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Pyrenophora tritici-repentis induces tan spot, one of the most important fungal diseases of wheat. At least eight races of the pathogen are known to occur based on their virulence on a wheat differential set. In 2009 and 2010, 87 commercial wheat fields were sampled for tan spot in the Southern Cone region of South America, namely Argentina, Brazil and Uruguay. An average of three isolates from each field were tested for their virulence on the differential wheat hosts ‘Glenlea’, 6B365, 6B662, ‘Salamouni’, ‘Coulter’ and 4B1149. Of 273 isolates tested, 52.4% were classified as race 1 and 47.6% were classified as race 2, with no other races identified. This low level of pathogenic diversity was somewhat expected, since the bread wheat grown in this region has a narrow genetic diversity with respect to tan spot resistance. To fully assess virulence patterns in the Southern Cone, use of this differential set is recommended. To our knowledge, this is the first report of the race structure of P. tritici-repentis in South America. [ABSTRACT FROM PUBLISHER]