In the United States, the annual value of the soybean crop is over $40 billion. Unfortunately, soybean suffers yield suppression valued at over $0.85 billion annually from the two serious pathogens, Fusarium virguliforme and Phytophthora sojae that causes sudden death syndrome (SDS) and Phytophthora root and stem rot diseases, respectively. Growing SDS and Phytophthora resistant soybean cultivars is the main method of controlling these two diseases. Therefore, identification and incorporation of novel SDS and Phytophthora resistance genes into the soybean cultivars is essential for fighting the two diseases. The long-term goal of this proposed research is to improve resistance of soybean against F. virguliforme and P. sojae. To reach our long-term goal, we proposed to (i) map the novel genes that govern SDS and Phytophthora resistance, (ii) identify molecular markers linked tightly to SDS and Phytophthora resistance genes, and (iii) generate desirable SDS and Phytophthora resistant soybean germplasm for breeding commercial soybean cultivars with SDS and Phytophthora resistance. Resources generated in this project are expected to facilitate breeding soybeans for SDS and Phytophthora resistance leading to reduction of annual soybean yield losses valued at 100s of million dollars. The outcomes of this project will contribute significantly towards long-term benefits of the soybean growers and sustainability of the U.S. soybean industry.
In this project we investigated 282 recombinant inbred lines (RILs) generated from each of the two crosses between Spencer x PI 393535 and Spencer and PI 407785 for segregation of genes conferring resistance against F. virguliforme and P. sojae isolates and identified six quantitative trait loci (QTL) for SDS resistance and three Rps genes conferring Phytophthora resistance. Three of the QTL for SDS resistance provide more than 10% of the variation for SDS resistance among the segregating RILs and are novel. We have identified molecular markers linked to these resistance genes. Additional markers associated very closely to the three novel QTL for SDS resistance and two novel Rps genes are being discovered and will be applied in introgressing these genes into a disease resistant soybean cultivar. In this project, we have identified four Phytophthora resistant RILs that are also highly resistant to F. virguliforme in two consecutive years under field conditions. The yield potential of these four lines were determined by harvesting and threshing of RILs on November 1, 2023. Two of the four most SDS and Phytophthora resistant RILs showed high yield/plant and we plan to release these two lines as germplasm lines to assist SDS and Phytophthora resistance breeding programs in other laboratories. Overall, we have met all four milestones proposed in our proposal including identification of map locations of SDS resistance QTL and Rps genes, identification of disease resistance gene-associated molecular markers, and SDS and Phytophthora resistant RILs to be released as germplasm lines.