Stem rust, (Puccinia graminis f.sp. tritici) is a catastrophic disease of barley. Previously stem-rust-tolerant genes were incorporated in genotypes of wheat grown in Kenya, but in 1999, a new race Ug99 that is virulent to the genes was detected in Uganda. The new race spread rapidly and, in 2001, stem rust Ug99 isolates were detected in Kenya. Although barley is an important crop affected by stem rust, limited work has been done to it. In this study 20 barley cultivars from Kenya and from International Centre for Agriculture Research in Dry Area (ICARDA) were screened in greenhouse environment and in the field with isolate of Puccinia graminis f.sp. tritici. At seedling stage, the infection levels ranged from 0 to 2, except in ICARDA-09 and ICARDA-11 that showed infection types 3 and 3, 4, respectively. At adult plant stage, genotypes ICARDA-01, Nguzo, and Karne were moderately resistant while the rest were susceptible or moderately susceptible. In the field, the new line 1512-5 showed the highest severity of 93% in season 1, with Sabini having the highest severity of 30% in the second season. The highest reduction in percent germination (54.1% and 38.3%) was recorded in 1385-13 and ICARDA-10 in season 1 and 2, respectively. The highest loss (9.00%) in protein content was observed in Sabini in season 1. It is evident that stem rust Ug99 is a threat to beer industry and there is need for regular monitoring of the stem rust pathogen, in particular isolates in the variable Ug99 lineage, as well as continued resistance breeding. The study has demonstrated the pathogenicity of PgtUg99 to barley despite the fact that it poses a great threat to wheat production in the world. 1. Introduction Barley (Hordeum vulgare L.) is one of the most important cereal crops, in Kenya. Currently about 30,000？ha of land is under the crop but potential for expansion remains [1]. It is a principal raw material used to process malt, a vital ingredient for beer brewing. The industry is one of the highest corporate taxpayers with annual turnover of Kshs. 28.9 billion and employs more than 1600 people across the region [1]. Diseases however reduce the quality of barley resulting in lower returns to the grower [2, 3]. Rust fungi, responsible for diseases like stem rust, yellow rust, and leaf rust are major contributors to the suboptimal yields realized by farmers. Breeding for tolerance has been used as the main method of protection against the fungi. The rust fungi can however overcome host tolerance genes and spread new strains through wind dispersal of spores [4]. Stem rust of
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