Agronomy Journal Grow Your Career With ASA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (10)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Nelson, K. A.
Right arrow Articles by Hammerschmidt, R.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Nelson, K. A.
Right arrow Articles by Hammerschmidt, R.
Agricola
Right arrow Articles by Nelson, K. A.
Right arrow Articles by Hammerschmidt, R.
Related Collections
Right arrow Soybean
Right arrow Agricultural Pesticides
Right arrow Crop Growth and Development
Right arrow Other Crop Management
Right arrow Plant Disease
Right arrow Production Agriculture
Agronomy Journal 94:1270-1281 (2002)
© 2002 American Society of Agronomy

PRODUCTION PAPER

Cultivar and Herbicide Selection Affects Soybean Development and the Incidence of Sclerotinia Stem Rot

Kelly A. Nelson*,a, Karen A. Rennerb and Ray Hammerschmidtc

a Dep. of Agron., Univ. of Missouri, Novelty, MO 63460
b Dep. of Crop and Soil Sci., Michigan State Univ., East Lansing, MI 48824-1325
c Dep. of Plant Pathol., Michigan State Univ., East Lansing, MI 48824-1325

* Corresponding author (nelsonke{at}missouri.edu)

Received for publication March 6, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.] is produced on a majority of U.S. soybean hectares. Concerns have been expressed regarding interactions of herbicides such as glyphosate and Sclerotinia stem rot [Sclerotinia sclerotiorum (Lib.) de Bary]. Research evaluated the effect of herbicides on growth, phytoalexin production, incidence of Sclerotinia stem rot, and grain yield of GR and isogenic nonresistant cultivars in 1998 and 1999. Lactofen at 70 g a.i. ha-1 delayed reproductive development, reduced leaf area index (LAI), reduced Sclerotinia stem rot lesion diameter 2 to 26 d after treatment (DAT), increased phytoalexin production 2 to 26 DAT, reduced disease severity index (DSI), and increased yield of ‘Great Lakes 2415’ (GL2415) by 510 kg ha-1 compared with untreated soybean. Glyphosate at 840 g a.e. ha-1 increased disease severity index in ‘Great Lakes 2600’ (GL2600) and ‘Pioneer 93B01’ (P93B01) GR compared with untreated soybean while yield of ‘Novartis S20-B9’ (S20-B9) GR treated with glyphosate was 650 kg ha-1 greater than untreated soybean. Thifensulfuron at 4.5 g a.i. ha-1 delayed reproductive development, reduced LAI, and did not affect phytoalexin production of ‘Novartis S 19-90’ (S 19-90) or S20-B9 GR but increased grain yield of ‘Novartis S 12-49’ (S 12-49) by 450 kg ha-1 compared with untreated soybean. S 12-49 and GL2415 treated with thifensulfuron had lower disease severity index and increased yield compared with GR isolines. Reduction of disease severity index following lactofen may be attributed to increased phytoalexin production, reduced LAI, and delayed reproductive development; however, increased yield was observed for only one of eight cultivars. Producers can reduce Sclerotinia stem rot with postemergence herbicides, but soybean yield did not necessarily increase.

Abbreviations: DAT, days after treatment • GL2415, ‘Great Lakes 2415’ • GL2600, ‘Great Lakes 2600’ • GR, glyphosate resistant • LAI, leaf area index • NIS, nonionic surfactant • P9281, ‘Pioneer 9281’ • P93B01, ‘Pioneer 93B01’ • S 12-49, ‘Novartis S 12-49’ • S14-M7, ‘Novartis S14-M7’ • S 19-90, ‘Novartis S 19-90’ • S20-B9, ‘Novartis S20-B9’ • TLC, thin-layer chromatography • UAN, 28% urea ammonium nitrate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
SCLEROTINIA STEM ROT or white mold [Sclerotinia sclerotiorum (Lib.) de Bary], a common disease in the North-Central region of the USA as well as other areas in the world, is caused by an ascomycete fungus (Marinelli et al., 1998; Thompson and Van der Westhuizen, 1979). The recent introduction of herbicide-resistant crops, including GR soybean, has raised concerns regarding the susceptibility of GR soybean to disease when glyphosate [N-(phosphonomethyl)glycine] is applied for weed control (Lee and Penner, 1999; Sanogo et al., 2000). Lévesque and Rahe (1992) reviewed the interaction between glyphosate and plant diseases. Glyphosate reduced or prevented phytoalexin production in bean (Phaseolus vulgaris L.) (Johal and Rahe, 1990), lucerne (Medicago sativa L.) (Latunde-Dada and Lucas, 1985), and soybean (Holliday and Keen, 1982) and enhanced mycelia growth of Alternaria cassiae, a biocontrol agent of sicklepod (Cassia obtusifolia L.) (Sharon et al., 1993). In addition, glyphosate increased the severity and frequency of Fusarium solani f. sp. glycines in the roots of GR and non-GR soybean cultivars (Sanogo et al., 2000). Research to date with glyphosate and the effects on phytoalexins has involved non-GR soybean cultivars; however, no research has reported the effects of glyphosate on phytoalexin production with GR cultivars. A genetic association between herbicide resistance and increased disease susceptibility in commercial crop cultivars could have detrimental effects on crop yield and the acceptance of a cultivar in the market. The interaction between herbicide treatments and the incidence of Sclerotinia stem rot could significantly affect soybean production practices and weed control recommendations in regions where Sclerotinia stem rot occurs.

