Virulence characterization of Puccinia striiformis f. sp. tritici collections from six countries in 2013 to 2020

Puccinia striiformis f. sp. tritici (Pst) causes wheat stripe rust (also called yellow rust, Yr), one of the most important diseases worldwide. Characterization of virulence in Pst populations is essential for developing wheat cultivars with effective and durable resistance to control the disease. A total of 138 Pst races, including 120 races that were not previously reported, were identified from stripe rust collections made from Canada, China, Ecuador, Egypt, Italy and Mexico in 2013-2020 using a set of 18 Yr single-gene differentials. Virulence of the resistance gene Yr5 or Yr15 was not found in isolates from any of the countries, indicating their effectiveness against the Pst populations. Virulence to 16 Yr genes was detected, but the frequencies varied greatly among countries. On average, the frequencies of virulence to Yr6, Yr7, Yr9, Yr43, Yr44 and YrExp2 were high (81.7-90.6%), those to Yr1, Yr8, Yr17, Yr27, YrSP and Yr76 were moderate (34.0-56.8%), and those to Yr10, Yr24, Yr32 and YrTr1 were low or very low (0.4-18.5%). The same races detected in different countries and different races from different countries clustered into the same virulence groups, indicating Pst migration among different countries, especially between eastern Asia and the Mediterranean region. These results should be useful for breeding wheat cultivars with effective resistance to stripe rust in these countries as well as globally.