Crown Rust Development and Selection for Virulence in Puccinia coronata f. sp. avenae in an Oat Multiline Cultivar

Oat crown rust, caused by Puccinia coronata f. sp. avenae, is considered to be the most serious disease of oat (Avena sativa L.) in North America (27). It is distributed worldwide but is most damaging where heavy nightly dews occur in conjunction with moderate to high temperatures during the growing season. Under ideal conditions, crown rust can result in complete crop failure. In the United States, the mean estimated loss to crown rust over the 10-year period of 1996 to 2005 was 2.2 and 3.2% for the upper Midwest (South Dakota, North Dakota, and Minnesota) and the southern plains (Texas and Louisiana), respectively (D. L. Long, personal communication). Losses in individual years and states were as great as 6% in the upper Midwest (Minnesota in 1996) and 20% in the southern plains (Louisiana in 1997). The use of host genetic resistance has been the primary means of controlling crown rust of oat. Breeders have primarily relied on race-specific seedling genes that are expressed as some form of hypersensitive reaction. Initially, resistance genes used were found in cultivated hexaploid oat, A. sativa. As these genes were deployed in oat cultivars, corresponding virulence in the crown rust population increased rapidly, to the point that virulence to most of these genes is nearly fixed in the North American population. Later efforts at finding effective resistance turned to exploiting genes found in the wild hexaploid animated oat, A. sterilis, which is the basis of most resistance in current oat cultivars. As these genes have been deployed, corresponding virulence has increased in the crown rust population, such that the effective life span of a resistant oat cultivar is often 5 years or less. Breeders and geneticists are now turning to diploid species, particularly black oat, A. strigosa, as sources of new resistance genes, but introgression of resistance into hexaploid oat is difficult due to differences in ploidy levels and the lack of homology of chromosomes between the two species (1). The University of Wisconsin–Madison and the Agriculture Agri-Foods Canada–Winnipeg oatbreeding programs have released cultivars with crown rust resistance genes from A. strigosa. Increasing intracrop diversity for disease resistance by the use of multiline cultivars or varietal mixtures has been proposed as a means of achieving more durable resistance to highly variable pathogens such as P. coronata f. sp. avenae (4,5,22,29). Multilines reduce the amount of initial inoculum and the rate of epidemic development because only a fraction of the pathogen population is virulent upon any component of the multiline (4,5,11,23). Disease spread is reduced because resistant plants act as a barrier to spore dispersal. Induced resistance caused by infection with a mixture of compatible and incompatible races may also serve to reduce disease in multilines (11,23). In addition to reducing disease development, multilines or cultivar mixtures may prevent or greatly delay the occurrence of “super races” with virulence ABSTRACT Carson, M. L. 2009. Crown rust development and selection for virulence in Puccinia coronata f. sp. avenae in an oat multiline cultivar. Plant Dis. 93:347-353.