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a Dep. of Plant Pathology, 495 Borlaug Hall, Univ. of Minnesota, 1991 Buford Circle, St. Paul, MN 55108
b Dep. of Plant Pathology, 284 Russell Labs, 1630 Linden Drive, Univ. of Wisconsin, Madison, WI 53706
c Dep. of Agronomy, 371 Moore Hall, 1575 Linden Drive, Univ. of Wisconsin, Madison, Madison, WI 53706
d Wisconsin Dep. of Agriculture, Trade, and Consumer Protection, Plant Pest and Disease Lab, 4702 University Ave., Madison, WI 53702
* Corresponding author (jkurle{at}puccini.crl.umn.edu)
Received for publication October 4, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: AUDPC, area under the disease progress curve • BSR, brown stem rot • CP, chisel plow • MP, moldboard plow • NT, no-till • SSR, Sclerotinia stem rot
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Infection of soybean plants by S. sclerotiorum occurs during the reproductive phase of soybean plant growth. Ascospores deposited on flower petals germinate when free water is present on plant surfaces, utilizing the petal as a nutrient base. Infection progresses with the growth of mycelium into plant tissue macerated by oxalic acid released by the advancing mycelium (Abawi and Grogan, 1979; Boland and Hall, 1988). Eventually, stems and petioles are infected, vascular tissues are disrupted, and stems, pods, or leaves beyond the site of infection die. As nutrients are exhausted, fungal mycelia aggregate into sclerotia that form both inside and outside the plant stem. These sclerotia then fall to the ground where they can survive for years (Schwartz and Steadman, 1978). When the soil is moist, sclerotia in the upper 5 cm of the soil profile germinate and form apothecia that release airborne ascospores, which initiate the disease cycle again. Low to moderate temperatures and abundant water are required for all stages of fungal growth and development. Apothecia develop most rapidly when soils are saturated and temperatures are in the range of 10 to 20°C (Abawi and Grogan, 1975). Fungal infection and mycelial growth is maximized when free water is present on plant surfaces (Abawi and Grogan, 1975, 1979; Boland and Hall, 1988).
Sclerotinia stem rot can be controlled successfully by fungicides in susceptible crops such as dry bean and canola (Steadman, 1979); however, chemical control of SSR in soybean has not proven economically feasible. As a result, strategies for controlling SSR in soybean emphasize cultivar selection and management practices that reduce canopy density (Hall and Nasser, 1996). Complete resistance to SSR has not been reported (Boland and Hall, 1987; Grau et al., 1982; Kim et al., 1999; Pennypacker and Risius, 1999; Yang et al., 1999). Planting in wide row spacings or at lower plant populations, by delaying canopy closure and reducing canopy density, causes less favorable humidity and temperature conditions for fungal development within the crop canopy (Blad et al., 1978; Buzzell et al., 1993; Grau and Radke, 1984; Steadman, 1979). There is little information available about the effect of crop sequence or tillage on soil inoculum density or on SSR development in soybean. Rotation with nonhost crops is recommended as a means of reducing sclerotial density by germination, senescence, or parasitism; however, studies of sclerotial population dynamics in dry bean suggest that attempts to manipulate sclerotial populations may have limited effect on either soilborne or airborne inoculum density (Schwartz and Steadman, 1978). Burial by tillage appears to have limited effectiveness in reducing the incidence of diseases caused by Sclerotinia spp. in vegetable crops, since viable sclerotia buried in earlier cropping cycles may be uncovered subsequently by tillage (Subbarao et al., 1996).
A better understanding of the effect of cultural practices on SSR incidence would enable more effective management of SSR by farmers. Our hypothesis was that the management practices that decrease soil inoculum density could be combined with cultivar selection to reduce SSR incidence and increase soybean yield. The objectives of this study were to examine the effect of crop rotation, tillage, and cultivar on sclerotial density, apothecial number, SSR severity, and soybean yield.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Planting methods were the same at all three locations. Soybean was planted in 19.1-cm rows at a rate of 556000 viable seeds ha-1 with a John Deere 750 no-till drill. Small grains were planted in 19.1-cm rows at a rate of 99 seeds m-1, and corn was planted in 76-cm rows at a rate of 70500 viable seeds ha-1. The area planted for each plot was 0.09 ha at Janesville and Waunakee and 0.08 ha at Sharon. Planting and harvest dates varied with location, crop, and year. Small grains and corn were planted from mid to late April while soybean were planted from late April to mid-May. Small grain harvest was completed in late August, whereas corn was harvested in October. In 1997 soybean planting was completed 10 May at Sharon, 11 May at Janesville, and 13 May at Waunakee.
