Wheat stem rust, caused by Puccinia graminis f. sp. tritici, has historically been a major limiting factor in wheat production. Identification of isolate Ug99 in Uganda in 1999 highlighted the vulnerability of a large proportion of the global wheat crop and raised international concerns. Since initial detection, seven races have been identified in the Ug99 lineage and occurrence has been confirmed in nine countries (4). During rust surveys of Eritrea undertaken in October 2010, stem rust was found to be widespread throughout the highland wheat-growing areas. Presence of P. graminis f. sp. tritici was recorded in 95% of the 92 cereal fields surveyed, with high disease severity (>40%) recorded at 50 sites. Collected stem rust samples were analyzed for race identity in a level 3 biocontainment laboratory in Canada. Nine collections yielded viable spores for infection studies. Virulence analysis with 20 differentials in the letter-code nomenclature system (1) identified two races from repeated experiments; TTKST (four confirmed isolates) and PTKST (five confirmed isolates). Both races belong to the Ug99 lineage and both exhibit combined Sr31 and Sr24 virulence. TTKST and PTKST differ only in their virulence or avirulence to Sr21, respectively. This first confirmation of TTKST and PTKST in Eritrea is important because it represents further geographical spread of Ug99-related races. Since first detection of a Sr24 variant of Ug99 (race TTKST) in Kenya in 2006 (1), these variants have become the predominant P. graminis f. sp. tritici pathotypes in most of eastern Africa. Race TTKST caused epidemics in Kenya in 2007, and race PTKST was first detected in Ethiopia the same year (T. Fetch, unpublished data). Recent detection of race PTKST in three additional southern Africa countries (South Africa, Zimbabwe, and Mozambique [2,3]) indicates on-going range expansion within the African continent. Sr24-virulent variants of Ug99 are a cause for concern since a high frequency of cultivars from South America, Australia, the United States, and the CIMMYT are known to possess the Sr24 resistance gene. On the basis of observed occurrence and postulated migration routes of the original Ug99 (race TTKSK), the confirmed presence of TTKST and PTKST in Eritrea increases the possibility for range expansion out of Africa by crossing the Red Sea and into the Arabian Peninsula. Future spread of TTKST and PTKST to western Asia is considered highly likely.
References: (1) Y. Jin et al. Plant Dis. 92:923, 2008. (2) F. Mukoyi et al. Plant Dis. 95:1188, 2011. (3) Z. A. Pretorius et al. Plant Dis. 94:784, 2010. (4) R. P. Singh et al. Annu. Rev. Phytopathol. 49:465, 2011.