Re-emergence of stem rust at epidemic levels in Argentina and Uruguay in 2014

Argentina and Uruguay are neighboring countries located in the same rust epidemiological area.  The last significant stem rust epidemic occurred in 1950. Since then, stem rust was frequently observed in experimental fields and off-season nurseries, but was mostly absent in commercial fields. During 2014, 4.6 million ha of wheat were grown, and there was a widespread incidence of stem rust, reaching levels of 80S on susceptible cultivars in both countries. Yield losses of 13 to 21% were estimated in experimental trials in Argentina. The epidemic was probably caused by the increasingly widespread cultivation of highly susceptible, but high yielding French cultivars during the last decade. In Argentina 42.3% of the commercial cultivars were susceptible to stem rust, and in Uruguay 23.0% were susceptible, 6.8% moderately susceptible and 20.3% were intermediate in reaction. However, the actual area sown to susceptible cultivars in Uruguay has continued to increase, from 22% of the wheat area in 2009 to 53.3% in 2014. Conductive weather conditions of high rainfall and warmer than average temperatures during the winter and spring, favored early infection. Cultivars with resistance genes Sr31 and Sr24 continue to be resistant in the region and are believed to be the most important genes currently providing resistance. Some Argentinean and Uruguayan cultivars that do not carry Sr31 and/or Sr24 were susceptible in 2011, but resistant in 2014, indicating a narrower range of virulence in 2014 compared to 2011. Both countries are working to improve resistance to local races and to the Ug99 race group. Disease modeling would be useful for understanding and predicting the occurrence and severity of this disease.

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National Institute of Agricultural Technology (INTA), Argentina
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