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Current Research on Sclerotinia Stem Rot

Research Overview: Sclerotinia stem rot remains as a major soybean production constraint in areas where the disease has been prominent over the years, as sclerotia are known to be long-time residers in soil. If conditions during soybean flowering are conducive for infection and disease development, then an epidemic will usually follow. Our main focus has been on field management of the disease, understanding more about epidemiology, and host resistance. Through our research, we continue to search for better sources of resistance as well as to further characterize isolates of the pathogen.

Publications

Host-Pathogen Interaction

Bent, A.F., T.K. Hoffman, J.C. Schmidt, G.L. Hartman, D.D. Hoffman, P. Xue, and M.L. Tucker. 2006. Disease- and performance-related traits of ethylene-insensitive soybean. Crop Science 46:893-901. [download] [view abstract]
Ethylene controls many beneficial responses in plants but also promotes chlorosis, senescence, disease, and fruit over-ripening. The present study compared previously isolated ethylene-insensitive lines of soybean [Glycine max (L.) Merr.] to their isogenic, ethylene-sensitive parent with respect to disease resistance, seed yield, and other field performance traits. In laboratory tests, ethylene insensitivity reduced root colonization by soybean cyst nematode. Using healthy young plants, ethylene-insensitivity also reduced ethylene-activated leaf chlorosis and abscission. However, in the field, leaf chlorophyll and late-season senescence were not altered, suggesting that ethylene is not a main determinant of late-season senescence of soybean leaves. Field studies also revealed no changes in susceptibility to Septoria brown spot disease (caused by Septoria glycines Hemmi), flowering date, plant height, or seed total protein and oil concentration. Field studies did demonstrate elevated susceptibility to white mold disease [Sclerotinia sclerotiorum (Lib.) deBary], poor stand establishment in some but not all environments, altered plant architecture, and earlier maturity date in the ethylene-insensitive lines. Seed yield was notably undependable, being similar to the parental line in some field locations but significantly reduced in most environments. To avoid these negative impacts on overall performance, manipulation of plant ethylene responses should be targeted to specific tissues, growth stages, or growth environments.
Calla, B., Voung, T., Radwin, O., Hartman, G.L., and Clough, S.J. 2009. Gene expression profiling soybean stem tissue early response to Sclerotinia sclerotiorum and in silico mapping in relation to resistance markers. Plant Genome 2:149-166.
Diers, B. W., Kopisch-Obuch, F. J., Hoffman, D. D., Hartman, G. L., Pedersen, W. L., Grau, C. R., and Wang, D. 2006. Registration of AxN-1-55 soybean germplasm with partial resistance to Sclerotinia stem rot. Crop Science 46:1403-1404. [download] [view abstract]
AxN-1-55 soybean [Glycine max (L.) Merr.] (Reg. no. GP-315, PI 640911) was developed by the Illinois Agricultural Experiment Station at the University of Illinois and the Michigan Agricultural Experiment Station at Michigan State University. AxN-1-55 was released in 2005 as a mid group II maturity germplasm line for use as a parent because of its high level of resistance to Sclerotinia stem rot [caused by Sclerotinia sclerotiorum (Lib.) de Bary].
Hartman, G. L., Gardner, M. E., Hymowitz, T., and Naidoo, G. C. 2000. Evaluation of perennial Glycine species for resistance to soybean fungal pathogens that cause Sclerotinia stem rot and sudden death syndrome. Crop Sci. 40:545-549. [download] [view abstract]
The cultivated soybean [Glycine max (L.) Merr.] has a relatively narrow genetic base and most commercial cultivars are susceptible to Sclerotinia sclerotiorum (Lib.) de Bary and Fusarium solani (Mart.) Sacc. f. sp. glycines, which, respectively cause Sclerotinia stem rot (SSR) and sudden death syndrome (SDS). The objective of this study was to screen all the available accessions of the perennial Glycine species for resistance to the pathogens that cause SSR and SDS. For SSR evaluations, five seedlings of each of 787 accessions were screened once in a series of eight non-replicated runs. Seedlings were inoculated with an agar plug cut from the edge of a 1-d-old fungal culture by placing the plug next to the stem. Of the 787 accessions, 183 had partial resistance with 144 of these accessions being G. tabacina (Labill.) Benth. A selected set of 53 accessions was further screened in two replicated trials with five plants per each of four replications. Glycine tabacina had several accessions that were consistently rated as partially resistant. For SDS evaluations, five plants of each of 767 accessions were screened once in a series of eight runs. Plants were inoculated by a layered technique in which infested sorghum seed were placed below the transplanted seedlings. In the initial evaluation of 767 accessions, 134 had partial resistance with 65 of these accessions being G. tomentella Hayata. In a replicated set of selected accessions, G. tomentella had several accessions that were consistently rated as partially resistant. These perennial Glycine species represent potential untapped sources for improving disease resistance in soybean.