Sclerotinia sclerotiorum causes premature soybean death, which results in the production of shriveled soybean pods with little or no seed (Hart, 1998). Several soybean growth characteristics, environmental conditions, and cultural practices that influence the infection and incidence of S. sclerotiorum have been evaluated in field and controlled environments (Boland and Hall, 1987; Buzzell et al., 1993; Grau and Radke, 1984). Numerous studies have shown that the management of this disease is complicated and depends heavily on the environmental conditions of a given year; adopted cultural methods; and most importantly, cultivar selection (Hoffman et al., 1998; Kim et al., 1999; Yang et al., 1999).

Soybean cultivars differ in Sclerotinia stem rot tolerance, and yield losses may not always occur under low levels of disease due to yield compensation of nearby soybean plants (Hart, 1998). Yield reductions caused by S. sclerotiorum may range from 147 to 370 kg ha-1 (2–5.5 bu acre-1) for every 10% increase in disease severity, depending on the environment and cultivar. A negative correlation between Sclerotinia stem rot disease severity or incidence and soybean yield has been reported in Illinois (Hoffman et al., 1998), Iowa (Yang et al., 1999), Michigan (Chun et al., 1987; Kim et al., 1999), and Wisconsin (Grau and Radke, 1984). Planting soybean cultivars that are tolerant to Sclerotinia stem rot is strongly recommended to reduce yield loss (Boland and Hall, 1987; Grau and Radke, 1984; Kim et al., 1999; Yang et al., 1999). In addition, avoiding cultivars with parentage from cultivars such as Williams or Asgrow A3127, which are sensitive to Sclerotinia stem rot, may also reduce the incidence of this disease (Kim et al., 1999).

No research has evaluated the effect of glyphosate or other postemergence herbicides on the incidence of Sclerotinia stem rot in GR soybean. Herbicide treatments may prevent disease infection or stimulate phytoalexin production (Dann et al., 1999; Levene et al., 1998), which may reduce the incidence of disease. Phytoalexins are antimicrobial compounds produced by plants after disease infection or treatment with biotic or abiotic elicitors (Hammerschmidt, 1999). Glycine spp. produce 12 known phytoalexins, with the amount and type of these compounds depending on the cultivar, disease, or stimulus evaluated (Ingham, 1982). The production of soybean phytoalexins in response to herbicides (Dann et al., 1999; Holliday and Keen, 1982; Keen et al., 1982; Kömives and Casida, 1983; Levene et al., 1998), other chemicals (Favaron et al., 1988; Ingham et al., 1981), or diseases (Ingham et al., 1981; Morris et al., 1991; Olah et al., 1985) and the effect of soybean phytoalexins on S. sclerotiorum (Sutton and Deverall 1984), Phytophthora megasperma f. sp. glycinea (Bhattacharyya and Ward, 1986), and nematodes (Heterodera glycines Ichonohe) (Kaplan et al., 1980; Levene et al., 1998) have been reported. These studies suggest that phytoalexins are an important component of soybean disease tolerance.

Postemergence herbicides, such as thifensulfuron {3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid}, lactofen {(+)-2-ethoxy-1-methyl-2-oxoethyl 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoate}, and acifluorfe{5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} cause chlorosis, necrosis, and/or stunting of soybean (Wichert and Talbert, 1993). Postemergence herbicide tank mixture treatments applied to V5 soybean reduced reproductive development 20 and 80 DAT and canopy development up to 52 DAT (Nelson and Renner, 1998). Changes in soybean morphology, time of maximum leaf area, and canopy development may affect the microclimate in the soybean canopy and potential for S. sclerotiorum infection and Sclerotinia stem rot development.