Weed control was maintained using herbicides selected for the weed problem present at each location. At Janesville, a preplant application of 2,4-D (2,4-dichlorophenoxyacetic acid) was made to chisel plow (CP), moldboard (MB), and no-till (NT). Pendimethalin (N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine) and imazethapyr ((±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid) and pendimethalin were applied preplant to CP and NT. A preplant-incorporated application of pendimethalin and a combination of imazethapyr and pendimethalin was applied to MB. At Sharon, a preplant application of pendimethalin was made to CP; preplant applications of glyphosate (N-(phosphonomethyl)glycine) and 2,4-D were made to NT. Postemergence applications of fomasafen (5-[2-chloro-4-(trifluoromethyl)phenoxy]-N-(methylsulfonyl)-2-nitrobenzamide) and sethoxydim (2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one) were applied in CP, MB, and NT. At Waunakee, pendimethalin and a combination of imazethapyr and pendimethalin were applied pre-plant to CP; glyphosate and 2,4-D were applied preplant to NT; and acifluorfen (5-[2-chloro-4-(trifluoromethyl)phenoxy]2-nitrobenzoic acid) and thifensulfuron (methyl-3-[[[[(4methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino] sulfonyl]-2-thiophenecarboxylate) were applied postemergence to CP, MB, and NT.
All three locations had been cropped previously in a corn–soybean rotation managed for optimum yields according to recommendations of the University of Wisconsin Extension Service. Before establishment of the studies, the fields at Janesville and Waunakee had been fall moldboard and spring chisel plowed, whereas a no-till system had been applied at Sharon. In 1994, soybean cultivar Kaltenberg 241 had been planted at all three locations.
Field Plot Design
A factorial treatment combination of three tillage systems—crops planted in 1995, crops and soybean cultivars planted in 1996, and soybean cultivars planted in 1997—was arranged in a split-split-split-split plot design with each location as a single replicate (Table 1). Main plots were tillage systems: fall moldboard plowed followed by spring field cultivation (MB); fall chisel plow followed by spring field cultivation (CP), or no-till (NT). Each tillage system was planted to three crops: corn, small grain, or soybean. The 1995 crop was the subplot. All soybean plots in 1995 were planted with the cultivar Sturdy. The 1996 crop was the sub-subplot; corn, small grain, or one of three soybean cultivars planted at right angles to each 1995 crop–tillage system combination. The small grain planted at Janesville and Waunakee was oat cultivar Dane while barley (Hordeum vulgare L.) cultivar Chilton was planted at Sharon. All soybean plots were split for planting to the three soybean cultivars; Northrup-King S19-90, a moderately resistant cultivar; BSR101, a moderately susceptible cultivar; or Sturdy, a susceptible cultivar. In 1997 the sub-sub-subplots were soybean cultivars S19-90, BSR101, and Sturdy planted across the 1996 plots. All combinations of tillage, 1995 crop, 1996 crop, and 1997 soybean cultivar were present in the sub-sub-subplots. There were 135 plots at each location in 1997.
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Weather Data
Weather data for the Waunakee location was provided by the University of Wisconsin Experiment Station located 6 km from the plots at Arlington, WI. Weather data for the Sharon location was obtained from an on-farm weather station located 4 km from the experiment. Twenty-year temperature normals recorded at the Arlington Experiment Station were used for comparison with temperatures at Waunakee. Twenty-year temperature normals recorded at Lake Geneva, WI, were used for comparison to temperatures at Sharon. Weather data for the Janesville location was recorded at Afton, WI, 15 km from the experiment. Temperature data for this station were incomplete because the station had been relocated within the past 20 yr.
Sampling
All soil and plant samples, epidemiological and disease observations, and crop yields were obtained from sample areas located adjacent to the center of the larger treatment plot. The plot center was located using the GPS map that had been developed in 1996. The size of the sample areas varied according to the sample or observation that was being obtained from the plot. Areas for samples and observations obtained after crop emergence were positioned to avoid damage from foot traffic and plant disturbance that might affect subsequent sclerotial or apothecial populations, disease incidence, and crop yields.