Hoffman, D. D., Diers, B. W., Hartman, G. L., Nickell, C. D., Nelson, R. L., Pedersen, W. L., Cober, E. R., Dorrance, A. E., Graef, G. L., Steadman, J. R., Grau, C. R., Nelson, B. D., del Rio, L. E., Helms, T., Poysa, V., Rajcan, I., and Stienstra, W. C. 2002. Selected soybean plant introductions with partial resistance to Sclerotinia sclerotiorum. Plant Disease 86:971-980. [download] [view abstract]
Sclerotinia stem rott caused by Sclerotinia sclemtiarum, is a major soybean (Glycine max) disease in north-central regions of the United States and throughout the world. Current sources of resistance to Sclerotinia stem rot express partial resistance, and are limited in number within soybean germ plasm. A total of 6,520 maturity group (MG) 0 to IV plant introductions (PIs) were evaluated for Sclerotinia stem rot resistance in the United States and Canada in small plots or in the greenhouse from 1995 to 1997. Selected PIs with the most resistance were evaluated for resistance in the United States and Canada in replicated large plots from 1998 to 2000. The PIs in the MG I to III tests in Urbanar IL were evaluated for agronomic traits from 1998 to 2000. The selected PIs also were evaluated with an excised leaf inoculation and petiole inoculation technique. After the 1995 to 1997 evaluations, all but 68 PIs were eliminated because of their susceptibility to Sclerotinia stem rot. In field tests in Urbana, higher disease severity in selected MG I to III PIs was significantly (P < 0,05) associated with taller plant heights and greater canopy closure. All other agronomic traits evaluated were not associated or were inconsistently associated with desease severity. MG I to III PIs 153.282, 189.931, 196.157, 398.637, 417.201, 423.818, and 561.331 had high levels of resistance and had canopies similar to the resistant checks. The resistance ratings from the petiole inoculation technique had a high and significant (P < 0.01) correlation with disease severity in the MG I and II field tests. The partially resistant PIs identified in this study can be valuable in incorporating Sclerotinia stem rot resistance into elite germplasm.
Hoffman, D. D., Hartman, G. L., Mueller, D. S., Leitz, R. A., Nickell, C. D., and Pedersen, W. L. 1998. Yield and seed quality of soybean cultivars infected with Sclerotinia sclerotiorum. Plant Disease 82:826-829. [download] [view abstract]
Sclerotinia stem rot (SSR) is one of the most important diseases of soybean in the United States. Five maturity group III cultivars, Asgrow A3304 STS (A3304), Pioneer Brand 9342 (P9342), Pioneer Brand 9381 (P938l), Probst, and Yale, grown in fields in east-central Illinois, were used to determine the relationship of SSR incidence to yield, 100-seed weight, seed protein and oil content, visual seed quality, and seed germination. In addition, the number of sclerotia in seed samples and the seedborne incidence of Sclerotinia sclerotiorum were determined. For each cultivar, at least 23 two-row plots, 3 m long, that represented a range of SSR incidence from low to high, were used to count the number of plants with and without SSR stem symptoms and were used to estimate yields and evaluate seed quality. Disease incidence ranged from 2 to 45% for Probst, 0 to 65% for P938l, 0 to 68% for P9342, 1 to 93% for Yale, and 0 to 95% for A3304. Regression of yields on SSR incidences for each cultivar was significant (P < 0.05); for every 10% increase in SSR incidence, yields were reduced by 147, 194, 203, 254, and 263 kg/ha for Probst, A3304, P9342, Yale, and P9381, respectively. Disease incidence was negatively correlated (P < 0.05) with seed germination for all cultivars but Probst, and to oil content and seed weight for P9381 and Yale. Disease incidence was positively correlated (P < 0.05) with seed quality for all cultivars and to the number of sclerotia in harvested seeds for P9342, P9381, and Probst. The seedborne incidence of S. sclerotiorum was 0.3, 0.3, 0.3, 0.4 and 0.7% in A3304, P9381, Yale, Probst, and P9342, respectively, and represents a significant potential for further spread of this pathogen and disease.
