The goal of this project is to provide new genetic materials and mechanistic insights into how soybeans build resistance to iron deficiency chlorosis (IDC). IDC is a major challenge to U.S. farmers across much of the soybean growing region. IDC is a notoriously complex symptom that can be caused by several factors and interactions, including high soil pH, high levels of soil calcium carbonate, high soil moisture, excessive residual soil nitrates, and microbial interactions. These are factors that are difficult for farmers to control. However, there are genes that can be bred into soybean varieties that confer enhanced resistance to IDC. We seek to better understand these genes and their stability for providing resistance across different environments. In this project, we are taking a multi-pronged approach towards this aim. We are: (1) Performing multi-environment IDC evaluations to test the stability of specific quantitative trait loci (QTL) for IDC resistance, (2) Developing near-isogenic lines (NILs) that will allow for the precise testing of previously uncharacterized QTL across the different environments, (3) Testing a candidate gene on chromosome 20 for its effect on IDC resistance. In this reporting period, we have made progress in all three objectives: (1) We have observed differential responses to IDC QTL across different environments; (2) We have initiated the development of QTL introgressions into lines adapted to NE, MN, and ND, respectively; (3) We have identified potential mutants for in-depth exploration of the Chr20 IDC QTL. We hope to leverage these resources and discoveries to enable efficient marker-assisted breeding strategies that introgress the best IDC resistance genes into elite soybean varieties for U.S. farmers.