Sclerotial Density
Sclerotial density was determined by extracting sclerotia from two separate groups of soil samples. The first group of samples enabled us to determine the effect of tillage, crop sequence, and soybean cultivar on the horizontal (spatial) distribution of sclerotia among all treatment plots. The second group of samples enabled us to determine the effect of tillage and crop sequence on the vertical distribution of sclerotia during the study.
The horizontal distribution of sclerotia was determined from a first group of samples obtained by sampling all plots in the spring of 1995, 1996, and 1997, and in the fall of 1997. Each sample consisted of 40 soil cores, 2.5 cm in diameter and 20 cm in depth, obtained with a standard soil probe from a 1.5 by 1.5 m sample area located at the center of each treatment plot. Samples were collected in plastic bags and stored at 8°C until they were processed to separate sclerotia using the technique described below. There were 135 samples taken at each location during each sampling.
The vertical distribution of soilborne sclerotia was determined from a second group of samples obtained from a 12-plot subset of all the treatment plots at each location in the spring of 1995, 1996, and 1997, and in the fall of 1997. The sampled plots represented all combinations of three tillage systems (CP, MB, and NT) and four crop sequences containing corn or soybean (soybean–soybean–soybean, corn–corn–soybean, soybean–corn–soybean, and corn–soybean–soybean). In each of these samples nine subsamples of 40 cores each were taken from an 8.2 by 8.2 m sample area in the center of each treatment plot. The location of each subsample was determined by subdividing the sample area into 25 squares, each 1.64 by 1.64 m in size. Subsamples were then taken in the 1st, 3rd, and 5th square of the 1st, 3rd, and 5th row. Each subsample consisted of 40 soil cores, each 2.5 cm in diameter and 20 cm in length obtained with a standard soil probe. Each core was then subdivided into sections from 0 to 2 cm, 2 to 10 cm, and 10 to 20 cm in the soil profile. All cores in each subsample were then grouped by depth and each group placed in a separate plastic bag and stored at 8°C until they were to be processed for separation of sclerotia. There were a total of 108 subsamples taken from the 12 sampled treatment plots at each location.
All soil samples were wet sieved to separate sclerotia from the soil and plant debris (Hoes and Huang, 1975). The volume of the soil sample was determined by the amount of water displaced after 2 min when the sample was placed in a graduated plastic beaker. The sample was then soaked for 20 min in a plastic dishpan in sufficient warm water to completely immerse the sample. After soaking, plant debris and sclerotia were separated from soil on a grain dockage sieve with circular openings of 0.31 mm (8/64 inch) (Seedburo Equipment Co., 1023 W. Jackson Blvd., Chicago, IL). All material remaining on the sieve was collected, air-dried at room temperature, and sclerotia separated from this material by hand. The sclerotia were counted and sclerotial density was calculated on the basis of the soil volume determined by water displacement.
Sclerotial viability was determined using the carrot disk assay method of Hoes and Huang (1975). Each sclerotium was cut in half transversely and one-half placed on a 0.3-cm carrot disk placed on moistened filter paper in a covered petri dish. The petri dish was then incubated for 5 d at 20°C under a 12 h/12 h, light/dark cycle with light from a combination of incandescent and flourescent sources. After 5 d, viability was indicated by the growth of white fluffy mycelium on the carrot disk or the cut end of the sclerotium. Viability was reported as percentage of sclerotia that were viable in the sample.
Apothecial Numbers
Apothecial populations were determined weekly in all treatment plots throughout the growing season beginning with crop emergence and ending at harvest in both 1996 and 1997. All apothecia in four 0.3 by 0.3 m quadrats located within 5 m of the plot center were counted. No attempt was made to return to the same quadrat for each observation. Apothecial numbers are reported for each observation and also as an average number of apothecia observed for a plot during the growing season.
