Stem rust resistance gene Sr25 has been effective (fully or partially) to wheat Puccinia graminis f. sp. tritici races including Ug99 race and its lineage. Sr25 gene is from Thinopyrum elongatum, located on chromosome 7DL and is linked with leaf rust resistance gene Lr19. Field phenotyping of stem rust differential sets in 2023 (off-season) at Kalro-Njoro rust screening platform showed a high severity score of 50MSS on wheat line carrying Sr25 gene. Rust spores were collected from the Sr25 line, purified and multiplied in the greenhouse. The single pure isolates were evaluated for virulence on stem rust differential lines (CIMMYT differentials and 20 North American differentials) and the line carrying Sr25 gene. Virulence on the line carrying Sr25 gene was observed, depicting high infection types (IT) of 3+ and 4. However, a low infection type of ;1 (fleck one) was observed on MISR 1 line, which carries Sr25 gene among other genes in its background. The observed low infection type on MISR line therefore warrants further studies on the effective gene (s) still providing resistance to the MISR 1 line. Results from the stem rust differential lines showed that the isolates were avirulent to genes Sr21 (depicting low IT 2) and combination Sr17, Sr13 (depicting low IT ;1). The isolates were virulent to genes Sr24, Sr30, Sr31, and SrTmp. Detection of virulence to Sr25 gene in Kenya for the first time is an important finding as this gene is widely used in breeding Ug99 resistant cultivars. Further studies on vulnerability mapping of breeding germplasm and varieties will be carried out using race screening and markers linked to Sr25. We therefore recommend breeding programs in Africa utilizing this gene to target other major and/or minor effective genes onto their wheat backgrounds, and emphasize on continuous monitoring for any change in pathogen virulence pattern.

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