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Virulence characterization of Puccinia striiformis f. sp. tritici collections from China, Italy, Mexico, and Ecuador

Xianming Chen: USDA ARS

<div><em>Puccinia striiformis</em> f. sp. <em>tritici</em> (<em>Pst</em>) causing stripe rust is one of the most important pathogens on wheat worldwide. The fungus evolves into different virulent races to overcome resistance. In 2014-2017, 443 isolates were recovered from collections of China (141), Italy (133), Mexico (123), and Ecuador (46). These isolates were tested with 18 wheat <em>Yr</em> single-gene lines that are used to differentiate <em>Pst</em> races in the U.S. A total of 132 <em>Pst</em> races were detected, including 105 new races. The number of unique races in China, Mexico, Italy, and Ecuador were 54, 25, 24, and 18, respectively. None of the races were shared by more than two countries, but 4 races were shared by China and Italy, 3 by China and Mexico, and 4 by Ecuador and Mexico. The top three Mexican races, PSTv-37 (40.7%), PSTv-52 (17.1%), and PSTv-198 (9.8), were similar to those identified in the US in 2016. Overall, virulences to <em>Yr9</em>, <em>Yr43</em>, <em>YrExp2</em>, <em>Yr6</em>, <em>Yr7</em>, and <em>Yr44</em> were most frequent (81.0-90.3%); those to <em>YrTr1</em>, <em>Yr8</em>, <em>Yr76</em>, <em>Yr27</em>, <em>Yr17</em>, <em>YrSP</em>, and <em>Yr1</em> were moderate (17.8-59.6%); and those to <em>Yr10</em>, <em>Yr32</em>, and <em>Yr24</em> were low (0.5-1.1%). None of the races were virulent on <em>Yr5</em> and <em>Yr15</em>. Significant variations in frequency were detected for virulences to <em>Yr1</em> (10.6% in Mexico to 85.8% in China); <em>Yr17</em> (11.3% in China to 71.4% in Italy); <em>Yr27</em> (3.0% in Italy to 84.6% in Mexico); and <em>Yr76</em> (6.5% in Ecuador to 83.0% in China). The information is useful for understanding epidemiology and controlling stripe rust.</div>