Kim, H. S., Hartman, G. L., Manandhar, J. B., Graef, G. L., Steadman, J. R., and Diers, B. W. 2000. Reaction of soybean cultivars to Sclerotinia stem rot in field, greenhouse, and laboratory evaluations. Crop Sci. 40:665-669. [download] [view abstract]
Sclerotinia stem rot of soybean [Glycine max (L.) Merr.], caused by the fungal pathogen Sclerotinia sclerotiorum (Lib.) de Bary, recently has increased in importance in the northern U.S. soybean production area. The objective of our study was to determine the effectiveness of three different inoculation techniques in predicting the field reactions of cultivars to sclerotinia stem rot. Eighteen soybean cultivars were field tested in six Michigan environments from 1994 to 1996 and tested in the greenhouse or laboratory with three inoculation methods. The cultivars were inoculated by placing infested oat (Avena sativa L.) seed or mycelial plugs on cotyledons or by placing mycelial plugs on detached leaves. There were significant (P < 0.05) differences in resistance to sclerotinia stem rot among cultivars at all but one field environment and for all inoculation methods. The disease severity ratings based on the inoculations were significantly correlated with the field results, with the exception of one method. Disease severity ratings for the three inoculation methods were significantly correlated with only two exceptions. Cultivars such as Novartis S19-90 and Corsoy 79 consistently had the lowest disease severity ratings in the field tests and for the inoculation methods. Similarly, a number of cultivars were rated as susceptible in all tests. Ratings for cultivars with intermediate reactions were not consistent across tests. The inoculation methods tested can provide some useful information on the resistance of soybean genotypes to sclerotinia stem rot. However, resistance identified by inoculation methods should be confirmed with field tests, since these methods can misclassify the resistance of some cultivars.
Kull, L. S., Pedersen, W. L., and Hartman, G. L. 2004. Mycelial compatibility and aggressiveness of Sclerotinia sclerotiorum. Plant Disease 88:325-332. [download] [view abstract]
Population variability of Sclerotinia sclerotiorum, the causal organism of Sclerotinia stem rot of soybean, was determined by mycelial compatibility grouping (MCG) and isolate aggressiveness comparisons. MCG and aggressiveness of S. sclerotiorum isolates from diverse hosts and geographic locations (Diverse Set, 24 isolates), from a soybean field in Argentina (Argentine Set, 21 isolates), and from soybean fields in DeKalb and Watseka, Illinois (DeKalb and Watseka Sets, 125 and 135 isolates, respectively) were assessed. Among 305 isolates tested, 42 MCGs were identified, and 61% were represented by single isolates observed at single locations. Within the Diverse Set, 17 MCGs were identified; one MCG consisted of six isolates, and 16 MCGs consisted of one isolate each. Each Illinois field was a mosaic of MCGs, but MCG profiles differed between the two fields. Nine MCGs were identified within the Argentine field with two MCGs composed of either five or six isolates, two MCGs composed of two isolates, and the remaining composed of one isolate each. MCGs were shared among the Diverse, DeKalb, and Watseka Sets, but MCGs within the Argentine Set were not shared with other sets. MCGs within the DeKalb and Watseka Sets were spatially aggregated based upon sampling location within the field. MCG 8 was the most frequently sampled and widely distributed MCG and occurred at a frequency of 29, 36, and 62% in the Diverse, DeKalb, and Watseka Sets, respectively. Variation in isolate aggressiveness was assessed using a limited-term, plug inoculation technique and area under the disease progress curve (AUDPC) was calculated for each isolate. Isolate aggressiveness varied (P=0.001) within the Diverse, DeKalb, Watseka, and Argentine Sets and varied (P<0.10) within MCGs composed of isolates from different locations, but did not vary within MCGs composed of isolates from a single field. Additionally, MCGs within the DeKalb and Watseka Sets differed in aggressiveness. A cultivar x isolate interaction was not detected, but resistant and susceptible cultivars performed similarly when inoculated with either less or highly aggressive isolates. Pathogen population structure and isolate variability may be important considerations in disease management systems.
