Puccinia coronata is a plant pathogen and causal agent of oat and barley crown rust. The pathogen occurs worldwide, infecting both wild and cultivated oats. It is a relatively new disease of barley in North America. It was first found in 1992 in a barley breeding nursery near Clay Center, Nebraska. Since then, crown rust has been found throughout the upper Midwest, with greatest incidence in the central Red River Valley of North Dakota and Minnesota. In that region the alternate host, common buckthorn (Rhamnus cathartica), grows abundantly in shelter belts and riparian areas. Since 1993, outbreaks of crown rust have occurred on barley and forage grasses at several localities in this region. The extent of yield losses in barley caused by this disease have not been determined. Crown rust pos