Virulence of Oat Crown Rust in Brazil and Uruguay

Oat (Avena sativa) is an important forage and grain crop in countries of the southern cone region of South America. Approximately 600,000 ha of oats are grown for grain in Argentina, Uruguay, and southern Brazil. Also, more than 4,000,000 ha are grown for forage (17,19,22), although in Brazil much of the forage oat is A. strigosa rather than A. sativa. In the subtropical to temperate climate of this region, oat is grown in the winter and is used as a winter cover crop, as food for both livestock and humans, and as raw material for industrial uses. It is the third most important winter crop in southern Brazil. Crown rust, caused by Puccinia coronata, is the most important disease of oat in Brazil, Argentina, and Uruguay. Grain yields are negatively correlated with crown rust severity (2) and may be reduced as much as 50% in susceptible cultivars (16). Although P. coronata produces teliospores, they have no known survival function in South America (15). The primary source of inoculum to infect oat in the fall comes from urediniospores produced on volunteer oat plants that survive the summer at the edges of fields of summer crops such as soybean, in fence rows, and along roadsides. Prevailing wind patterns annually distribute urediniospores of P. coronata in a cyclical pattern throughout the oat growing regions of Brazil, Argentina, and Uruguay as a shared epidemiological system. Epidemics typically start early, sometimes while A. sativa is still at the tillering stage of development. A. strigosa is less susceptible than A. sativa, but rust severities of 5 to 10% are common during the crop season. Typically, no chemical control is used for crown rust. Attempts to breed crown rust resistant oat cultivars in these countries have been frustrated by the buildup of new virulent races of P. coronata, often within a few years of the release of cultivars with new types of race-specific resistance. According to Federizzi and Stuthman (5), the rapid breakdown of resistance is due to the large population of P. coronata maintained in the uredinial stage throughout the year and the large numbers of mutations to virulence that occur annually in the pathogen population. Although a large number of Pc genes for race-specific resistance to crown rust are available to oat breeders, information on occurrence of virulence to these genes in the South American populations of P. coronata was largely lacking. The objectives of this study were (i) to determine the frequencies of virulence to oat lines with known Pc genes in collections of P. coronata from oat in Brazil and Uruguay in relation to potential effectiveness of breeding oat cultivars for racespecific resistance, and (ii) to compare results in Brazil and Uruguay with virulence diversity found previously in North America. In this study, a collection of oat crown rust isolates from western Siberia in Russia was used as an external reference for comparisons with levels of virulence found in the South American isolates of P. coronata.