Kull, L. S., Vuong, T. D., Powers, K. S., Eskridge, K. M., Steadman, J. R., and Hartman, G. L. 2003. Evaluation of resistance screening methods for Sclerotinia stem rot of soybean and dry bean. Plant Dis. 87:1471-1476. [download] [view abstract]
Data from three screening methods used to identify levels of resistance to Sclerotinia sclerotiorum for soybean (Glycine max) and dry bean (Phaseolus vulgaris) genotypes were compared by five statistical methods. Analysis of variance and mean separation tests were used to separate genotypes by resistance level and isolates by aggressiveness level. Six S. sclerotiorum isolates of known relative aggressiveness, and three soybean and three dry bean genotypes with varying levels of resistance to S. sclerotiorum were utilized. The three screening methods were mycelial plug inoculations of cotyledons, cut stems, and detached leaves. For soybean, all three inoculation methods accurately identified isolate aggressiveness levels irrespective of cultivar, but identification of susceptible and partially resistant soybean cultivars was influenced by isolate. For dry bean, the cotyledon and cut stem methods accurately identified isolate aggressiveness levels, but identification of susceptible and partially resistant dry bean cultivars was influenced by isolate and inoculation method. The cut stem method had the smallest coefficient of variation, was more precise at detecting interactions, and correlated with the other two methods. The sensitivity ratio showed the cut stem method to be superior to both other methods. The cotyledon method approached the technical merit of the cut stem method for both soybean and dry bean. When considering all five statistical analyses of resistance screening methods under controlled environmental conditions, the cut stem method was superior to the cotyledon method which was superior to the detached leaf method.
Marzano, S., Hobbs, H. A., Nelson, R. L., Hartman, G. L., Eastburn, D. M., McCoppin, N. K., and Domier, L. L. 2015. Transfection of Sclerotinia sclerotiorum with in vitro transcripts of a naturally occurring interspecific recombinant of Sclerotinia sclerotiorum hypovirus 2 significantly reduces virulence of the fungus. Journal of Virology 89:5060-5071. [view abstract]
A recombinant strain of Sclerotinia sclerotiorum hypovirus 2 (SsHV2) was identified from a North American Sclerotinia sclerotiorum isolate (328) from lettuce (Lactuca sativa L.) by high-throughput sequencing of total RNA. The 5'- and 3'-terminal regions of the genome were determined by rapid amplification of cDNA ends. The assembled nucleotide sequence was up to 92% identical to two recently reported SsHV2 strains but contained a deletion near its 5' terminus of more than 1.2 kb relative to the other SsHV2 strains and an insertion of 524 nucleotides (nt) that was distantly related to Valsa ceratosperma hypovirus 1. This suggests that the new isolate is a heterologous recombinant of SsHV2 with a yet-uncharacterized hypovirus. We named the new strain Sclerotinia sclerotiorum hypovirus 2 Lactuca (SsHV2L) and deposited the sequence in GenBank with accession number KF898354. Sclerotinia sclerotiorum isolate 328 was coinfected with a strain of Sclerotinia sclerotiorum endornavirus 1 and was debilitated compared to cultures of the same isolate that had been cured of virus infection by cycloheximide treatment and hyphal tipping. To determine whether SsHV2L alone could induce hypovirulence in S. sclerotiorum, a full-length cDNA of the 14,538-nt viral genome was cloned. Transcripts corresponding to the viral RNA were synthesized in vitro and transfected into a virus-free isolate of S. sclerotiorum, DK3. Isolate DK3 transfected with SsHV2L was hypovirulent on soybean and lettuce and exhibited delayed maturation of sclerotia relative to virus-free DK3, completing Koch's postulates for the association of hypovirulence with SsHV2L.
