Kimani N. C^{1, 2,}, Onguso J.¹, Njau P.²

Stem rust disease of wheat caused by Puccinia graminis f. sp. tritici is of major concern because of its devastating effects on wheat. It can cause yield loss of up to 100% in susceptible varieties. East Africa has been designated as a “hot spot” of the stem rust pathogen as evidenced by the emergence of a new race of stem rust designated as TTKSK or better known as Ug99 and several of its variants. This pathogen therefore poses a threat to wheat production and hence to food security in Kenya. The frequent use of fungicides to control the disease also poses a potential adverse effect on the environment. The objective of this study was to screen a core collection of Kenyan bread wheat varieties to determine those with natural resistance to stem rust disease hence reduce the risk posed to food security and the environment. Twenty Kenyan commercial bread wheat varieties were screened for stem rust resistance under artificial disease epidemic simulation in the International Stem Rust Screening Field at Kenya Agricultural and Livestock Research Organisation, Food Crops Research Centre-Njoro, Kenya. The disease notes were taken using the Modified Cobb’s Scale and the Area Under Disease Progress Curve (AUDPC) values computed. Thirteen random samples of stem rust fungi were collected from the trial plot and analyzed using Ug99 race group-specific Single Nucleotide Polymorphism (SNP) markers. The varieties fell in three disease categories of resistant, intermediate and susceptible, with the most susceptible being Pasa and Kenya Swara being the most resistant. The mean AUDPC computed showed that there was variation in the AUDPC values among the varieties with the variety K.Swara having the lowest AUDPC value of 78.33 and variety Pasa having the highest value of 478.67. Analysis of Variance (ANOVA) showed that both AUDPC and disease scores had significant variation (P<0.0001) among the varieties. From the analysis of stem rust fungi samples two genotypes of stem rust race TTKSK (AF-001ad and AF-001aa) were detected indicating mutations within the same race variant. In conclusion there are Ug99 resistant Kenyan bread wheat varieties which hold a promise for food security. There is also evidence of further mutation within the TTKSK race variant and hence a possible increased virulence on the wheat genotypes.

Stem rust, AUDPC of stem rust, Puccinia graminis f. sp. tritici, Emerging strains of Race Ug99, SNP markers