Stem rust disease has the potential to devastate wheat production worldwide. In the 1950s, large epidemics spread across North America and through other parts of the world.

Developing a stem rust resistant gene stopped the spread of the disease. In 1999, a new race of stem rust was discovered in Uganda and identified as Ug99. Previously developed stem rust resistant genes are no longer effective against Ug99.

Fortunately, researchers at the University of California-Davis, Kansas State University, and the USDA Cereal Disease Laboratory in Minnesota have mapped and characterized a gene resistant to Ug99 and its derivatives, known as Sr35.

Scientists identified molecular markers closely flanking the gene on the long arm of one of the gene’s chromosomes, and then used comparative genomics to identify a small set of candidate genes among the collinear genes in rice and the model grass species. These candidate genes and molecular markers can be used to accelerate the deployment of Sr35 in wheat breeding programs.

Having a precise genetic map of Sr35 is the first step towards potentially cloning the gene. Cloning Sr35 will provide a perfect tool for the understanding of the resistance mechanisms against Ug99. However, certain genetic imperfections would need to be reduced before it is introduced to commercial wheat varieties.

While Sr35 is effective against Ug99, its derivatives, and another broadly virulent strain of stem rust originally found in Yemen; it cannot counter all the known forms of the disease. Since stem rust is airborne, outbreaks can spread in very little time. Therefore, Sr35 should be deployed with other stem rust resistant genes to successfully defend wheat production.

According to Jorge Dubcovsky, the author of the study, “The presence of multiple resistance genes is expected to extend the durability of resistance, since the probability of simultaneous mutations in the pathogen to overcome multiple resistance mechanisms is much lower than the probability to overcome individual mutations.”

The study was funded by the USDA Agriculture and Food Research Initiative and by the Bill and Melinda Gates Foundation. Results of the study can be reviewed in full detail in the November-December 2010 issue of Crop Science.

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