ABSTRACT
More than 2,000 isolates of Puccinia coronata f. sp. avenae collected in national virulence surveys were analyzed to determine regional patterns of virulence on 28 single-gene oat lines. From 1990 to 2000, frequencies of virulence to eight differentials increased significantly in Texas, and virulence frequencies of four of the eight also increased in the northern Great Plains. No significant changes occurred for the other 20 differentials. Isolates from northern states from the Dakotas to New York had similar patterns of virulence frequencies on these 20 differentials. The pattern of virulence frequencies in the north differed from that of southern states from Texas to the Carolinas. There was a gradient of similarity values in state by state comparisons from Texas to the Atlantic Coast. Isolates from California were distinctly different from those elsewhere in the United States. Isolates from the South and from California had greater mean virulence complexity than isolates from northern states. Collections from the central Great Plains were intermediate between northern and southern isolates. Uredinial isolates from oat and aecial isolates from the alternate host Rhamnus cathartica in Minnesota had similar frequencies of virulence, as did isolates from cultivated oat and wild Avena fatua in North Dakota. Maintenance of large numbers of stable virulence polymorphisms with different regional allele frequencies is consistent with the operation of some form of stabilizing selection.
