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First Report of Puccinia graminis f. sp. tritici Race PTKSK, a Variant of Wheat Stem Rust Race Ug99, in South Africa

Terefe, T., Pretorius, Z. A., Visser, B., Boshoff, W. H. P.
Plant disease 2019 v.103 no.6 pp. 1421
Puccinia graminis, Triticum aestivum, breeding lines, crop production, cultivars, genes, genetic similarity, greenhouses, microsatellite repeats, monitoring, races, seedlings, stem rust, surveys, temperature, triticale, virulence, wheat, Ethiopia, Kenya, South Africa, Yemen
Stem rust caused by Puccinia graminis f. sp. tritici (Pgt) is an important disease of bread wheat (Triticum aestivum L.) in South Africa (SA). The frequent emergence of new Pgt races with increased virulence remains a major constraint when genetic control is considered. Monitoring the occurrence, distribution, and pathogenicity of Pgt is essential to quickly detect and track new races. During the annual rust survey conducted in 2017, a Pgt isolate (S42-17) collected from wheat in the Free State province near Bethlehem (28.15603°S, 28.28852°E) differed in its virulence profile from known SA Pgt races. Two single-pustule isolates obtained from collection S42-17 were independently phenotyped at the rust laboratories of University of the Free State and ARC-Small Grain, SA. Race analysis was performed using a modified North American (NA) differential set (Jin et al. 2008). In addition to differential lines containing the standard set of Sr genes, Barleta Benvenuto (Sr8b), Coorong triticale (Sr27), Kiewiet triticale (SrKiewiet), and Satu triticale (SrSatu) were included. Seedling inoculation, incubation, and rust evaluation were conducted as previously described (Terefe et al. 2016). Additional tests with lines Cns_T.mono_deriv. and T.mono_deriv/*8LMPG, both containing Sr21, were conducted in greenhouse cubicles at mean temperatures of 16.7 and 20.4°C, respectively. All seedling assays were repeated. The two S42-17 isolates were avirulent (infection types 0; to 2) for Sr24, 27, 36, Kiewiet, Satu, and Tmp and virulent (infection types 3 to 4) on Sr5, 6, 7b, 8a, 8b, 9a, 9b, 9d, 9e, 9g, 10, 11, 17, 30, 31, 38, and McN. The 2 to 3 = infection type range produced by Cns_T.mono_deriv. and T.mono_deriv/*8LMPG was not deemed as conclusive evidence for virulence to Sr21 in the isolates tested. Both S42-17 isolates coded to race PTKSK (2SA42; SA race notation). This is the first report of Pgt race PTKSK in SA. This race has been previously reported as one of the Ug99 variants from Ethiopia (2007), Kenya (2009), and Yemen (2009) (Singh et al. 2015). Except for avirulence for Sr24, PTKSK is similar to race PTKST, previously identified in SA. From 112 SA bread wheat cultivars and advanced breeding lines tested in the seedling stage, PTKSK was virulent on 45 entries as opposed to 71 entries being susceptible to race PTKST. Simple sequence repeat (SSR) comparison of PTKSK with 12 other SA Pgt races using 10 SSR markers confirmed that PTKSK grouped with all the SA Ug99 race group members, sharing at least 85% genetic similarity compared with the 25% genetic similarity shared with non-Ug99 SA races. At present, it is not known if PTKSK arose from PTKST or whether it represents a new introduction into SA. Because PTKSK is less virulent on SA wheat cultivars compared with PTKST, it should not pose a significant threat to local wheat production. Its detection, however, indicates the continued emergence of new Pgt races in SA and emphasizes the importance of sustained stem rust surveillance in the region.