New Races with Wider Virulence Indicate Rapid Evolution of Puccinia striiformis f. sp. tritici in the Southern Cone of America.

Wheat yellow (stripe) rust, caused by Puccinia striiformis f. sp. tritici ( Pst ), is one of the most devastating diseases of wheat worldwide. Pst populations are composed of multiple genetic groups, each carrying one or more races characterized by different avirulence/virulence combinations. Since the severe epidemics in 2017, yellow rust has become the most economically important wheat foliar disease in Uruguay. A set of 124 Pst isolates collected from wheat fields in Uruguay between 2017 and 2021 were characterized phenotypically, and 27 of those isolates were subsequently investigated in-depth by additional molecular genotyping and race phenotyping analyses. Three genetic groups were identified, PstS7 , PstS10 , and PstS13 , with the latter being the most prevalent. Two races previously reported in Europe, Warrior ( PstS7 ) and Benchmark ( Pst S10), were detected in four and two isolates, respectively. A third race, known as Triticale2015 ( PstS13 ), that was first detected in Europe in 2015 and in Argentina in 2017 was detected at several locations. Additional virulence to Yr3 , Yr17 , Yr25 , Yr27 , or Yr32 was detected in three new race variants within PstS13 . The identification of these new races, which have not been reported outside South America, provides strong evidence of the local evolution of virulence in Pst during the recent epidemic years.