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Crop Science 41:1726-1731 (2001)
© 2001 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Inoculum Rate Influences Selection for Field Resistance to Soybean Sudden Death Syndrome in the Greenhouse

V. N. Njiti*,a, J. E. Johnsona, T. A. Tortoa, L. E. Grayb and D. A. Lightfoota

a Dept. of Plants, Soil, and General Agriculture, Southern Illinois Univ., Carbondale, IL 62901-4415
b USDA, ARS, Dept. of Crop Science, Univ. of Illinois, Urbana, IL 61801

* Corresponding author (vicnji{at}siu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Effective selection of field resistance to soybean sudden death syndrome (SDS) caused by Fusarium solani (Mart.) Sacc. f. sp. glycines (Fsg) (Roy, 1997), measured by disease index (DX), requires multiple environments. Current greenhouse assays reduce genotype x environment interactions, but fail to predict field resistance. Our objective was to compare selection for field resistance to SDS in the greenhouse among recombinant inbred lines (RILs) inoculated with Fsg at three rates. Thirty soybean [Glycine max (L.) Merr.] RILs with characterized field resistance to SDS were evaluated in the greenhouse for scorch severity at three inoculum rates in four experiments. Ten cultivars with characterized field resistance were compared using disease severity (DS) readings from one experiment at one inoculum rate. The heritability of DS among RILs in the greenhouse was 46% at the low, 66% at the moderate, and 37% at the high inoculum rates. Reduced inoculum rates in the greenhouse (3500 to 5000 spores cm-3 plant growth medium) provided DS values that explained {approx}65% of variation in the field DX. Using a Fsg inoculum rate of 5000 spores cm-3 plant growth medium and greenhouse midparent DS as criterion for selection, the number of lines potentially resistant to SDS within a segregating population could be reduced by 53%. Errors caused {approx}10% of field resistant lines to be eliminated. Among unrelated soybean cultivars, greenhouse DS values from an inoculum rate of 4000 spores cm-3 plant growth medium explained 81 and 73% of variations in field DS and DX, respectively. Therefore, the method is an effective tool for inheritance studies and cultivar evaluation for SDS.

Abbreviations: DS, disease severity • DX, disease index • Fsg, Fusarium solani f. sp. glycines • RIL, recombinant inbred line • SDS, sudden death syndrome


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOYBEAN SUDDEN DEATH SYNDROME significantly reduces soybean yield in the midwestern USA and South America (Wrather et al., 1997; Roy et al., 1997; Njiti et al., 1998b). Protection against yield loss derives from the use of SDS-resistant cultivars (Gibson et al., 1994; Njiti et al., 1998b).

In the greenhouse, monogenic resistance to leaf scorch was identified in ‘Ripley’ (Stephens et al., 1993a) and bigenic resistance to leaf scorch in ‘P9451’ (Ringler and Nickell, 1996). Resistance to SDS in the field is partial (Njiti et al., 1996, 1997, 1998a; Iqbal et al., 2001), multi-genic (Hnetkovsky et al., 1996; Chang et al., 1996), and derived from both root resistance loci (Njiti et al., 1997, 1998a; Prabhu et al., 1999) and leaf scorch resistance loci (Gibson et al., 1994; Meksem et al., 1999). Selection for SDS resistance in the field is complicated by the quantitative nature of the trait and interactions between resistance loci and the environment (Njiti et al., 1996). Selection for stable and durable resistance to SDS might be improved using controlled environmental conditions in the greenhouse or growth chambers (Stephens et al., 1993a,b). However, the existing greenhouse assays with seedlings have not accurately predicted field responses of mature plants in inheritance studies (Torto et al., 1996).