The objectives of this research were to determine (i) if GR cultivars differed in reproductive development, canopy development, flowering characteristics, phytoalexin production, Sclerotinia stem rot disease severity, and soybean yield compared with glyphosate-sensitive near isolines and (ii) if postemergence herbicides such as glyphosate, lactofen, and thifensulfuron influenced soybean response, reproductive development, canopy development, flowering characteristics, phytoalexin production, Sclerotinia stem rot severity, and soybean yield compared with untreated, weed-free soybean. If the incidence of Sclerotinia stem rot varied by cultivar or herbicide treatment, we would be able to determine if the reproductive development, canopy development, or phytoalexin production would be a key component influencing the incidence of this disease.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Field research was conducted at the S. sclerotiorum nursery, Michigan State University Research Farm, East Lansing (42°43' N, 84°33' W) in 1998 and 1999. The soil was a Capac sandy loam (fine-loamy, mixed mesic Aeric Ochraqualf) with 1.5% organic matter and pH 6.5 in 1998. The 1998 field was fall chisel-plowed and soil-finished (Sunflower Manufacturing Co., Beloit, KS) three times in the spring and fertilized with 170 kg ha-1 of 0–0–60 before planting. In 1999, the soil was a Capac loam with pH 6.8 and 1.7% organic matter. The field was soil finished–tilled twice in the spring and fertilized with 170 kg ha-1 of 6–24–24 before planting. Plots were maintained weed-free by manual weed removal throughout the season.

Research was arranged in a split-plot design with three replications. Cultivar was the main plot and herbicide treatment the subplot. Plots were 1.5 by 6.1 m with no open borders between plots to facilitate the development of Sclerotinia stem rot. Plots were trimmed back to 1.5 by 4.3 m to facilitate harvest. Near isolines of GR and non-GR soybean—‘Novartis S 12-49’ (S 12-49), ‘Novartis S14-M7’ (S14-M7; GR), ‘Novartis S 19-90 (S 19-90), ‘Novartis S20-B9’ (S20-B9; GR), ‘Great Lakes 2415’ (GL2415), ‘Great Lakes 2600’ (GL2600; GR), ‘Pioneer 9281’ (P9281), and ‘Pioneer 93B01’ (P93B01; GR)—were planted 13 May 1998 and 11 May 1999 in 38-cm rows at 543000 seeds ha-1, with a final stand of 474 000 plants ha-1. Glyphosate-resistant cultivars were backcross derivatives of a glyphosate-susceptible cultivar. P9281 and P93B01, GL2415 and GL2600, S12-49 and S14-M7, and S 19-90 and S20-B9 were first, fifth, sixth, and sixth backcross derivatives, respectively (personal communication with representatives from Pioneer, Great Lakes Hybrids, and Syngenta, 1998). Herbicide treatments included an untreated control, thifensulfuron at 4.5 g a.i. ha-1 plus 0.25% (v/v) nonionic surfactant (NIS) (Activator-90, a mixture of alkyl polyoxyethylene ether and free fatty acids, Loveland Industries, Greeley, CO) plus 28% urea ammonium nitrate (UAN) at 2.3 L ha-1, lactofen at 70 g a.i. ha-1 plus 0.25% (v/v) NIS plus UAN at 2.3 L ha-1, and glyphosate (formulated as Roundup Ultra) at 840 g a.e. ha-1 plus UAN at 2.3 L ha-1.

Herbicide treatments were applied on 25 June 1998 and 20 June 1999 to 20- to 25-cm-tall soybean in the V5 to V6 stage of development and before R1 for all cultivars (Fehr and Caviness, 1977). Treatments were applied with a CO2–propelled hand boom calibrated to deliver 178 L ha-1 at 207 kPa, traveling 6.3 km h-1, and equipped with 8003 flat-fan nozzles (Spraying Syst. Co., Wheaton, IL) spaced 51 cm apart and 48 cm above the soybean canopy. The air temperature was 32°C with 52% relative humidity in 1998 and 28°C with 40% relative humidity in 1999. Supplemental overhead irrigation was provided in the evenings beginning 5 July 1998 and 3 July 1999 at 10 and 13 d after herbicides were applied, respectively. Approximately 3 mm of water was provided daily during soybean flowering to encourage Sclerotinia stem rot development.

Soybean injury from 0 to 100% (0 = no visual crop injury and 100 = complete crop death) was evaluated 3, 7, 14, 28, and 35 DAT based on the combined visual effects of the herbicides on necrosis, chlorosis, and stunting. Reproductive development of soybean was classified according to flowering, pod development, seed development, and maturity (Fehr and Caviness, 1977). Soybean in R1 is in beginning flower, R2 in full flower, and R3 in beginning pod. Flowering has almost completely ceased by R4 (full pod), and most of the flowers at this stage are concentrated near the top of the plant. Flower number of three randomly selected plants in each plot was recorded and averaged to determine if flowering was delayed or reduced by the herbicide treatments. Relative humidity and temperature at the soil surface were measured 19 DAT in all plots at solar noon because the greatest differences in microclimate conditions were reported at this time of the day in other research (Blad et al., 1978).

Light measurements were recorded at 7- to 14-d intervals from the time of herbicide application until 85 DAT with a SunScan Canopy Analysis System (Dynamax, Houston, TX). Five measurements were recorded with the 1-m light measurement device diagonal to the soybean row. Incident and diffused light measurements have been utilized as an effective nondestructive method to measure soybean LAI and determine if canopy development was delayed or reduced by the herbicide treatments (Walker et al., 1988).