Disease Observations
Incidence of SSR was surveyed in all plots five times at 3-d intervals beginning at the R6 (one pod containing green seeds that fill the pod cavity at one of the four uppermost nodes) and continuing into the R7 (one pod on the main stem that has reached its mature pod color) growth stage. Observations of SSR incidence were expressed as a percentage of symptomatic plants in the 7.6 by 1.5 m area that was to be harvested for grain yield. Final SSR incidence was expressed two ways, as an AUDPC and as a one-time SSR incidence observation noted for a particular observation date. Because AUDPC was highly correlated with SSR incidence at the second observation date, only the values for the second observation date are reported.
Field studies involving plant diseases can be complicated by the presence of more than one disease. Because brown stem rot (BSR) caused by Phialophora gregata (Allington and Chamberlain) W. Gams occurred in the plots at Janesville, an assessment of BSR severity was made simultaneously with the assessment of SSR incidence.
Brown stem rot is a commonly occurring disease throughout the north central USA. Its incidence is increased under reduced tillage, in monocultures, or in crop rotations with a limited number of crops (Mengistu and Grau, 1987; Gray and Grau, 1999). Inoculum of P. gregata survives on soybean crop residue. The disease is favored by growing season temperatures between 15 and 27°C and by heavy precipitation (Gray and Grau, 1999). Because BSR is favored by environmental conditions similar to those favoring SSR and because of the widespread occurrence of BSR inoculum in the north central states, the two diseases frequently occur together.
We felt that it might be possible to account for the effects of BSR on yield and avoid confounding yield loss caused by BSR with that caused by SSR because the three soybean cultivars included in the study differ in susceptibility to BSR. Sturdy is considered to be susceptible, S19-90 is considered to be partially resistant, and BSR101 is considered to be resistant to BSR. Disease severity was estimated based on a visual estimate of the percentage of leaf area affected by BSR (Mengistu and Grau, 1987).
Soybean Plant Density
Soybean populations were determined at all locations in the spring at the V4 (completely unrolled leaf at the fourth node above the cotyledon node) growth stage and at harvest in the 12 plots sampled for vertical distribution of sclerotia. Plant density was determined by counting the number of plants present within a 1-m diameter counting hoop.
Soybean Yield
In 1997 soybean seeds were harvested from the center of each plot with a small-plot combine in an area 7.6 by 1.5 m for yield determination. Soybean harvest was completed 26 September at Sharon, 29 September at Janesville, and 14 October at Waunakee. Soybean yields were adjusted to 13.0% moisture.
Statistical Analysis
Values of sclerotial density, apothecial numbers, SSR incidence, and grain yield were analyzed for homogeneity of variance and normal distribution. Sclerotial density, apothecial numbers, and grain yield were analyzed using PROC GLM of the Statistical Analysis System (SAS) to perform an ANOVA. Sclerotinia stem rot incidence and percentage of viable sclerotia were transformed using an arcsine square root transformation before analysis. Because there was no disease present at the Sharon location, only Janesville and Waunakee values were included in the ANOVA of apothecial numbers, SSR incidence, and grain yield. Grain yield, SSR incidence, apothecial numbers, and sclerotial density were first analyzed as a split-split-split-split plot design that included a factorial level for location, tillage, crop in 1995, crop in 1996, and crop in 1997. The main effect, location, and interactions that included location were considered to be random effects. Grain yield, SSR incidence, apothecial numbers, and sclerotial density were then analyzed using single degree of freedom contrasts that compared the effect of the crop grown in 1996 on yield, SSR incidence, apothecial number, and sclerotial density.
The relationship of yield to transformed SSR incidence and BSR severity was analyzed using Pearson's correlation in PROC CORR and regression analysis in PROC REG of SAS.
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RESULTS |
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Sclerotinia stem rot incidence was influenced by the interaction of tillage with soybean cultivar (P = 0.0031). Incidence of SSR was highest in MB for both S19-90 and Sturdy, and in CP for BSR101 (Table 2). Sclerotinia stem rot incidence in CP was similar to that in MB and averaged >70% when either BSR101 or Sturdy were grown (Table 2). When S19-90 was grown, SSR incidence averaged <28% in all tillage systems (Table 2), was 21% less in NT, and 17% less in CP than SSR incidence in MB (Table 2).
