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ARS Home » Midwest Area » St. Paul, Minnesota » Cereal Disease Lab » Research » Publications at this Location » Publication #183876


item Leonard, Kurt

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2005
Publication Date: 8/1/2005
Citation: Leonard, K.J., Martinelli, J.A. 2005. Virulence of oat crown rust in Brazil and Uruguay. Plant Disease. 89:802-808.

Interpretive Summary: Oat crown rust is the most important disease of oat in Brazil and Uruguay, where it can cause yield losses of up to 50% in susceptible cultivars. Attempts to breed crown rust resistant oat varieties in these countries have been frustrated by the rapid buildup of new virulent races of the crown rust fungus. Often the resistance of new varieties is overcome by the appearance of new virulent races within a few years after the new variety is released. Although there are many known genes for race-specific resistance to crown rust, little was known about the virulence of crown rust races in South America. We analyzed the virulence of 144 isolates of the crown rust fungus from Brazil and 36 isolates from Uruguay against a set of 27 oat lines each with a different gene for race-specific resistance. Most isolates of the crown rust fungus had virulence to overcome between 30 and 70% of these resistance genes. The average level of virulence of the South American isolates of the oat crown rust fungus was greater than that of crown rust isolates from the U.S. The oat crown rust population in Brazil and Uruguay was highly diverse even though the sexual stage of the fungus does not occur there. The high virulence and great diversity of the crown rust fungus in Brazil and Uruguay make it unlikely that race-specific resistance can be effective there. Therefore, breeding efforts should focus on increasing the levels of non-spcific partial resistance that will be effective against all races of the fungus.

Technical Abstract: Race-specific resistance to crown rust, the most important disease of oat in Brazil, often fails within a few years of use in Brazilian cultivars. Virulence of 144 isolates of Puccinia coronata from cultivated oat in Brazil in 1997-1999 and 36 isolates from Uruguay in 1994-95 and 1998 was tested on a set of 27 oat crown rust differentials lines, each with a different Pc gene for race-specific resistance. Frequencies of virulence and mean virulence complexity were compared among these 5 collections from Brazil and Uruguay as well as with mean virulence complexity for a collection of 17 isolates from cultivated oat in western Siberia in Russia. Virulence/avirulence for each of the 27 Pc genes was polymorphic in both Brazil and Uruguay. Virulence frequencies were similar for collections from Brazil in 1998 and 1999 and for the collection from Uruguay from 1998, but there were large differences between the 1997 collection and the 1998 and 1999 collections from Brazil. Mean virulence complexity in both Brazil and Uruguay was greater than reported in the United States and much greater than in the Russian collection of P. coronata. A large number of races of P. coronata were found with no more than five isolates of any race found in a single year in Brazil or Uruguay. The high virulence complexity and great diversity of virulence polymorphisms in Brazil and Uruguay make it unlikely that race-specific resistance can be effective there even though the South American populations of P. coronata are apparently entirely asexual.