Phytoalexin production was measured in the trifoliolate leaves located at the fifth node (fourth trifoliolate) 0, 2, 4, 7, 12, and 26 DAT and at the ninth node 26 DAT. Leaves were harvested from three separate plants in each untreated and glyphosate- or lactofen-treated plot. Leaves were harvested from S 19-90 and S20-B9 cultivars in the thifensulfuron-treated plots only. Trifoliolates located at the ninth node (eighth trifoliolate) only were harvested from the aforementioned plots 26 DAT to compare phytoalexin production in treated and nontreated leaves. One leaf from each trifoliolate was evaluated in a detached leaf bioassay, and the other leaves were stored at -20°C and evaluated for antifungal compounds using a thin-layer chromatography (TLC) phytoalexin bioassay.

Excised leaves used in the detached leaf bioassay were placed in a 150 by 15 mm Petri dish (VWR Sci. Products, Batavia, IL) with moistened filter paper to ensure a humid environment for S. sclerotiorum growth. A potato (Solanum tuberosum L.) dextrose agar plug, 2.4 mm in diameter, from the margin of S. sclerotiorum was placed on the leaves, and the diameter of growth was measured with an electronic digital caliper after incubating for 48 h. The TLC phytoalexin bioassay utilized Cladosporium cucumerinum to determine and quantify total antifungal activity (phytoalexin production) produced by soybean and is referenced in Nelson et al. (2002) and Keen et al. (1971). Photo copies (Cannon imageRUNNER 330S, Lake Success, NY) of the plates were made, and clear areas with no C. cucumerinum growth were quantified with a LI-3000 leaf area meter (LI-COR, Lincoln, NE). Total areas were calculated that corresponded to retention factor (Rf) values for the methyl ester of glyceofuran (Rf 0.71); isoformonoetin and a mixture of glyceollin I, II, and III (Rf 0.5–0.56); and glyceofuran and the precursor of glyceollin II and III (Rf 0.25) previously reported using chloroform–acetone–NH4OH (50:50:1 v/v) as the mobile phase and a silica-gel TLC plate as the stationary phase (Ingham et al., 1981; Keen et al., 1982).

Near physiological maturity, 30 plants plot-1 were evaluated for the incidence of Sclerotinia stem rot. The disease severity index was calculated according to the scale (0 = no symptoms, 1 = lesions on the lateral branches only, 2 = lesions on the main stem but no effect on pod fill, and 3 = lesions on the main stem and pod fill reduced) described by Grau and Radke (1984). Soybean was harvested with a Massey 10 (Kincaid Equipment Manufacturing, Haven, KS) small-plot harvester and moisture adjusted to 13%.

An analysis of variance was conducted and percentage data for visual injury were transformed to the arcsine before the analysis. The transformation did not affect the conclusions, so original data are presented. Data were combined over years, and main effects of cultivar or herbicide treatment are presented where interactions were not observed. The TLC phytoalexin bioassay data were subjected to an F max test for homogeneity (Kuehl, 1994), and data were combined over years because variances for both years were homogenous. Means were separated using Fisher's Protected LSD at p < 0.05. Disease severity index and yield means were separated at p <= 0.10.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Soybean Response
Soybean response to herbicide treatments differed by cultivar (Table 1). Thifensulfuron was more injurious to P93B01 (GR) compared with P9281 at 3 and 7 DAT and to S20-B9 compared with S 19-90 at 7 DAT. Glyphosate did not injure GR soybean while thifensulfuron injury to soybean was minimal (Table 1). Lactofen caused 8 to 12% injury 3 DAT, with greater injury in S 19-90 compared with S20-B9. By 7 DAT, injury increased to 13 to 17%, with no difference between isolines. Injury was no longer visually evident 35 DAT from any herbicide treatments (data not presented).


View this table:
[in this window]
[in a new window]
  		Table 1. Soybean injury from postemergence herbicides 3 and 7 d after treatment (DAT) in 1998 and 1999.{dagger}

 
Reproductive and Canopy Development
The reproductive stage of soybean at 14 and 21 DAT (Table 2) as well as canopy development (Fig. 1) varied by cultivar and year. In 1998, all cultivars were at the same reproductive stage except P93B01 at 14 DAT, and all cultivars were at full bloom, with some of the late group I (S12-49 and S14-M7) or early group II (S 19-90 and S20-B9) soybean producing pods by 21 DAT (Table 2). One week later, S 12-49 started producing pods while the other cultivars, except P93B01, were primarily in full bloom. Drier growing conditions 2 wk before and 1 wk following herbicide application (5.4 cm in 1998 and 2.3 cm in 1999), coupled with cooler (6–7°C) minimum and maximum average temperatures in the week before herbicide application, delayed reproductive development in 1999 compared with 1998. A delay in flowering in 1999 could reduce the number of infection sites in the 2-wk period following herbicide application, which could provide less protection from Sclerotinia stem rot.


