Cucurbit downy mildew, a disease caused by the oomycete pathogen Pseudoperonospora cubensis (Berk. & Curt.) Rostov., is a serious threat to cucumber (Cucumis sativus L.) production worldwide and can result in 100% yield losses in affected environments. In the last decade, strains of the pathogen have overcome the resistance of commercial cultivars in the United States, and currently no cultivar has robust resistance to the disease. This lack of resistance has been especially problematic for cucumber growers seeking to capture the late-season market, when downy mildew is ubiquitous throughout Eastern and Great Lakes production environments. Our objectives were to identify sources of resistance genes and to introgress these genes into high-quality, high-yielding breeding material. Using the moderately resistant cucumber cultivars Marketmore 97 and Ivory Queen as well as the Cornell-developed cultivars Platinum and Salt & Pepper, we have developed lines with excellent disease resistance. In a trial of 27 lines that included Cornell breeding material and the most resistant cultivars and U.S. Department of Agriculture (USDA) accessions identified in previous studies, the Cornell breeding line DMR-NY264 had the highest level of downy mildew resistance and the highest yields under disease pressure. In New York, plants of DMR-NY 264 produced fruit until frost without fungicide application.