The two most common greenhouse assays for SDS inoculate seedling roots with Fsg-infested oat seeds (Lim and Jin, 1991; Hartman et al., 1997) or a Fsg-infested sand and cornmeal mixture (Killebrew et al., 1988; Gray and Achenbach, 1996). In both assays, disease is rated in seedlings 2 wk after inoculation (Stephens et al., 1993a, b; Melgar and Roy, 1994; Torto et al., 1996). In the field, plants are exposed to the pathogen from planting through the reproductive period before leaf symptoms appear (Gibson et al., 1994; Njiti et al., 1997). The latent period of the disease differs between the field and greenhouse because the inoculum rate is higher (>10 000 spores cm-3 plant growth medium) in the greenhouse (Torto et al., 1996) than in the field (<5000 spores cm-3 plant soil; Roy et al., 1997). High pathogen rates overcome both partial and complete plant resistance to disease pathogens (Parleviet, 1979; Tooley and Grau, 1982) including the soybean–Fsg interaction (Torto et al., 1996; Gray and Achenbach, 1996; Hartman et al., 1997). The objective of this study was to compare selection for field resistance to SDS in the greenhouse by leaf DS at three inoculum rates.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The genetic material included 30 F5:10 RILs (a subset of a population of 100 lines) from the cross of ‘Essex’ (Smith and Camper, 1973) x ‘Forrest’ (Hartwig and Epps, 1973) and 10 soybean cultivars of diverse genetic background. Essex is susceptible, while Forrest is resistant to SDS (Gibson et al., 1994; Hnetkovsky et al., 1996). One of the RILs (ExF78) has been released as germplasm under the name LS-G96 (Schmidt et al., 1999). LS-G96 is resistant to the soybean cyst nematode (Heterodera glycine Ichinohe) and soybean SDS. The RILs were selected by DX mean (Gibson et al., 1994) to include three classes (10 RILs per class) that contrasted for SDS DX and DS scores by the mean of five field environments. The classes were (i) field resistant, the 10 most resistant of the 100 lines, eight of which were significantly more resistant than Forrest; (ii) field partially-resistant, the 10 lines around the population median; and (iii) field SDS-susceptible; the 10 least resistant lines, all of which were more susceptible than Essex (Hnetkovsky et al., 1996). Although SDS resistance in the field is an incomplete resistance in all cultivars and is controlled by several genes, resistance and partial resistance will be used to refer to SDS Classes 1 and 2, respectively, in this paper. The 10 diverse soybean genotypes included four cultivars that were susceptible to SDS [Flyer (McBain et al., 1990), Spencer (Wilcox et al., 1989), A5403, and CM497]; five cultivars and one plant introduction that were resistant to SDS in the field [Ripley (Cooper et al., 1990), ‘Jack’ (Nickell et al., 1990a), ‘Manokin’ (Kenworthy et al., 1996), ‘Hartwig’ (Anand, 1992), ‘Hamilton’ (Nickell et al., 1990b), and PI 520733].

The Fsg isolate (ST90) was isolated from SDS-infected roots of the soybean cultivar Spencer in Stonington, IL, in 1990 by single spore isolation (Stephens et al., 1993a). The strain, stored on Bilays medium, was subcultured on potato dextrose agar medium and used to infest a 1:1 (v/v) mixture of cornmeal and SiO2. After incubation at room temperature for 10 d (O'Donnell and Gray, 1995), 5 cm3 of the inoculum was added to 250 mL of sterile water, and the average count of spores (in 10 samples of 1 mL) determined on a hemocytometer under a microscope. Spore counts were used to calculate the volume of culture necessary for each inoculum rate. The plant growth medium consisted of a 1:1 (v/v) mixture of sterile sand and soil in all experiments.

Five experiments were conducted in the greenhouse at the Southern Illinois University Horticulture Research Center in Carbondale, IL. To evaluate the effect of inoculum rate on leaf scorch resistance by SDS DS among RILs, four experiments were conducted using three inoculum rates (high = 104, moderate = 5 x 103, and low = 3.3 x 103 spores cm-3 plant growth medium). Two experiments were replicated two times and two were replicated five times, for a total of 14 replications per genotype per treatment. To test the robustness of the greenhouse assay, ten soybean genotypes of diverse genetic background were evaluated at 4000 spores cm-3 plant growth medium in a fifth experiment. Experiments were conducted between 1 Nov. 1996 and 15 Mar. 2000. Plants were grown with a 14-h photoperiod, and the air temperature ranged from 20 ± 2°C at night to 27 ± 2°C during the day in the greenhouse.

