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Zymoseptoria tritici (P. Crous; syn. Mycosphaerella graminicola, Septoria tritici), causal agent of Septoria tritici blotch (STB) disease, is one of the most important foliar diseases of wheat in Iran and across the world. To identify resistance sources than can be relevant to breeding programs, it is necessary to determine the virulence factors of the pathogen. In this study, 26 differential wheat cultivars (carrying Stb1-18 genes) were inoculated as seedlings in a greenhouse with 10 individual isolates of Z. tritici collected from different regions of Iran under different environmental conditions. Iranian Z. tritici isolates showed new virulence patterns. Among wheat differentials, none of the cultivars were resistant to all the Iranian isolates used in this study. Oasis (carrying Stb1), Sulivan (carrying Stb2), and Bulgaria 88 (carrying Stb1 and Stb6) showed susceptibility to all the isolates. Stb5, Stb7, Stb13, and Stb14 possibly do not have a good resistance as most of the isolates were virulent to these genes. Therefore, these genes cannot be considered as effective sources of resistance to Z. tritici. Shafir (carrying Stb6) revealed a resistant reaction to most of the isolates. M3 (carrying Stb16 and Stb17) was susceptible to four Z. tritici isolates, and this is the first report of susceptibility reaction for this cultivar.