Vuong, T. D., and Hartman, G. L. 2003. Evaluation of soybean resistance to Sclerotinia stem rot using reciprocal grafting. Plant Dis. 87:154-158. [download] [view abstract]
Sclerotinia stem rot of soybean is one of the major soybean diseases in the North Central region of the United States. One disease management option is to plant cultivars that have partial resistance. Although sources of partial resistance have been identified, information pertaining to the nature of resistance to Sclerotinia stem rot is limited. The objective of this study was to determine the expression of resistance by using combinations of susceptible and partially resistant shoots. Resistant and susceptible soybean genotypes were grafted in different shoot and rootstock combinations of self-, single- or double-scion grafts. After successful grafts were made, cotyledons were inoculated with a plug of fungal mycelium. Based on seven grafting combinations of cross- and self-grafted plants using two soybean cultivars, grafts with NKS19-90 (partially resistant) as shoots had greater (P < 0.05) plant survival than the other graft combinations. In another experiment, a total of 17 graft combinations were generated using resistant plant introductions, PI549.076A, PI503.336, and PI194.639, and two susceptible cultivars, Alpha and Merit. Resistant self-grafts of PI194.639, PI503.336, and PI549.076A, had greater (P < 0.05) plant survival than those of Alpha or Merit. Most inter-genotypic grafts with resistant shoots had greater plant survival than those in reciprocal combinations. In a double-graft experiment, shoots each of NKS19-90 and Williams 82 were grafted to either NKS19-90 or Williams 82 rootstocks. Regardless of the rootstock, the shoots of Williams 82 died while shoots of NKS19-90 survived. A resistant or susceptible response was consistently associated with resistant or susceptible shoots regardless of the rootstock, type of grafting technique, or inoculation method, indicating that shoot are expression of partial resistance to S. sclerotiorum is not affected by root characteristics.
Vuong, T. D., D. D. Hoffman, B. W. Diers, J. F. Miller, J. R. Steadman, and G. L. Hartman. 2004. Evaluation of soybean, dry bean, and sunflower for resistance to Sclerotinia sclerotiorum. Crop Sci. 44:777-783. [download] [view abstract]
Many inoculation methods have been used to evaluate resistance of different crops to Sclerotinia sclerotiorum (Lib.) de Bary. Only a few of these methods have been used to evaluate more than one crop. This study compared disease evaluations of soybean [Glycine max (L.) Merr.], dry bean (Phaseolus vulgaris L.), and sunflower (Helianthus annuus L.) inoculated in the greenhouse (cut stem inoculation method) to field evaluations. In one experiment, stems of two soybean cultivars, ‘Williams 82’ (susceptible) and ‘NKS19-90’ (partially resistant), were severed and inoculated with a colonized mycelial plug of S. sclerotiorum placed on top of the plant at the cut point of the stem. Stem lesion lengths on these two cultivars were used to determine what affect plant age and post-infection temperature had on disease development. There was a significant (P<0.05) difference in lesion lengths between inoculated 5-wk-old plants compared to 6- or 7-wk-old plants within each cultivar. At different post-infection temperatures, lesions developed at 25°C but not at 30°C. In another experiment, disease rating of 15 soybean cultivars evaluated in the greenhouse and field had significant (P<0.05) correlation coefficients from 0.53 to 0.79. In addition to soybean, two experiments were completed on dry bean and sunflower. There were significant (P<0.05) differences in lesion lengths among 14 genotypes of each, dry bean and sunflower. The correlation between greenhouse and field evaluations of dry bean and sunflower were 0.74 and 0.50 (P<0.05), respectively. In summary, disease assessments from the cut stem inoculation compared favorably to disease assessments in the field for soybean, dry bean, and sunflower.
