LAUSR

A total of 353 urediniospore isolates of Puccinia striiformis f. sp. tritici (Pst) collected in Israel during 2001-2019 were analyzed. Pst pathogenicity was studied with a set of 20 differentials (17 Avocet and 3 other lines). Three periods were compared: 2001-2007, 2009-2016, and 2017-2019. No virulence to Yr5 and Yr15 was detected. Virulence frequencies on Yr4, Yr10, Yr24, and YrSp genes rose to the moderate level (0.28-0.44) in 2017-2019. Virulences to Yr2 and Yr9 decreased. One Pst phenotype was identified in all three periods, but its frequency drastically decreased from 0.74 in 2001-2016 to 0.21 in 2017-2019. The most probable scenario of emergence of wheat yellow rust in Israel is wind dissemination of Pst urediniospores from the Horn of Africa. Variability of the Pst population increased amid considerable evolution with two major transformations in 2009 and 2017. The first modification can be attributed to changes in wheat genetic background in Israel due to deployment of new cultivars resistant to yellow rust since 2004. The second shift in 2017 can be primarily explained by intensive deployment of wheat cultivars resistant to the stem rust race Ug99 in the 2010s in the Horn of Africa. This led to changing genetic background of the cultivated wheats in the donor region, development and long-distance spread of new Pst phenotypes to Israel. Two singular multi-virulent Pst phenotypes were identified in 2019, one of them being closely related to the aggressive Warrior race. Such phenotypes may potentially defeat existing resistances.