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a Dep. of Agriculture, Alcorn State Univ., 1000 ASU Dr. #352, Alcorn State, MS 39096-7500
b Dep. of Plant, Soil, and General Agriculture, Southern Illinois Univ. at Carbondale, Carbondale, IL 62901-4415
c Monsanto
* Corresponding author (vicnji{at}lorman.alcorn.edu).
Received for publication December 13, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Abbreviations: CFU, colony forming unit • DI, disease incidence • DS, disease severity • DX, disease index • IS, infection severity • Fsg, Fusarium solani f. sp. glycines • MG, maturity group • RR, Roundup Ready
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Glyphosate-resistant soybean has been developed via genetic engineering (Padgette et al., 1995). Monsanto markets and licenses glyphosate-resistant soybean under the trade name Roundup Ready (RR) soybean. During 1997, the first year of widespread use of RR soybean cultivars, environmental conditions were unusually wet and the southern Midwest had a severe epidemic of soybean sudden death syndrome (SDS) (Wrather et al., 2001). Soybean SDS is caused by the soil-borne fungus Fusarium solani (Mart.) Sacc f. sp. glycines (Fsg) (Roy et al., 1989; Rupe, 1989; Roy, 1997). Soybean SDS was identified more frequently on RR soybeans (Myers et al., 1999), suggesting that either RR soybeans were more susceptible to SDS, or that glyphosate application increased the severity of SDS.
Sudden death syndrome significantly reduces soybean yield in midwestern USA and South America (Wrather et al., 1997; Roy et al., 1997; Njiti et al., 1998b). Management of SDS is mainly from the use of SDS-resistant cultivars (Gibson et al., 1994; Njiti et al., 1998a). Although SDS is identified by foliar symptoms, the causal agent Fsg (Roy et al., 1989) infects only the roots and crowns. The leaf scorch or SDS leaf symptoms are thought to be a result of a toxin or toxins produced in the root by the fungus and transported to the leaves (Jin et al., 1996). Although very few cultivars are resistant to root infection (Njiti et al., 1997), several have good resistance to leaf scorch (Gibson et al., 1994). Root resistance is measured as infection severity (% of root segments with Fsg colonization) or colony forming units per gram of dry root tissue (Njiti et al., 1997; Luo et al., 2000) whereas leaf resistance is measured using a disease index (Gibson et al., 1994).
The objective of this study was to evaluate the effects of glyphosate on root colonization by Fsg and development of SDS on RR soybean. Results of this study will help soybean growers determine whether glyphosate application on RR soybean increases the risk of SDS problems in Fsg-infested fields.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Each maturity group was considered a separate experiment. In this study cultivar pairs within each experiment were evaluated at two locations per year for 2 yr. In 1998, MG II and III were evaluated in central Illinois in a field near Bloomington on Sable silty clay loam soil (fine-silty, mixed, superactive, mesic Typic Endoaquolls), and in a field near Pontiac on Andres loam soil (fine-loamy, mixed, superactive, mesic Aquic Argiudolls); MG IV, V, and VI were evaluated in southern Illinois in a field near Harrisburg on Reesville silt loam soil (fine-silty, mixed, superactive, mesic Aquic Hapludalfs), and in a field near Ullin on Bonnie silt loam soil (fine-silty, mixed, active, acid, mesic Typic Fluvaquents). In 1999 the MG II and III were evaluated in central Illinois in a field near Pontiac on Tama silt loam soil (fine-silty, mixed, superactive, mesic Typic Argiudolls), and in field near Mahomet on Drummer silty clay loam soil (fine-silty, mixed, superactive, mesic Typic Endoaquolls); MG IV, V, and VI were evaluated in southern Illinois in a field near Valmeyer on Ambraw silty clay soil (fine-loamy, mixed, superactive, mesic Fluvaquentic Endoaquolls), and in a field near Harrisburg on Patton silty clay loam soil (fine-silty, mixed, superactive, mesic Typic Endoaquolls). All fields were selected based on a history of uniform SDS leaf symptom expression. All plots were established using conventional tillage.
