Diseases that infect sunflower frequently occur in North Dakota, but the impact they have on yield is unclear. The objectives of this research are to 1) evaluate fungicide efficacy, application timing and yield impact of Phoma black stem of sunflower and 2) determine the impact of diseases on sunflower yield in North Dakota and Minnesota. Results of 14 fungicide trials conducted between 2017 and 2019 showed that yield losses to Phoma black stem were infrequent, but the disease could be managed by application of several available and efficacious fungicides applied at growth stage R1. Analysis of survey data collected over 11 years from 1,003 sunflower fields revealed that when diseases were determined to be a production-limiting factor, mean yield was 427 kg/ha less than in fields where no was production-limiting factor was reported. Results of these studies may help sunflower growers make decisions that optimize yield on their farms.