2008 Annual Report 1a.Objectives (from AD-416) To evaluate soybean breeding lines for levels of resistance to Sclerotinia stem rot and agronomic performance, and to characterize the genes underlying partial resistance in two newly identified sources of resistance. 1b.Approach (from AD-416) Fourteen high yielding soybean lines derived from crosses with resistant parents will be evaluated for Sclerotinia sclerotiorum resistance in the field using either advanced lines in an infected disease nursery or using a field cut-petiole inoculation method. Over 2000 breeding lines that are a subset of high yielding lines will be evaluated for resistance to Sclerotinia stem rot. A linkage map approach will be used to determine the locations of resistance genes in the two new sources of resistance. 3.Progress Report This project was initiated on June 1, 2004, research is ongoing, and the overall goal of this project is to develop soybean varieties or germplasm that have a high level of resistance to Sclerotinia stem rot. ADODR monitoring activities to evaluate research progress included phone calls, meetings with the cooperator, and an annual meeting held each year in January. Sclerotinia stem rot is an important soybean disease in the northern soybean production states of the U.S. The disease was estimated to cause a total yield loss of over 175 million bushels from 1996 to 2006. The best way to control this disease is to use soybean varieties with partial resistance to the disease. The long-term goal of this research is to develop soybean varieties or germplasm with high level of resistance to Sclerotinia stem rot. Eighteen advanced lines with potential resistance to Sclerotinia were selected for further evaluation. These 18 lines were tested at four locations in 6-row plots with two replications at each location for yield and other agronomic traits. Five of the 18 lines were among the top 10 highest yielding lines and were higher yielding than the check IA2021 and IA2052. The 18 lines were also evaluated for Sclerotinia stem rot resistance in our disease nursery where we test commercial soybean varieties annually for Sclerotinia stem rot resistance. Of the top 5 highest yielding lines, two had a DSI (disease severity index, ranging from 0% to 100% with 0% as the most resistant) less than 1.7% in the test where the average DSI of all 61 lines were 9.9% and the maximum DSI was 45%. Three hundred ninety two lines from seven populations segregating for resistance from five new PIs were evaluated in single 3-foot row plots with two replications for agronomic traits such as yield and lodging. The 392 lines were also evaluated for Sclerotinia stem rot resistance in the greenhouse using the spray-mycelium method we developed previously. These 392 lines are currently being evaluated with DNA markers that showed association with resistance to Sclerotinia stem rot in other studies. The National Sclerotinia Initiative contributes to the goals of ARS National Program 303–Plant Diseases. Publications: Chen, Y., and D. Wang. 2005. Two convenient methods to evaluate soybean for resistance to Sclerotinia sclerotiorum. Plant Dis. 89:1268-1272. Diers, B.W., F.J. Kopisch-Obuch, D. Hoffman, C.R. Grau, and D. Wang. 2006. Registration of ‘A X N-1-55’ soybean germplasm with partial resistance to Sclerotinia stem rot. Crop Sci. 46:1403-1404. Guo, X., D. Wang, S.G. Gordon, E. Helliwell, T. Smith, S.A. Berry, S.K. St. Martin, and A.E. Dorrance. 2008. Genetic mapping of QTLs underlying partial resistance to Sclerotinia sclerotiorum in soybean PI 391589A and PI 391589B. Crop Sci. 48:1129-1139. Wang, D., J. Boyse, and B.W. Diers. 2006. Registration of 'Skylla' Soybean. Crop Sci. 46:974-975.