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Reaction of Iraqi barley cultivars and differential varieties to four isolates of Ustilago hordei was thoroughly investigated. Results confirmed pathogen variation in Iraq, which was reflected in clear variation in the pathogenicity of isolates used on some differential varieties. Resistant gene carried on barley cultivar Excelsior (Ruh2) was still effective regardless of the isolates used. Infection types observed on some differential varieties indicated that the isolates used carried at least the following genes: Avr4: Avr5, Avr2, avr1, in isolates 1 and 4; avr4: avr5, Avr2, avr1 in isolate 2; and Avr4: Avr5, Avr2, Avr1 in isolate 3. Although isolates 1 and 4 were identical concerning those genes, they were different when inoculated to other differential varieties such as Lion, Rihan 03 and Tichdert.