All greenhouse experiments were planted in a randomized complete block design. Parents and noninoculated control plants were included in the experiments. Two-week-old seedlings were transplanted onto Fsg-infested plant growth medium in four-inch styrofoam cups, and kept saturated to 5.08 cm with water for 4 wk. Sudden death syndrome DS was rated at 21 d after inoculation, determined on the basis of the degree of leaf damage (chlorosis/necrosis) on each plant, and was rated on a scale of 1 to 9 (1 = 0–10%/1–5%, 2 = 10–20%/6–10%, 3 = 20–40%/10–20%, 4 = 40–60%/20–40%, 5 = >60%/>40% of leaf surface chlorosis/necrosis, respectively, 6 = up to 33% premature defoliation, 7 = up to 66% premature defoliation, 8 = >66% premature defoliation, and 9 = premature death of plant).

Field resistance was previously evaluated in Villa Ridge in 1990 with soil type Bonnie silt loam (Fine-silty, mixed, active, acid, mesic Typic Fluvaquents); Pulaski in 1991 with soil type Dupo silt loam (Coarse-silty over clayey, mixed over smectitic, superactive, nonacid, mesic Aquic Udifluvents); Cora in 1991 with soil type Gorham silty clay loam (Fine-silty, mixed, superactive, mesic Fluvaquentic Endoaquolls); Cora in 1992 with soil type Dupo silt loam (Coarse-silty over clayey, mixed over smectitic, superactive, nonacid, mesic Aquic Udifluvents); and Ridgway in 1993 with soil type Patton silty clay loam (Fine-silty, mixed, superactive, mesic Typic Endoaquolls); for a total of five southern Illinois environments. Plots were rated for SDS disease incidence and DS. Disease incidence was the percentage of plants in the plot with visible SDS leaf symptoms. Disease severity was rated using the same scale as for greenhouse-grown plants, except it was average over all disease plants in each plot. Disease index was then calculated as , with a possible range of 0 (no disease) to 100 (death of all plants).

Data were subjected to ANOVA (SAS Institute, Cary, NC)1. The interaction terms (experiment x inoculum rate, experiment x SDS class, and experiment x inoculum rate x SDS class) were tested to determine pooling data for mean comparisons. The heritability (h2) of DS within inoculum rate was calculated from variance component (Nyquist, 1991) as

[1]
where {sigma}2g = genotypic variance, {sigma}2e = plant-to-plant error variance, {sigma}2gE = Genotype x experiment interaction error variance, R = total number of replications, and E = number of experiments. Mean comparisons were made by LSD (Gomez and Gomez, 1984). A simple regression analysis (SAS Institute, Cary, NC) of greenhouse DS on field DX was used to derive predictive equations for field DX at each inoculum rate.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Sudden death syndrome leaf symptoms were not observed on control plants grown on non-Fsg-infested plant growth medium. Among plants grown on infested media, there was a significant effect of inoculum rate (Table 1). Disease severity means were 1.6, 2.6, and 4.4 for low, moderate, and high inoculum rates, respectively. Higher Fsg inoculum rates were expected to cause more severe SDS leaf symptoms and greater levels of root rot (Gray and Achenbach, 1996). Means of DS at the high inoculum rate in the first experiment could not be determined because of the death of numerous plants. Soil compaction contributed to the death of this set of plants (treatments were planted by different individuals), and was monitored closely in subsequent experiments.


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  		Table 1. Mean squares of sudden death syndrome disease severity in 30 soybean lines across four greenhouse experiments using three inoculum rates.**

 
Analysis of variance across experiments and inoculum rates found a significant interaction between inoculum rate and experiment (Table 1). Therefore, comparison of inoculum rates was limited to within experiment (Table 2). No significant interaction was found between experiment and SDS class or inoculum rate and SDS class. However, there was a significant three-way interaction of experiment x inoculum rate x SDS class (Table 1). Therefore, comparison of SDS class means was conducted within experiment and inoculum rate (Table 2). This interaction resulted from the differential response of the first two experiments compared with the second two experiments. At a given inoculum rate, DS was higher for Experiments 1 and 2 (conducted earlier) than for Experiments 3 and 4 (conducted later). These sets of experiments were conducted 4 yr apart, using the same isolate of Fsg. Factors such as, but not limited to, soil temperature, effective inoculum rate, and changes in the pathogen's ability to cause damage may be responsible for the reduced DS observed in Experiments 3 and 4.


