The pathogenicity of P. coronata isolates from Texas [USA] was compared with the pathogenicity of isolates from oats and from buckthorn (Rhamnus catharticus) in a nursery in Minnesota in which the fungus had cycled for many years between the buckthorn and the oats planted adjacent to it. Pathogenicity was tested on 24 differential cultivars and lines of oats (Avena sativa) known or assumed to have single genes for reaction to this pathogen. In 1975, a collection of 107 uredial isolates from Texas had 14 virulence patterns, and included virulence on 17 of the 24 differentials. Collections of 17 aecial and 51 uredial isolates from the Minnesota nursery had 15 and 39 virulence patterns, respectively, and both sets of material included virulence on 19 differentials. In 1976, a collection of 73 uredial isolates from Texas had 16 virulence patterns and virulence on 16 differentials. Collections of 49 aecial and 138 uredial isolates from the Minnesota nursery had 22 and 73 virulence patterns, respectively, and virulence on 18 and 21 differentials, respectively. In 1975 and 1976, the Texas material averaged 8.9 and 9.0 virulence genes per isolate, respectively. Corresponding values for the Minnesota uredial isolates were 9.2 and 9.4.