Vuong, T., Diers, B. W., and Hartman, G. L. 2008. Identification of QTL for resistance to Sclerotinia stem rot (Sclerotinia sclerotiorum) in plant introduction 194639. Crop Science 48:2209–2214. [download] [view abstract]
Sclerotinia stem rot of soybean [Glycine max (L.) Merr.], caused by Sclerotinia sclerotiorum (Lib.) de Bary, is a diffi cult disease to manage, although some gains have been made through breeding for quantitative resistance. The objective of the present study was to map quantitative trait loci (QTL) controlling partial resistance to Sclerotinia stem rot from the soybean plant introduction (PI) 194639. The resistance QTL were mapped in a population of 155 F4:5 recombinant inbred lines (RILs) developed from the hybridization of the partially resistant parent, PI 194639, to the susceptible cultivar Merit. The population was evaluated for Sclerotinia stem rot resistance using a cut stem inoculation technique and was genotyped with 134 simple sequence repeat (SSR) markers. Broad-sense heritability of lesion length (LL) after inoculation with the cut stem technique in the population was 0.57. Two putative QTL-controlling LL were identified by composite interval mapping (CIM) and mapped to linkage groups (LGs) A2 and B2, with likelihood of odds scores of 5.6 and 3.5, respectively. The LG A2 QTL was linked to the marker Sat_138 and explained 12.1% of the phenotypic variation for LL. The LG B2 QTL was proximal to the marker Satt126 and explained 11.2% of the phenotypic variance. Two minor QTL also were mapped onto LGs K and L, explaining 5.5% of the total phenotypic variation. A multivariate model that included all significant QTL explained 27% of the observed phenotypic variation of LL. These results suggest that SSR markers associated with resistance QTL mapped in this study for Sclerotinia stem rot resistance may be useful for marker-assisted breeding programs in soybean.

Epidemiology and Management

Hartman, G. L., Kull, L., and Huang, Y. H. 1998. Occurrence of Sclerotinia sclerotiorum in soybean fields in East-Central Illinois and enumeration of inocula in soybean seed lots. Plant Disease 82:560-564. [download] [view abstract]
Sclerotinia stem rot (SSR) of soybean is an important disease in the northern soybean-production areas of the United States. In Illinois, the occurrence of SSR has been limited to the northern-most soybean-production areas. In this study, soybean fields in east-central Illinois were surveyed for incidence of SSR. Of 68 fields surveyed, 31 contained SSR. Of the fields with SSR, 25 had between 1 and 5% SSR incidence, while 6 fields had more than 5% SSR incidence. The pattern of the disease in one field was mapped, and the spatial distribution of SSR was aggregated with a Lloyd's index of 1.4. In addition to field incidence of SSR, seed lots suspected of being contaminated with Sclerotinia sclerotiorum sclerotia were obtained from the Illinois Crop Improvement Association. Sclerotia and seeds were separated, and seeds were germinated to determine the level of seedborne infection by S. sclerotiorum. Sclerotia were recovered from 71 of 81 seed lots. Most of the seed lots were from the north-central area of Illinois, but one seed lot was from Madison County in the southwestern area of the state, and three and six seed lots contaminated with sclerotia were from Iowa and Wisconsin, respectively. Sclerotia counts ranged from 0 to 363 per seed lot. Normal-appearing seeds from the 81 seed lots (100 to 200 seeds per lot) were tested for germination and incidence of seedborne infection by S. sclerotiorum. Eight seed lots had at least one infected seed, and the incidence of seed infection ranged from 0.07 to 0.1%. From 10 randomly selected seed lots of the 81, discolored, shriveled seeds were selected and germinated to determine the incidence of seedborne infection. Seed infection rates ranged from 0. to 70%. The occurrence of SSR throughout Illinois and the importance of seedborne infection as a source of inoculum dispersal need further documentation.
