Enhancing soybean for resistance to Sclerotinia stem rot Ramkrishna Kandel and Dechun Wang Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48823 Introduction: Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is an economically important disease of soybean (Glycine max) in the north-central United States. Cultural practices like use of narrow row spacing, higher plant density, and Optimal fertilizer application create a dense plant temperature leading to fungal infection and disease outbreak (Mueller et al. 2002). Use of partial resistance varieties is promising factor for enhancing the yield of soybean (Kim, H.S. and B.W. Diers, 2000). The mode of inheritance of partial resistance to sclerotinia stem rot in soybean is found to be quantitative (Vuong et al. 2008). Our project aims at evaluating the three-hundred and ninety-two F4:6 lines for resistance to Sclerotinia stem rot and to identify novel QTLs or validate the QTLs already reported. Materials and Methods: These lines were grown in greenhouse at Michigan State University, East Lansing. The plants were arranged in randomized complete block design with three replicates in Dec, 2008, and January and October, 2009. Six plants per line in each replication were grown and inoculated by drop-mycelium at V3 growth stage. Immediately after inoculation, chamber was closed and humidifier was operated to maintain humidity to enhance the disease development. Ten days after inoculation, total number of dead plants were counted for each line and the percentage of plant mortality was calculated. The seven parental combinations were tested for polymorphisms with simple sequence repeats (SSRs) markers flanking the 33 reported QTLs according to Song et al.( 2004). On an average, 15 SSRs were found to be polymorphic for each parental combination. Population one (E00290 x PI089001) has been genotyped and correlation with phenotypic data is yet to come. Results and Discussions: Significant (p < 0.05) differences were found among lines within populations and between populations. Figure 1 explains that population (Skylla x PI89001) showed maximum resistance to sclerotinia stem rot among seven populations followed by the population (Skylla x PI437764). These lines can be further used for deriving the cultivars with partial resistance to sclerotinia stem rot. Conclusions: The differential distribution of resistance genes from two parents allowed us to select the resistance lines both from within and between the populations. The results from this phenotypic data will be correlated with marker data to validate the resistance lines and subsequently the QTLs associated with the partial resistance to sclerotinia stem rot. The QTLs that would be identified in these populations will allow us to use out PIs as a new source of resistance lines. Literature Cited: Kim, H. S. and B. W. Diers. 2000. Inheritance of partial resistance to sclerotinia stem rot in soybean. Crop Sci 40:55-61 Mueller, T. D., A. E. Dorrance, R. C. Derksen, E. Ozkan, J. E. Kurle, C. R. Grau, J. M. Gaska, G. L. Hartman, C. A. Bradley and W. L. Pedersen. 2002. Efficacy of fungicides on sclerotinia sclerotiorum and their potential for control of sclerotinia stem rot on soybean. Phytopathology 86: 26-31 Vuong, T. D., B. W. Diers and G. L. Hartman. 2008. Identification of QTL for resistance to sclerotinia stem rot in soybean plant introduction 194639. Crop Sci 48: 2209-2214 Song, Q. J., L. F. Marek, R. C. Shoemaker, K. G. Lark, V. C. Concibido,X. Delannay, J. E. Specht, P. B. Cregan. 2004. A new integrated genetic Linkage map of the soybean. Theor Appl Genet 109:122-128 Fig 1. Population-wise reaction of lines to sclerotinia stem rot Fig 2. Soybean plants heavily infected with Sclerotinia stem rot Fig 3. population showing maximum resistance to sclerotinia stem rot