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  		Table 2. Disease severity (DS) means among sudden death syndrome (SDS) resistance classes within and across four greenhouse experiments using 30 soybean lines.

 
Analysis of variance across experiments within inoculum rate indicated that at the moderate inoculum rate (5000 spores cm-3 of plant growth medium), no significant interaction occurred between SDS class and experiment. In addition, only the moderate inoculum rate produced significant variation among SDS classes. However, comparison of SDS class means within experiments indicated that while the low inoculum rate was effective at separating SDS classes in Experiments 1 and 2, the moderate inoculum rate was effective in Experiments 1, 3, and 4. The spore rate in the low inoculum treatment (3300 spores cm-3 of plant growth medium) was similar to that in field hot spots (Roy et al., 1997).

Lines within SDS class did not vary significantly in SDS DS in the greenhouse (Table 1). Since class members were selected by similarity of response to SDS in the field, low variation within class was expected for an effective assay. There was a significant interaction between inoculum rate and line within SDS class. The ability of a resistant cultivar to withstand high pathogen populations in the field may depend on the number of beneficial alleles contributing to the resistance (Iqbal et al., 2001).

Variance component heritabilities of SDS DS for seedlings in the greenhouse were 46, 66, and 37% for low, medium, and high inoculum rates, respectively (Table 3). These estimates were all lower than those obtained from replicated field studies for plants at the R6 growth stage (Hnetkovsky et al., 1996) using the Essex x Forrest RIL population. The modest heritability (<=66%) indicated that the greenhouse assay did not control all environmental and developmental factors that can influence SDS occurrence and severity in seedlings. Air temperature varied (±2 °C) from bench to bench, and could have influenced soil temperature, which has been a major factor influencing SDS occurrence and severity in the field (Rupe et al., 1993). The use of growth chambers and water baths may enable us to test the effect of soil temperature on heritability.


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  		Table 3. Variance components and heritabilities of soybean sudden death syndrome (SDS) disease severity among recombinant inbred lines at three Fusarium solani f. sp. glycines inoculum rates and four experiments in the greenhouse.

 
Within each SDS class, SDS DS increased with increasing inoculum rate (Table 4). At the high inoculum rate, all three classes were highly susceptible (Table 4), as expected (Stephens et al., 1993b; Torto et al., 1996; Gray and Achenbach, 1996). Therefore, high inoculum rates may be responsible for the breakdown of field SDS resistance in the greenhouse (Torto et al., 1996; Stephens et al., 1993b; Gray and Achenbach, 1996; Hartman et al., 1997).


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  		Table 4. Comparison of individual soybean line means for disease severity (DS) at three inoculum rates in the greenhouse with sudden death syndrome (SDS) field disease index (DX), and field DS.

 
At the low inoculum rate in Experiments 1 and 2, and moderate inoculum rate in Experiments 3 and 4 (Table 2), the DS means of the field SDS-susceptible class were significantly higher than those of both the partially-resistant and the resistant classes. However, the field partially-resistant and resistant classes were not significantly different from each other (Table 2), although in the field, the class of genotype with partial resistance to SDS had a significantly higher DX mean (7.4%) than the class of genotypes with resistance to SDS (1.2%). Therefore, the greenhouse assay was not as effective as an equally-replicated field study for separating cultivars with partial resistance from those with resistance.