View this table:
[in this window]
[in a new window]
  		Table 2. Reproductive stage of development for soybean cultivars combined over herbicide treatments in 1998 and 1999 at 14 and 21 d after treatment (DAT).{dagger}

 


View larger version (40K):
[in this window]
[in a new window]
  		Fig. 1. Soybean leaf area index for cultivar combined over the untreated, lactofen, and thifensulfuron herbicide treatments in 1998 and 1999. Vertical lines indicate the LSD (p <= 0.05).

 
Soybean cultivar LAI was similar between GR and non-GR near isolines for all measurement dates except S 12-49 and S14-M7 and GL2415 and GL2600 21 DAT in 1998 and P9281 and P93B01 isolines 7 DAT in 1999 (Fig. 1). Therefore, the microclimate in the canopy of isolines should be similar. Peak canopy development for all of the cultivars was 35 DAT in 1998 and 35 to 48 DAT in 1999.

Glyphosate did not affect reproductive development compared with the untreated control (data not presented). However, thifensulfuron and lactofen delayed soybean reproductive development 3 to 35 DAT (Fig. 2) and reduced LAI 3 to 28 DAT (Fig. 3) in 1998 and 1999. These results support previous research conducted with postemergence tank mixtures that included thifensulfuron or lactofen (Nelson and Renner, 1998). Thifensulfuron and lactofen caused a greater reduction in canopy development in 1999 than 1998. Yearly differences in herbicide effects on canopy ground-cover ratings have been attributed to rainfall differences following herbicide application (Donald, 1998). In 1999, only 1.8 cm of rain fell from 0 to 7 DAT, which decreased LAI of treated soybean compared with the untreated control. Irrigation was not initiated until 10 and 13 d after herbicides were applied in 1998 and 1999, respectively (Fig. 3). Lack of canopy development could reduce the incidence of Sclerotinia stem rot by allowing more air movement through the soybean canopy. However, there was no difference in relative humidity 19 DAT due to cultivar or herbicide treatment, and air temperature elevated from 25.6 to 26.2°C in the lactofen and thifensulfuron treatments (data not presented). Postemergence herbicide treatments such as lactofen and thifensulfuron applied at the V5 stage of development altered canopy development and delayed reproductive development; however, the microclimate was similar between treatments.



View larger version (19K):
[in this window]
[in a new window]
  		Fig. 2. Soybean reproductive stage for the untreated control, thifensulfuron at 4.5 g ha-1 plus 0.25% (v/v) nonionic surfactant (NIS) plus 28% urea ammonium nitrate (UAN) at 2.3 L ha-1, and lactofen at 70 g a.i. ha-1 plus 0.25% (v/v) NIS plus UAN at 2.3 L ha-1 combined over cultivars in 1998 and 1999. Vertical lines indicate the LSD (p <= 0.05).

 


View larger version (33K):
[in this window]
[in a new window]
  		Fig. 3. Soybean leaf area index for the untreated control, thifensulfuron at 4.5 g ha-1 plus 0.25% (v/v) nonionic surfactant (NIS) plus urea ammonium nitrate (UAN) at 2.3 L ha-1, and lactofen at 70 g a.i. ha-1 plus 0.25% (v/v) NIS plus UAN at 2.3 L ha-1 combined over cultivars in 1998 and 1999. Vertical lines indicate the LSD (p <= 0.05).

 
Soybean Flowering
Postemergence herbicide treatments influenced flower no. plant-1 and peak flowering for all cultivars (Fig. 4) . Interactions were expected and observed due to the maturity differences of the cultivars evaluated; therefore, data were analyzed for paired isolines separately. Flower no. plant-1 in GR cultivars was no different than non-GR soybean (data not presented). Peak flowering was 14 DAT for S 12-49, S14-M7, S 19-90, and S20-B9 and 28 DAT for GL2415, GL2600, P9281, and P93B01. Lactofen and thifensulfuron reduced the flower no. of group I soybean (S 12-49, S14-M7, S 19-90, and S20-B9) 7 and 14 DAT and had no effect on midgroup II soybean flower no. (GL2415 and GL2600), and lactofen only increased late group II flower no. 28 DAT (P9281 and P93B01) compared with untreated soybean. The first appearance of S. sclerotiorum apothecia under the soybean canopy was observed 21 DAT when the mid- and late group II soybean were in full bloom and had peak flowering 1 wk later (Table 2). Thus, the effect of lactofen and thifensulfuron on soybean flowering increased when applied at or near the time of flowering because the reproductive stage of group I cultivars 14 DAT indicated an earlier initiation and beginning of full flowering. We conclude that soybean injury caused by thifensulfuron and lactofen, which delayed reproductive development, reduced canopy development (i.e., LAI), and influenced flower no. plant-1, is a factor that could reduce the severity of Sclerotinia stem rot in GR and non-GR cultivars.