In 1998, experiments were planted during the period of 15 to 19 May in central Illinois and on 15 to 16 May in southern Illinois. In 1999, experiments were planted during the period of 11 to 15 May in central Illinois and on 10 and 11 May in southern Illinois. All experiments were planted in a split-plot design with cultivar as the main plot and glyphosate as the subplots. There were four replications per experiment. Subplots consisted of four rows 3 m long and spaced 0.75 m apart. The center two rows were evaluated for SDS leaf symptoms and grain yield and the outside two rows were evaluated for Fsg root IS and CFU.
All plots were treated with a preemergence herbicide combination of 0.84 kg a.i. ha-1 trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)-benzamine] and 0.07 kg a.i. ha-1 imaziquin (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic). Treatments consisted of glyphosate-sprayed and a nonsprayed control. At the V3 growth stage (Fehr et al., 1971), the treated subplots were sprayed with 1.12 kg a.i. ha-1 glyphosate. Sprays were applied on wind-free days with necessary precautions to prevent drift.
At the R6 growth stage, subplots were rated for SDS disease incidence (DI) and disease severity (DS). Disease incidence was the percent of plants in the subplot with visible SDS leaf symptoms. Disease severity (percentage leaf surface chlorotic/necrotic) was rated on a scale of 1 to 9 as described: (1 = 0–10%/1–5%, 2 = 10–20%/6–10%, 3 = 20–40%/10–20%, 4 = 40–60%/20–40%, 5 = >60%/>40%, 6 = up to 33% premature defoliation, 7 = up to 66% premature defoliation, 8 = >66% premature defoliation, and 9 = premature death of plant). Disease index (DX, expressed as %) was then calculated as (DI x DS)/9, with a possible range of 0 (no disease) to 100 (death of all plants).
Eight taproots were harvested from each subplot at R6 growth stage. Four roots were used for IS and four for CFU determination. For IS determination, taproots were washed and surface sterilized in 0.5% (v/v) sodium hypochlorite. Each taproot was cut into several 1-cm segments. Ten root segments were randomly selected and placed on a Petri plate containing a selective medium composed of 20 g agar, 20 g sucrose, 1 g KH2PO4, 0.5 g MgSO4·7H2O, 1 g KNO3 and 1 L distilled water; autoclaved at 110°C for 40 min and cooled to 42°C before 0.3 g streptomycin sulfate, 0.1 g neomycin, 0.1 g chlortetracycline, 0.05 g rifampicin, and 0.23 g pentachloronitrobenzene (PCNB) (Terrachlor, Uniroyal Chemical Co., Vaugntuk, CT) were added. Infection severity was determined as the percentage of root segments yielding slow-growing, blue fungus (Njiti et al., 1997). A total of 40 root segments from 4 taproots per subplot were used in IS determination.
For determination of CFU, the four taproots from each subplot were treated as one sample. The roots were washed, surface-sterilized in 0.5% (v/v) sodium hypochlorite, oven-dried at 28°C for 48 h, ground using a Wiley cutting mill (Thomas Scientific, Swedesboro, NJ) to pass through a 40-mesh screen, weighed, and suspended in 2 L of distilled water. One mL of the suspension was transferred onto each of five Petri plates containing the selective medium described above. Plates were inoculated and incubated at room temperature for 14 d. The number of slow-growing, appressed, and blue fungal colonies (Rupe et al., 1996) were counted as CFUs. The CFU per gram of root tissue was calculated (Luo et al., 2001).
At the R8 developmental stage, the middle two rows for each plot were harvested with a two-row combine. The seeds were cleaned without loosing broken seeds. Yield was calculated for each plot at 13 g kg -1 seed moisture content.
Data were collected for root IS in all test environments, CFU in 10 environments (2 per MG) in 1998, disease index in 4 environments for MG II and III; 3 environments for MG V and VI and 0 environment for MG IV, and grain yield in 4 environments in MG II, III, IV, and V; and 3 environments in MG VI.
