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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Leaf rust is the most important wheat disease in Argentina; 4.2 M ha of wheat were grown in 2014. The objective of the study was to identify avirulence/virulence phenotypes of the Pt population in leaf rusted samples collected in wheat-growing areas during 2013. Single uredinial isolates were taken from samples and tested on Thatcher near-isogenic lines and some local varieties. Resistance genes in sets of four included: Lr1, Lr2a, Lr2c, Lr3; Lr9, Lr16, Lr24, Lr3ka; Lr11, Lr17, Lr30, Lr10; Lr14a, Lr19, Lr20, Lr21; Lr23, Lr25, Lr26, Lr27+31; Lr29, Lr36, Lr39/41, Lr42, Lr43; Lr44 and Lr47. Race designations were based on the first three sets proposed by Long and Kolmer (1989, Phytopathology, 79: 525-529) and the gene designations Lr10 and/or Lr20 were appended to indicate virulence on lines with those genes. Among 141 single uredinial isolates, 18 races were identified. Race MFP, was the most frequent, accounting for 27% of isolates; race MDP was second at 22.7%. Both races were isolated for the first time in 2005. The first three races were present in similar frequencies to 2012. Two new races were found, MKJ 10 and MGJ 10. Virulence was not found for genes Lr19, Lr21, Lr25, Lr29, Lr36, “Lr43”, Lr44 and Lr47. Race DBB 10,20 was the most frequent race on durum wheat. Virulence to Lr16 appears to be increasing.