The use of race-specific seedling genes for resistance is the primary means of controlling crown rust of oat ( Puccinia coronata f. sp. avenae) in the United States. To better utilize those resistance genes, knowledge of the occurrence and frequency of corresponding virulence in the population of P. coronata f. sp. avenae in the United States is essential. In total, 571 single-pustule isolates of oat crown rust were collected from cultivated and wild oat ( Avena sativa and A. fatua, respectively) in the major oat production areas of the United States from 2006 through 2009. They were tested for virulence on seedlings of 31 differential oat lines in the greenhouse. In all, 201 races were found among the 357 isolates from the spring oat region of the north-central United States, and 140 races were found among 214 isolates from the southern winter oat region. The crown rust populations from the winter and spring oat regions were clearly differentiated from one another, differing in the frequency of virulence for 24 of the 31 differentials. Some virulence associations previously reported in the U.S. oat crown rust population were also found in both regions in this survey, even when the dataset was clone corrected. Associations between virulence to the Pc genes were predominately positive in both regions but both positive and negative associations occurred more frequently in the winter oat region, where sexual reproduction does not occur. Some of the virulence diversity in the oat crown rust population in the United States can be related to the deployment of resistance genes in commercial oat cultivars and virulence associations existing in the oat crown rust population. When data from a previous report covering 2001 through 2005 is combined with data reported in this article, the mean virulence of the U.S. populations of crown rust continued to increase from 2001 to 2009. Virulence to Pc38, Pc39, Pc45, Pc48, Pc52, Pc55, Pc56, Pc57, Pc59, Pc62, Pc63, Pc64, Pc68, and Pc96 significantly increased in one or both regions during this time period. No significant declines in virulence frequency were found in either region. Genes for crown rust resistance derived from A. sterilis appear to be as rapidly defeated as has happened to Pc genes from A. sativa. There is an urgent need to find additional sources of effective resistance to P. coronata f. sp. avenae and introgress it into adapted oat cultivars.
