Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2007
Publication Date: 2/25/2008
Citation: Carson, M.L. 2008. Virulence Frequencies in Oat Crown Rust in the United States from 2001 through 2005. Plant Disease. 97:379-384. Interpretive Summary: Crown rust is considered the most important disease of oat in the United States. Isolates of the crown rust fungus were collected from oat fields in the U.S. from 2001 through 2005. The effectiveness of different genes for resistance in oat to each of the crown rust isolates was tested in the greenhouse and races of the fungus identified. Many races were found in both the winter and spring oat production areas of the U.S. No resistance gene in oat was found to be effective against all the isolates collected in the survey. Crown rust isolates from the winter oat region differed in their virulence from those from the spring oat region. The oat crown rust population in the U.S. is becoming more virulent and resistance genes used by oat breeders rapidly become ineffective when varieties containing them are widely planted. This research emphasizes the need for oat breeders and pathologists to find alternative types of resistance to oat crown rust that the pathogen cannot easily overcome.
Technical Abstract: A total of 680 single pustule isolates of oat crown rust, Puccinia coronata f. sp. avenae, were collected from cultivated and wild oat (Avena sativa and A. fatua, respectively) in the major oat production areas of the United States from 2001 through 2005. They were tested for virulence on seedlings of differential oat lines in the greenhouse. A total of 171 races were found among the 357 isolates from the winter oat region of the US, whereas 212 distinct races were found among 323 isolates from the spring oat region. The crown rust population derived from winter oat in the southern US was distinct from the spring oat population in the upper Midwest, although there was no virulence unique to either population. Virulence to Pc48 and Pc52 increased significantly in both regions during the 2001-2005 time period. Virulence to Pc59 increased and virulence to Pc53 decreased in the winter oat region during the same period. Many of the virulence associations reported by Leonard et al. (2005) in the US oat crown rust population in the early 1990’s were also found in both regions in this survey. 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. Much 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. The mean virulence of the U.S. population of crown rust continued to increase from 2001 to 2005. Genes for crown rust resistance derived from A. sterilis appear to be rapidly defeated as has happened to Pc genes from A. sativa.