Sr31-Virulent Races (TTKSK, TTKST, and TTTSK) of the Wheat Stem Rust Pathogen Puccinia graminis f. sp. tritici are Present in Tanzania.

Since the first detection of race TTKSK (syn. Ug99) in Uganda in 1999 (2), the migration and evolution of Sr31-virulent races of Puccinia graminis f. sp. tritici [Pgt] have been closely monitored, particularly in Kenya and countries north, along the likely trajectory of migration to major wheat-producing regions of Asia. More recently, surveillance efforts have been undertaken to the south as well, and Ug99-related races have been detected in South Africa and Zimbabwe (3,4). Here we report for the first time results of a survey conducted in Tanzania. Systematic race surveillance provides data not only on the current distribution of the Ug99 race group, but also on the possible points of origin as well as the pace and probable paths of dispersal of future races from the region. In this context, the presence or absence of the Ug99 group of wheat stem rust races in adjacent countries like Tanzania assumes regional, and possibly global, relevance. A preliminary survey conducted in September 2006 indicated the presence of Sr31-virulent races of Pgt outside Slahhamo Village (3°15'S, 35°48'E) in the Ngorongoro highlands of northern Tanzania, based on compatible reactions with cv. K-Mamba (a.k.a. Mwamba), a cultivar whose pedigree indicates the presence of Sr31. A broader survey was conducted in August 2009, during which infected tissue was collected from currently-grown cultivars in research plots and on large estates, as well as from the mixes of older cultivars common on smallholder farms. In all, Pgt-infected samples were collected from one site in the Arusha region [Monduli (3°16' S, 36°24'E)], three sites in the Ngorongoro highlands [Karatu (3°20' S, 35°40' E), Upper Kitete (3°14' S, 35°53' E), and Slahhamo], one site in the Manyara region [Hanang (4°43' S, 35°40' E)], and one site in the Mbeya region [southern highlands (8°87' S, 33°40' E)], thereby giving representation to all four major wheat growing areas in the country. Sample storage, inoculation, incubation, disease assessment, and derivation of single-pustule cultures were all performed according to the methods described by Jin et al. (1). In addition to the 20 differentials in the expanded Pgt differential set of North America, we included two supplemental tester lines: Siouxland (Sr24 + Sr31) and Sisson (Sr31 + Sr36). Each single-pustule-derived isolate was evaluated for virulence on the differential and supplemental lines at least twice. A total of 39 single-pustule isolates were derived from the six collection sites. All 39 isolates were identified as belonging to the Ug99 race group, with six identified as TTKSK (all four regions), 30 identified as TTKST (Sr31 + Sr24 virulence; Arusha region and the Ngorongoro highlands), and three identified as TTTSK (Sr31 + Sr36 virulence; Manyara region and the Ngorongoro highlands). The results of this study suggest that, to more precisely locate the "hot spots" and thereby gain a better understanding of the mechanisms of novel race emergence in East Africa, it would be prudent to include Tanzania, heretofore a blank area on the wheat rust surveillance map, in future systematic race monitoring efforts. References: (1) Y. Jin et al. Plant Dis. 92:923, 2008. (2) Z. A. Pretorius et al. Plant Dis. 84:203, 2000. (3) Z. A. Pretorius et al. Plant Dis. 94:784, 2010. (4) Z. A. Pretorius et al. Plant Dis. 96:590, 2012.