The majority of United States sunflower production is in seven Midwestern states, but hybrid planting seed is almost exclusively produced in California. Due to the lack of summer rains and furrow irrigation, California-produced seed is relatively disease free and thus it regularly meets phytosanitary restrictions imposed by many countries. For the 15-year period from 1997 to 2011, 7231 seed fields in northern California were inspected and samples processed at the state diagnostic laboratory (California Department of Food and Agriculture). Rust (Puccinia helianthi) was the most prevalent quarantine disease, found in 4.3% of fields. Stalk rot (Sclerotinia sclerotiorum) and downy mildew (Plasmopara halstedii) were the only other quarantine pathogens observed, found in 2.6% and 0.5% of the 7231 fields, respectively. Many sunflower pathogens have never been recorded in California, including Phoma macdonaldii, Phomopsis helianthi, or any virus. North Dakota, the state with the highest US sunflower production, had quarantine pathogens in 88% of 1263 fields surveyed from 1995 to 2011. Phoma macdonaldii, Sclerotinia sclerotiorum, Puccinia helianthi, Phomopsis helianthi, Plasmopara halstedii, and Verticillium dahliae were recorded in 62, 54, 37, 33, 14, and 12%, respectively, of North Dakota fields. Accepted for publication 5 November 2012. Published 14 December 2012.