In this study, environment was considered a random factor and cultivar and glyphosate were considered fixed factors. To avoid violation of the assumptions for analysis of variance, the CFU values were subjected to logarithmic transformation using the formula log10(CFU + 1). The values of DX and IS were subjected to square root transformation using the formulae 
(DX + 0.5) and (IS + 0.5), respectively. Transformed data were subjected to analysis of variance (SAS Inst., 1985). Analysis of variance was conducted for combined environments to test possible interaction of treatments with environments.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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0.05) variation among environments in MG III, IV, V, and VI for IS; MG II and VI for CFU; MG II, III, and VI for DX (Table 1). There was no significant Glyphosate x Environment interactions for any disease measure within any maturity groups (Table 1).
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For each disease measure, significant variation between cultivars was only observed in some maturity groups (Table 1). There was significant variation between cultivars for IS in MG IV and VI, for CFU in MG VI, and for DX in MG III and VI (Table 1). The SDS-resistant cultivar generally had less disease (as measured by IS, CFU, and DX) than the SDS-susceptible cultivar except for IS in MG III, V, and VI (Fig. 1 and 2) .
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The MG IV, the earliest of the cultivars evaluated in southern Illinois, escaped SDS symptoms (DX = 0). It has been observed that earlier cultivars may progress past the critical growth stages (<R6) for SDS leaf symptoms before favorable conditions for symptom occurrence become available (Rupe et al., 1993; Gibson et al., 1994; Hnetkovsky et al., 1996). Environmental dependence of leaf symptom expression highlights the importance of root infection as a measure of SDS resistance.
Development of SDS symptoms occurrence is influenced by both the genotype of the soybean cultivar and the environment. Significant differences in DS means among environments were consistent with previous studies (Gibson et al., 1994; Rupe et al., 1993). Disease development and severity in any given environment are functions of planting date, genotype, and soil factors including but not limited to soil moisture and temperature (Hershman et al., 1990; Rupe et al., 1993). As a result, some cultivars show inconsistent responses from one environment to another. However, there are a few cultivars (susceptible and resistant) that show low Genotype x Environment interaction (Gibson et al., 1994; Njiti et al., 2002). Significant differences between cultivars were expected in this study since the cultivars were selected to contrast for disease resistance. However, in some environments (data not shown) and maturity groups, cultivar differences were not significant. This is an indication of Genotype x Environment interaction (G x E). Genotype x Environment interaction is a large component of SDS resistance (Njiti et al., 1996) but the magnitude of G x E varies from cultivar to cultivar. The magnitude of G x E is influenced by the number of beneficial alleles for SDS resistance possessed by each cultivar (Iqbal et al., 2001) as well as the environmental factors.
In this study, the application of glyphosate to RR soybean did not increase susceptibility to soybean root infection by Fsg or soybean SDS leaf symptoms caused by toxin produced in response to root infection. It has been suggested that there might be a relationship between the application of glyphosate to RR soybean and increased fungal diseases (Kremer, unpublished, 2001). This study found no relationship between the application of glyphosate to RR soybean and SDS. No increase in fungal colonization was noted. It has been reported that RR soybean plants inoculated with Fsg in the greenhouse had more root colonization by the fungus and more severe leaf symptoms when sprayed with glyphosate (Sanogo et al., 2000). However, the response was not limited to glyphosate herbicide. The difference between the greenhouse and field response may be related to the stage of evaluation and inoculum concentration (Njiti et al., 2001). Some of the cultivars, including Resnik (McBlain et al., 1990a), Flyer (McBlain et al., 1990b), and A5403 used as parents in the development of RR soybean cultivars have been shown to be very susceptible to SDS (Gibson et al., 1994; Prabhu et al., 1999). Alleles contributed by these parents may explain why some RR cultivars are very susceptible to SDS.
In most MG, the SDS-resistant cultivars had higher grain yield than the SDS-susceptible cultivars. It is not clear whether this was the effect of yield genes or SDS resistance genes.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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ACKNOWLEDGMENTS |
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