1995, 31 newly identified SCN-3 resistant PIs (Rao-Arelli, unpublished data, 1994) were field evaluated at Ullin and Ridgway in Southern Illinois for their response to SDS and agronomic performance. A rectangular lattice design with two replicates was used at each location. Plots were 4 m long and 0.76 m apart, and were later end-trimmed to 3 m, as previously described (Gelin and Arelli, 2003). Besides SCN resistance, we also used SDS response, seed yield (t ha–1), and seed coat color as additional selection criteria for the identification of superior PI genotypes. Maturity group (MG) was not considered an important selection factor despite its potential effect on seed yield (Heatherly, 2005). Generally, plant introductions are screened not for their yield but primarily for the disease resistance genes they carry. When identified, these new resistance sources can be incorporated into breeding programs across maturity groups.

Disease incidence (DI) for SDS was recorded per plot as the percentage of plants showing visible leaf symptoms. Disease severity (DS) was also rated from symptomatic plants only, using a scale of 1 to 9, where 1 = 0–10% total of leaf area necrotic, 2 = 10–20% chlorotic or any necrosis up to 10%, 3 = 20–40% chlorotic or 10–20% necrotic, 4 = 40–60% chlorotic or 20–40% necrotic, 5 = greater than 60% chlorotic or greater than 40% necrotic, 6 = premature leaf drop up to 1/3 defoliation, 7 = premature leaf drop from 1/3 to 2/3 defoliation, 8 = premature leaf drop greater than 2/3 defoliation, and 9 = premature death. The disease index (DX) was later estimated according to Gibson et al. (1994) using the formula DX = (DI x DS)/9, with a range of 0 (no disease) to 10 (all plants dead before R6). The square root of the disease index (SQRTDX) was calculated and used in the statistical analysis. Low SQRTDX values indicate resistance, and high values indicate susceptibility to SDS.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Twenty-four of the 31 lines had a yellow seed coat (77%), the seven remaining were either black or a green seed coat. Among those 24 yellow seeded genotypes, 11 had a good response to SDS (SQRTDX < 3.0) combined with a relatively good seed yield (range: 1.5–2.7 t ha–1) (Table 1). Four lines belong to MG II, two to MG III, four to MG IV, and one to MG V. Several lines had an SQRTDX value similar to that of the SDS resistant control cultivar Pharaoh (Schmidt et al., 1993). Seed yield was relatively low compared to Pharaoh, and similar to that of the SDS susceptible control ‘Spencer’ (Wilcox et al., 1989).


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  		Table 1. Maturity group, across location means for yield (t ha–1) and SDS response (SQRTDX) of two controls and 11 PI soybean lines grown in 1995 at Ullin and Ridgway, IL.

 
The genotypes selected in this study are characterized by resistance to SCN-3, a yellow seed coat, a relatively good response to SDS in the field, and a moderate seed yield. A low seed yield is generally expected from PI lines since they are landraces and unadapted genotypes. From a conventional breeding standpoint, however, these superior PI genotypes offer some hope of combined resistance to both SCN and SDS. The yellow seed of these new resistance sources is a great advantage to soybean breeders because the association of SCN resistance with black seed coat has been broken. This will significantly reduce the amount of backcrosses that would otherwise be necessary to remove this undesirable trait from the progeny. These genotypes can therefore directly be incorporated into breeding programs aimed at developing improved soybean genotypes with a combined field resistance to SCN and SDS. Independent culling has proved to be a good selection strategy to identify these potential parental genotypes.

From a molecular perspective, the relatively high degree of genetic similarity of these genotypes has been established (Gelin and Arelli, 2003). The microsatellite marker Satt038, used also in that study and whose potential role in detecting soybean genotypes with dual resistance is known (Prabhu et al., 1999), can be very useful in evaluating crosses derived from these superior PI lines. A dual resistance to SCN and SDS is therefore a reachable goal for soybean breeders, and these elite soybean plant introductions identified through independent culling can be used for that purpose.


   ACKNOWLEDGMENTS
 
The authors express their gratitude to Drs. Paul Gibson, Oval Myers, Jr., and David Lightfoot for their assistance and guidance during this study conducted at Southern Illinois University at Carbondale, IL.

Received for publication December 29, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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