Crown rust, caused by Puccinia coronata f. sp. avenae is the most important disease of cultivated oat in North America. Numerous race-specific (Pc) genes for crown rust have been found in Avena spp. but this type of resistance has not been durable when used in oat cultivars. Increasing diversity for resistance within a crop by the use of multiline cultivars or varietal mixtures has been proposed as a means of achieving durable resistance to highly variable pathogens such as P. coronata f. sp. avenae. Multiline cv. E77 was evaluated over multiple seasons in the University of Minnesota buckthorn nursery in St. Paul. Buckthorn (Rhamnus cathartica, the alternate host of P. coronata) supports a sexually recombining, highly diverse crown rust population in the St. Paul nursery. Crown rust severity on flag leaves was measured multiple times on E77 and its 10 component lines during grain filling. Single-urediniospore isolates taken from crown rust samples during early stages of the epidemic and at the end of the epidemic were tested for virulence on the 10 component lines of E77 in greenhouse seedling tests. Crown rust development was reduced in E77 compared with the weighted mean of the component lines at all stages of the crown rust epidemics. The mean virulence of single-urediniospore isolates tended to increase late in the epidemic on E77. These data suggest that multilines may select for complex virulence or "super races" and any resistance effect may not be durable.