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Single degree of freedom contrasts indicated that the soybean cultivar or the crop planted in 1996 affected SSR incidence that occurred in 1997 (Table 3). The highest incidence of SSR developed in 1997 following the cultivar Sturdy, the most susceptible cultivar planted in 1996 (Table 4). Lower SSR incidence occurred following BSR101, and still lower SSR incidence developed following the moderately resistant cultivar, S19-90. Among previous crops or soybean cultivars, the lowest SSR incidence developed following oat (Tables 3 and 4). Incidence of SSR following either corn or oat differed significantly from disease incidence that occurred following soybean (Tables 3 and 4). Incidence of SSR following the moderately resistant cultivar, S19-90, was significantly different from that following the susceptible cultivar, Sturdy (Table 3).
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Soybean Yield
Soybean yields in 1997 averaged 3110 kg ha-1 at Janesville, 3843 kg ha-1 at Sharon, and 2135 kg ha-1 at Waunakee (Fig. 2). ANOVA of results from Janesville and Waunakee showed that grain yield was affected significantly by tillage (P = 0.0058), the 1995 crop (P = 0.0273), the 1996 crop (P = 0.0396), and soybean cultivar planted in 1997 (P = 0.0001). Grain yield was greater in NT and CP than in MB (Table 5). The greatest grain yield was produced by the moderately resistant cultivar, S19-90, while the lowest yield was produced by the moderately susceptible cultivar, BSR101 (Table 5).
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Plant density differed among tillage systems in both spring and fall (Fig. 3). Plant density decreased significantly in MB and CP in the interval between June and October (Fig. 3) because of mortality associated with SSR and Phytophthora root rot caused by Phytophthora sojae M.J. Kaufmann & J.W. Gerdemann (Hansen and Maxwell, 1991). The highest early season plant density occurred in MB and the lowest occurred in NT (Fig. 3). At harvest, the highest plant density occurred in NT (Fig. 3).
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Sclerotial density in the upper 20 cm of soil averaged 1.3 sclerotia L-1 of soil at Janesville, 0.7 sclerotia L-1 of soil at Sharon, and 0.8 sclerotia L-1 of soil at Waunakee. In the analysis of results from Janesville and Waunakee, the density of sclerotia was influenced by the interaction of tillage with the crop planted in 1995 (P = 0.0949) and by tillage (P = 0.0004) (Table 6). In NT, sclerotial density was lower when the crop planted in 1995 was corn or oat than when it was soybean (data not shown). In MB, sclerotial density was higher when oat or corn was planted in 1995 compared with the areas where soybean had been planted. The greatest density of sclerotia was found in MB, while the lowest density was found in NT (Table 6).
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Although there were consistent patterns in values of sclerotial viability, no main effect or interactions affected sclerotial viability at the P 
0.05 level of significance. Average sclerotial viability in MB was 35%, in CP was 34%, and in NT was 52%. In 1997, sclerotia from the 0- to 2-cm layer of NT and CP possessed greater sclerotial viability than sclerotia from the same layer in MB (Fig. 5). Sclerotial viability was similar at all three depths in MB, but decreased with increasing depth in CP (Fig. 5). In NT, sclerotial viability was similar in the 0- to 2- and 2- to 10-cm layer of the soil profile, but was sharply reduced in the 10- to 20-cm layer. However, at this depth the percentage was based on a relatively low number of sclerotia (Fig. 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Planting S19-90, a moderately resistant cultivar, was the most effective management practice available for controlling SSR. The average reduction in SSR incidence when S19-90 was planted was >40%. This was greater than the reduction in SSR incidence when NT was used instead of MB or CP or the reduction in SSR incidence when soybean was planted following oat or corn.
Rotation with nonhost crops and use of NT were both effective in reducing SSR incidence. When these practices were used in combination with S19-90 their effectiveness in reducing SSR was mutually complementary. For example, SSR was reduced by 40% in NT when compared with CP or MB. When either BSR101 or Sturdy was planted in NT, SSR incidence was 33% lower than when either cultivar was planted in either CP or MB. When S19-90 was planted in NT, SSR incidence was reduced by >67% compared with the incidence of SSR in MB or CP planted to either BSR101 or Sturdy. Rotation with nonhost crops also reduced SSR incidence, although the reduction in SSR incidence was less than that obtained by planting S19-90. The benefit obtained from planting corn or oat as a rotational crop the previous year was increased when S19-90 was planted.