Simple regression analyses of greenhouse SDS DS with field DX indicated that the amount of variation in field DX explained by greenhouse DS was 40, 60, and 42% at the low, moderate, and high inoculum rates, respectively (Fig. 1) . Predicted values of field DX were calculated for each inoculum rate (Fig. 1) using the predictive equation:

[2]
where Y1 = the predicted value of field DX, a = y-intercept, b = slope of the regression line, and x = greenhouse DS. The y-intercepts were 12.7, -8.2, and -9.9 for low, moderate, and high inoculum rates, respectively. The slopes of regression lines were 13.3, 6.6, and 4.3 for low, moderate, and high inoculum rates, respectively.



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  		Fig. 1. Regression lines for soybean sudden death syndrome disease index (DX) from field experiments with disease severity (DS) from four greenhouse experiments among 30 soybean lines. The greenhouse experiments were conducted at three Fusarium solani f. sp. glycines inoculum rates: (a) low (3.3 x 103), (b) moderate inoculum rate (5 x 103), and (c) high (104). While the moderate inoculum rate explained more variation in field DX and produced predicted values that were close to those observed in the field, the low and high inoculum rates produced predicted field DX values that were higher and lower than those observed in the field in this population, respectively.

 
When DS values from the low and moderate inoculum rates were pooled for each genotype, the amount of variation in field DX explained by greenhouse DS was 65%. Therefore, reduced inoculum rates (3500–5000 spores cm-3 plant growth medium) will provide greenhouse DS values that are a better predictor of field DX. Field DX was held constant for all three regression lines. However, greenhouse DS depended on inoculum rate, causing differences in slopes among regression lines (Fig. 1).

The strong association between field and greenhouse disease severities contrasted with the absence of correlations between field and greenhouse DS in previous tests (Torto et al., 1996).

The midparent value of DS within inoculum rate was used as the cut-off point for resistance in the greenhouse. At the low inoculum rate, while 20% of field resistant lines were eliminated, there was a high rate of retention of field partially-resistant (90%) and field susceptible (20%) lines. At the moderate inoculum rate, only 10% of field resistant lines were eliminated, and the proportion of lines retained was lower for field partially-resistant (50%) and field susceptible (0%). At the high inoculum rate, the lower proportion of field partially-resistant (30%) and susceptible lines (0%) retained was offset by a higher proportion of field resistant lines eliminated (Table 4). Therefore, the highest selection efficiency of 42% [(selected field resistant/total selected) * 100] was achieved with the moderate inoculum rate.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
While significant variability was observed among SDS classes at the low inoculum rate in Experiments 1 and 2 and at moderate inoculum rate in Experiments 2, 3, and 4, the moderate inoculum rate was more consistent in separating SDS classes. The moderate inoculum rate also resulted in more efficient selection with the lowest rate of elimination (10%) of field resistant lines. Hence, it is recommended that inoculum rates of 4000 to 5000 spores cm-3 of plant growth medium be used in greenhouse assays to predict field performance for SDS.

In a greenhouse cultivar trial, two out of four field susceptible genotypes were significantly more susceptible than all six field resistant genotypes. Three field SDS-susceptible cultivars showed significantly (P <= 0.05) higher SDS DS means than Ripley when evaluated in the greenhouse on plant growth medium infested with {approx}4000 Fsg spores cm-3 (Table 5). Among these 10 cultivars, greenhouse DS was significantly associated with field DS and field DX . Given that most of these cultivars are not directly related to Essex and Forrest, and because of the diverse germplasm they contain, we conclude that the low Fsg inoculum rate greenhouse assay can select for field SDS resistance in many genetic backgrounds.


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  		Table 5. Comparison of soybean cultivars for sudden death syndrome (SDS) following seedling inoculation by Fusarium solani f. sp. glycines at low inoculum rate (about 4000 spores per cm3 of plant growth medium) in the greenhouse and SDS disease index (DX) and DS from field experiments.

 
The assay described above will allow for multiple cycles of testing per season and reduce the time to both produce and verify new resistant cultivars. The assay will reduce the costs of data loss due to field variability. Finally, the assay will facilitate the isolation of SDS resistance genes by rapid characterization of recombinants during fine mapping.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
1 The USDA neither guarantees nor warrants the standard of the product and the use of the name by USDA implies no approval of the product to the exclusion of the others that may also be suitable. Back

Received for publication September 5, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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