Mueller, D. S., Dorrance, A. E., Derksen, R., Ozkan, E., Grau, C. R., Gaska, J. M., Kurle, J. E., Hartman, G. L., Bradley, C. A., and Pedersen, W. L. 2002. Efficacy of fungicides on Sclerotinia sclerotiorum and their potential control of Sclertoinia stem rot on soybean. Plant Disease 86:26-31. [download] [view abstract]
Sclerotinia stem rot (SSR) of soybeans, caused by Sclerotinia sclerotiorum, is a major disease in north central growing regions of the United States. One approach to control SSR on soybeans is with the use of fungicides. Azoxystrobin, benomyl, tebuconazole, thiophanate methyl, and vinclozolin were tested to control S. sclerotiorum in pure cultures on agar medium, inoculated soybean seedlings, detached inoculated leaves, and in the field. To evaluate the efficacy of five foliar fungicides in vitro, potato dextrose agar (PDA) was amended with different concentrations of fungicide and fungal radial growth in culture was measured. Vinclozolin was the most effective in controlling S. sclerotiorum by inhibiting mycelial growth at 1.0 ug a.i./ml. Mean mycelial growth of 91 isolates of S. sclerotiorum in the thiophanate methyl and vinclozolin amended agar was reduced 18-93% and 93-99%, respectively. Greenhouse grown seedlings treated with benomyl, thiophanate methyl, and vinclozolin prevented S. sclerotiorum from colonizing leaf tissue. Azoxystrobin and tebuconazole significantly (P<0.01) reduced disease severity ratings, but did not provide complete control. Detached leaves sprayed with thiophanate methyl and then inoculated with mycelial plugs of S. sclerotiorum did not become infected, and the area under disease progress curve was significantly less (P<0.01) than the nontreated control for leaves treated with azoxystrobin, benomyl, and tebuconazole. In field plots in 1997 and 1999, all plots had less than 1% disease, while in 1998, all fungicide treated plots had similar levels of disease as the nontreated control.
Mueller, D. S., Hartman, G. L., and Pedersen, W. L. 1999. Development of sclerotia and apothecia of Sclerotinia sclerotiorum from infected soybean seed and its control by fungicide seed treatment. Plant Disease 83:1113-1115. [download] [view abstract]
Seed coats were removed from soybean seeds infected with Sclerotinia sclerotiorum and the seed coats and cotyledons were surface sterilized with NaOCl (0.53%), rinsed in sterile distilled water, and placed on water agar. After 48 hours, mycelia of S. sclerotiorum were observed on both seed coats and cotyledons. Infected soybean seeds also were placed in aluminum pans containing field soil and placed in soybean fields near Urbana, IL and Clinton, WI. In 1997, a total of 553 sclerotia, 20 stipes, and 10 apothecia were produced from 500 infected seeds. In 1998, 201 sclerotia, and 22 stipes were produced, but no apothecia were observed from the 500 infected seeds. Fludioxonil was the most effective fungicide for reducing radial growth of S. sclerotiorum on PDA plates and suppressed 99% of the radial growth at 0.1 ug a.i./ml. When seed lots containing infected seeds were treated with several fungicides, captan + pentachloronitrobenzene + thiabendazole and fludioxonil completely inhibited mycelial growth from infected seeds; thiram and thiabendazole reduced infection by 90%. In a similar study in the field, thiram, fludioxonil, and captan + pentachloronitrobenzene + thiabendazole reduced sclerotia formation from infected seeds by more than 98%.
Mueller, D. S., Hartman, G. L., and Pedersen, W. L. 2002. Effect of crop rotation and tillage on Sclerotinia sclerotiorum on soybean. Canadian Journal of Plant Pathology 24:450-456. [download] [view abstract]
Sclerotinia stem rot (SSR) of soybean is a major disease In the North Central region of the United States. A 3-year study was done to determine if crop rotation and tillage, moldboard plowing then mulch tillage (MP + MT}, mulch tillage (MT), and no-till (NT), affected the population density of sclerotia and apothecia, incidence of SSR, and soybean yield. Crop rotation did not significantly affect the number or distribution of sclerotia or SSR incidence, but did affect the number of apothecia and the yield. The highest number of apothecia per square metre was observed in the plots in the continuous soybean rotation. Tillage affected both the number and distribution of sclerotia in the soil profile. Moldboard plowing lowered the number of sclerotia per litre of soil, compared to MT and NT plots, and buried the sclcrotia deeper than 10 cm into the soil. Tillage did not affect the total number of apothecia observed, but moldboard plowing did delay emergence of apothecia compared to no-till. The MP + MT plots had the lowest disease incidence and the highest yield, while the NT plots had the highest disease incidence and (he lowest yields. One year of moldboard plowing will bury sclerotia at least 10 cm in soil and delay the production of apothecia. How this affects SSR development depends on the other factors involved with disease development.