Increased yields accompanied reduced SSR incidence when S19-90 was planted following corn or oat as a previous crop or in NT at Waunakee, where BSR severity was very low. In addition, the relationship of yield loss to SSR incidence was similar to that reported by Yang et al. (1999). At Janesville the relationship between SSR incidence and yield loss was obscured by the presence of BSR. This may explain the absence of yield differences among the three soybean cultivars at this location, since BSR101 is resistant to BSR, whereas S19-90 and Sturdy are considered to be more susceptible. The yield advantage expected from resistance to SSR was absent because of BSR. However, we were unable to develop a satisfactory regression model to explain the combined effect of SSR and BSR on yield at this location.
The reduction in SSR incidence following either corn or small grain does not appear to be caused by a reduction in soilborne inoculum of SSR, because there was no significant difference in sclerotial density or apothecial numbers in 1997 following corn, oat, or any one of the three soybean cultivars planted in 1996. Possible explanations are a better seedbed and improved emergence or more readily available N following soybean. Either might contribute to a denser canopy and a more favorable environment for SSR.
Management practices such as tillage may have reduced SSR incidence by lowering either soilborne or airborne inoculum density. Correlation analysis indicated that soilborne inoculum density and apothecial numbers explain a small portion of SSR incidence; however, the weakness of this relationship suggests that other factors were more important in determining SSR incidence. This is also illustrated by the relationship of sclerotial density and sclerotial viability to SSR incidence. The three tillage practices affected sclerotial distribution and sclerotia viability differently. Sclerotia were buried and then redistributed throughout the profile by MB and to a lesser extent by CP. Sclerotial viability throughout the soil profile was reduced in MB. Sclerotial viability in the uppermost 2 cm of soil was reduced under both MB and CP when compared with NT. There was no reduction in sclerotial density and little reduction in sclerotial viability in NT. As a result, the greatest density of viable sclerotia was found in the upper 2 cm of soil in NT. In contrast, the greatest number of apothecia and the highest SSR incidence developed in MB.
Reduced sclerotial viability present in CP and MB may have been caused by parasitism of buried sclerotia. Parasitism of sclerotia lying on the soil surface was probably reduced by periodic drying. A possible explanation for the decline in sclerotia numbers that occurred during the study is the activity of Sporidesmium sclerotivorum Uecker, Ayers, and Adams (D.R. Fravel, personal communication, 1997) that was present in sclerotial samples from Waunakee. Sporidesmium sclerotivorum has been used as a biocontrol agent for SSR in horticultural crops (Adams and Ayers, 1983).
Differences in plant stand and canopy density among tillage systems are more important than differences in the density of viable sclerotia as a cause for differences in apothecial numbers and SSR incidence seen in our study. The three tillage systems affected SSR incidence by their effect on soybean emergence, plant population, and crop canopy structure rather than their effect on soilborne inoculum density. Spring stand counts indicated that higher soybean plant populations were present in MB and CP than in NT early in the growing season. In addition to low plant populations, canopy development in NT was so poor that complete canopy closure did not occur. Consequently, microclimatic conditions within the crop canopy in NT were not favorable for either formation of apothecia or SSR development. Previous research in dry bean has emphasized the role of canopy density in promoting SSR development by maintaining high humidity, prolonging leaf wetness, and lowering temperature within the canopy (Blad et al., 1978).
Differences in plant architecture and canopy density may also explain the difference in apothecial numbers observed under the different soybean cultivars in the study (Schwartz and Steadman, 1978). Lower apothecial numbers observed in S19-90 may be a consequence of lower humidity, more rapid evaporation, and soil drying that occurs under the more upright open canopy developed by this cultivar. Lower apothecial numbers also indicate that conditions are unfavorable for other stages of infection and disease development.
Our original hypothesis was that tillage, rotation with crops other than soybean, or planting a more resistant cultivar would be effective strategies for reducing the incidence of SSR in soybean and maintaining soybean yield. Planting a moderately resistant soybean cultivar was the most effective single management practice available for controlling SSR. Its effectiveness was enhanced by crop rotation and by NT. An unexpected effect of tillage was higher SSR incidence in MB than in NT because of reduced stand and a less dense canopy in NT. Our results suggest that management practices that reduce canopy density, such as wider row spacing or lower plant population, would be more effective in controlling SSR than management practices intended to reduce soilborne inoculum density.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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