View larger version (25K):
[in this window]
[in a new window]
  		Fig. 4. Soybean flowers plant-1 for thifensulfuron at 4.5 g ha-1 plus 0.25% (v/v) nonionic surfactant (NIS) plus UAN at 2.3 L ha-1 and lactofen at 70 g a.i. ha-1 plus 0.25% (v/v) NIS plus UAN at 2.3 L ha-1 combined over glyphosate-resistant (GR) and non-GR isolines in 1998 and 1999. Group I soybean included S 12-49, S14-M7 (GR), S 19-90, and S20-B9 (GR). Group II soybean included GL2415, GL2600 (GR), P9281, and P93B01 (GR). Vertical lines indicate the LSD (p <= 0.05).

 
Detached Leaf and Thin-Layer Chromatography Phytoalexin Bioassays
Non-GR soybean cultivars varied in S. sclerotiorum lesion diameter throughout the sample period (Fig. 5) . Sclerotinia sclerotiorum lesion diameter was greater for GL2415 and P9281 compared with S 12-49 and S 19-90 before herbicide treatments. This cultivar difference continued from 2 to 12 DAT. There were no recurrent differences in the development of Sclerotinia stem rot between GR and non-GR isolines in the detached leaf bioassay (data not presented).



View larger version (19K):
[in this window]
[in a new window]
  		Fig. 5. Sclerotinia sclerotiorum lesion diameter of soybean cultivars using a detached leaf bioassay of the fourth trifoliolate (treated). Data were combined over herbicide treatments including the untreated control, lactofen, and thifensulfuron for soybean cultivars not resistant to glyphosate in 1998 and 1999. Vertical lines indicate the LSD (p <= 0.05).

 
Sclerotinia sclerotiorum lesion diameter was reduced by lactofen averaged over eight cultivars from 2 to 26 DAT compared with the untreated control averaged over eight cultivars or glyphosate treatments averaged over four cultivars (Fig. 6) . Thifensulfuron did not reduce lesion diameter compared with the untreated control for the two cultivars evaluated, S 19-90 and S20-B9 (data not presented). None of the herbicide treatments affected S. sclerotiorum lesion diameter of the eighth trifoliolate (untreated leaf) 26 DAT (data not presented). Therefore, it is evident from this bioassay that some soybean cultivars are less inclined to develop S. sclerotiorum lesions. Furthermore, it appears that only lactofen reduces lesion diameter of S. sclerotiorum, and this effect occurs only on the treated leaf for 26 DAT and is not evident in new trifoliolates 26 DAT.



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 6. Sclerotinia sclerotiorum lesion diameter using a detached leaf bioassay of the fourth trifoliolate treated with herbicides. Herbicide treatments included an untreated control, glyphosate at 840 g ha-1 plus urea ammonium nitrate at 2.3 L ha-1, and lactofen at 70 g a.i. ha-1 plus 0.25% (v/v) nonionic surfactant plus urea ammonium nitrate at 2.3 L ha-1. Data were combined over sampled cultivars in 1998 and 1999. Vertical lines indicate the LSD (p <= 0.05).

 
The TLC phytoalexin bioassay indicated the presence of antifungal compounds that corresponded to Rf values previously reported for the methyl ester of glyceofuran (Rf 0.71); isoformonoetin and a mixture of glyceollin I, II, and III (Rf 0.5–0.56); and glyceofuran and the precursor of glyceollin II and III (Rf 0.25) (Ingham et al., 1981; Keen et al., 1982). Total soybean phytoalexin production did not differ due to cultivar from 0 to 7 DAT (data not presented). However, S 12-49 (glyphosate sensitive) inhibited C. cucumerinum growth more than S14-M7 (GR) while P9281 (glyphosate sensitive) inhibited C. cucumerinum growth more than P93B01 (GR) 12 DAT (Table 3). By 26 DAT, there was no difference in total antifungal compound production between cultivars for the fourth trifoliolate (herbicide treated) (data not presented). However, S 12-49 and S20-B9 produced more phytoalexins in the eighth trifoliolate (untreated leaves) than GL2415, GL2600, P9281, or P93B01 (Table 3). This indicates increased phytoalexin production in S 19-90 an S20-B9 throughout the leaves of the plant.


