ABSTRACT Stem rust, caused by (Puccinia graminis f.sp. tritici) is a major disease of wheat in Kenya. The disease was previously contained by the rpg1, sr31, 24 and 36 genes for resistance that were incorporated in the genotypes of barley and wheat grown in Kenya. In 1999, a new race Ug99 was detected in tritici. The germplasms showed varying levels of resistance to stem rust. 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; Nguzo had the lowest disease severity of 16% and 5% in season 1 and 2 respectively. The highest reduction in percent germination (54.1% and 38.3%) was recorded in 1385-13 and ICARDA-10 in season 1and 2 respectively. The highest loss (9.00 %) in protein content was observed in Sabini in season 1 and a loss of 4.0 and 16.3% in zeleny content was noted in season 1 and 2 respectively. From the results in this experiment most of the Kenyan grown cultivars were susceptible to the new race of stem rust. This emphasizes the 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