View this table:
[in this window]
[in a new window]
  		Table 3. Inhibited area with no Cladosporium cucumerinum hyphal growth. The thin-layer chromatography phytoalexin bioassay determined the relative phytoalexin production of the fourth (treated) trifoliolate 12 d after treatment (DAT) and the eighth trifoliolate 26 DAT for glyphosate-resistant (GR) and non-GR soybean cultivars averaged over herbicide treatment in 1998 and 1999.{dagger}

 
Extracts from lactofen-treated plants inhibited C. cucumerinum growth in the TLC phytoalexin bioassay from 2 to 26 DAT, thus indicating increased phytoalexin production with this treatment from the time of application until peak flowering (Fig. 7) . Phytoalexins reduced S. sclerotiorum growth in vitro (Sutton and Deverall, 1984) and have been related to increased Sclerotinia stem rot tolerance of soybean treated with lactofen in the field (Dann et al., 1999). However, the duration of phytoalexin production with lactofen was unknown. Glyphosate and thifensulfuron did not affect phytoalexin production compared with the untreated control. The diphenyl ether herbicides like lactofen inhibit protoporphyrinogen oxidase, which causes peroxidative destruction of membrane fatty acids and subsequent cell death (Scalla and Matringe, 1994). This could mimic a hypersensitive response of a plant to disease infection (Baker and Orlandi, 1995; Bhattacharyya and Ward, 1986; Sutton and Deverall, 1984) or abiotic factors (Ingham et al., 1981), which has resulted in phytoalexin production. An increase in phytoalexin production should reduce the incidence of Sclerotinia stem rot.



View larger version (23K):
[in this window]
[in a new window]
  		Fig. 7. Thin-layer chromatography phytoalexin bioassay using Cladosporium cucumerinum. The untreated control, thifensulfuron at 4.5 g ha-1 plus 0.25% (v/v) nonionic surfactant (NIS) plus urea ammonium nitrate (UAN) at 2.3 L ha-1, glyphosate at 840 g ha-1 plus UAN at 2.3 L ha-1, and lactofen at 70 g a.i. ha-1 plus 0.25% (v/v) NIS plus UAN at 2.3 L ha-1 treatments were combined over sampled cultivars in 1998 and 1999. S 19-90 and S20-B9 were the only cultivars evaluated with thifensulfuron. Glyphosate-resistant cultivars were the only cultivars evaluated with glyphosate. Vertical lines indicate the LSD (p <= 0.05).

 
Sclerotinia Stem Rot Disease Severity
A linear response between the detached leaf bioassay of S. sclerotiorum lesion diameter 0 to 26 DAT and disease severity at the end of the season was established by averaging the S. sclerotiorum lesion diameter mean for the six harvest dates of the fourth trifoliolate. Linear regression analysis indicated soybean with small lesion diameters had a low incidence of disease late in the season (P = 0.0001) (Fig. 8) .



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 8. Linear regression of the detached leaf bioassay of Sclerotinia sclerotiorum lesion diameter of the fourth trifoliolate (treated) averaged over harvest date (0–26 d after treatment) and disease severity index for glyphosate-resistant (GR; solid shapes) and non-GR (open shapes) cultivars. Data were combined over all herbicide treatments in 1998 and 1999.

 
Disease severity was greater for untreated S14-M7 (GR), thifensulfuron-treated S14-M7 (GR), and thifensulfuron-treated GL2600 (GR) than for the isogenic glyphosate-susceptible isolines (Table 4). Thifensulfuron did not increase the incidence of Sclerotinia stem rot compared with the untreated control while Sclerotinia stem rot disease severity for each cultivar was reduced by lactofen compared with the untreated control. Glyphosate increased the incidence of Sclerotinia stem rot of GL2600 (GR) and P93B01 (GR) compared with the untreated control. Therefore, cultivars treated with thifensulfuron or glyphosate may have dissimilar response to Sclerotinia stem rot. This research does not support research suggesting that acetolactate synthase (ALS)–inhibiting herbicides increase the incidence of disease; however, it does support research suggesting that glyphosate may increase the incidence of disease of some cultivars (Holliday and Keen, 1982; Keen et al., 1982; Sanogo et al., 2000) and that a postemergence application of lactofen reduces the incidence of disease (Dann et al., 1999; Sanogo et al., 2000).


View this table:
[in this window]
[in a new window]
  		Table 4. Sclerotinia stem rot disease severity index and soybean yield for glyphosate-resistant (GR) and non-GR soybean isolines treated with postemergence herbicides in 1998 and 1999.{dagger}

 
Soybean Yield
Grain yield of untreated S 19-90, thifensulfuron-treated S 12-49 and GL2415, and lactofen-treated S 19-90 and GL2415 was 350 to 720 kg ha-1 greater than isogenic GR soybean (Table 4). S 12-49 treated with thifensulfuron, GL2415 treated with lactofen, and S20-B9 (GR) treated with glyphosate had grain yields 450 to 640 kg ha-1 greater than those of untreated soybean. Grain yield of thifensulfuron-treated S 12-49 and GL2415 indicated reduced incidence of disease and increased grain yield compared with the isogenic GR cultivar. A reduction in Sclerotinia stem rot did not always indicate an increase in grain yield. Soybean response to a postemergence herbicide and disease needs to be considered as part of a complete soybean production system.


   SUMMARY
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
Apothecia, the fruiting bodies that produce ascospores, which are responsible for S. sclerotiorum infection of soybean plants, were first observed (personal observation) in the field approximately 21 DAT (mid-July). Soybean LAI during this time was 6.5 to 8.0 in 1998 and 6.5 to 7.5 in 1999. The cottony white mold appeared approximately 2 wk later on the lower portion of the soybean stem and at other locations on the plants during the peak LAI period of the growing season (Fig. 1). Glyphosate-resistant and non-GR soybean cultivars had similar canopy development (Fig. 1), flower no. (data not presented), S. sclerotiorum lesion diameters (data not presented), and phytoalexin production (Table 3). Cultivar selection had a large influence on reducing the incidence of Sclerotinia stem rot in the field (Table 4). Lesion diameter (Fig. 5) and phytoalexin production (Table 3) indicated an inherent difference in cultivar susceptibility to S. sclerotiorum; however, maturity (Table 2), canopy development (Fig. 1), and timing of peak flowering (Fig. 4) may also help to reduce Sclerotinia stem rot in more tolerant cultivars like S 12-49, S14-M7, S 19-90, and S20-B9. In our research, later-maturing cultivars had a greater incidence of Sclerotinia stem rot than earlier-maturing cultivars, which was similar to other studies (Boland and Hall, 1987; Buzzell et al., 1993; Chun et al., 1987; Grau et al., 1982; Yang et al., 1999). However, grain yield of untreated cultivars with a reduced incidence of disease did not yield greater than cultivars with a high incidence of Sclerotinia stem rot.

Glyphosate did not affect soybean response, reproductive development, canopy development, flower no. plant-1, S. sclerotiorum lesion size, or phytoalexin production. However, the incidence of Sclerotinia stem rot was greater in late group II GR soybean [GL2600 (GR) and P93B01 (GR)] treated with glyphosate compared with untreated soybean. Table 5 summarizes the effects of thifensulfuron and lactofen on the parameters evaluated in this research compared with no herbicide treatment. Thifensulfuron and lactofen reduced or delayed soybean development, but only lactofen reduced S. sclerotiorum lesion diameter, increased phytoalexin production, and reduced disease severity compared with untreated soybean. Therefore, increased phytoalexin was related to reduced disease in lactofen-treated soybean, but grain yield was only increased in one of the eight cultivars evaluated. The impact of herbicide treatments on the incidence of disease and ultimately on an increase in grain yield depends on the cultivar. Recommendations for Sclerotinia stem rot management with herbicides may need to be cultivar specific. Lactofen consistently reduced the incidence of Sclerotinia stem rot but did not consistently increase grain yield of the cultivars evaluated. Thifensulfuron treatments that reduced the incidence of Sclerotinia stem rot of glyphosate-susceptible cultivars compared with GR cultivars also increased grain yield, which indicates that the acetolactate synthase (ALS) herbicides influenced glyphosate-susceptible cultivars differently compared with glyphosate-resistant cultivars; however, no phytoalexin levels were evaluated for these cultivars. Farmers with a moderate level of Sclerotinia stem rot may reduce the incidence of this disease with a lactofen treatment but may not always increase grain yield.


View this table:
[in this window]
[in a new window]
  		Table 5. Summary of the effects of thifensulfuron and lactofen on soybean development, physiology, incidence of Sclerotinia stem rot, and yield compared with untreated soybean.{dagger}

 
A reduction in disease severity could reduce sclerotia production. Buzzell et al. (1993) reported a reduction in the number of sclerotia present in the soil when the incidence of Sclerotinia stem rot was reduced. Herbicide treatments and cultivars evaluated in this research that reduced the incidence of Sclerotinia stem rot may therefore reduce sclerotia production. Integrated pest management will help soybean growers to make informed decisions on how weed management programs impact soybean development, incidence of Sclerotinia stem rot, and soybean yield.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 




This article has been cited by other articles:


Home page
 Agron. J.Home page
K. A. Nelson, G. E. Rottinghaus, and T. E. Nelson
Effect of Lactofen Application Timing on Yield and Isoflavone Concentration in Soybean Seed
Agron. J., April 4, 2007; 99(3): 645 - 649.
[Abstract] [Full Text] [PDF]

Home page
 J. Environ. Qual.Home page
A. L. Cerdeira and S. O. Duke
The Current Status and Environmental Impacts of Glyphosate-Resistant Crops: A Review
J. Environ. Qual., August 9, 2006; 35(5): 1633 - 1658.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (10)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Nelson, K. A.
Right arrow Articles by Hammerschmidt, R.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Nelson, K. A.
Right arrow Articles by Hammerschmidt, R.
Agricola
Right arrow Articles by Nelson, K. A.
Right arrow Articles by Hammerschmidt, R.
Related Collections
Right arrow Soybean
Right arrow Agricultural Pesticides
Right arrow Crop Growth and Development
Right arrow Other Crop Management
Right arrow Plant Disease
Right arrow Production Agriculture

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome