First Report of a Puccinia graminis f. sp. tritici Race Virulent to the Sr24 and Sr31 Wheat Stem Rust Resistance Genes in South Africa.

Isolates of Puccinia graminis f. sp. tritici belonging to the Ug99 race group are virulent to a broad spectrum of resistance genes, rendering most of the world's wheat germplasm susceptible to stem rust (3). Following the initial detection of Ug99 (TTKSK, North American [NA] race notation) in Uganda, virulence to the widely used Sr31 resistance gene has been reported from Kenya, Ethiopia, Sudan, and Iran (2,3). In November 2009, a wheat genotype suspected to carry Sr31 showed a susceptible response to stem rust in a disease nursery (29°08'05.02''S, 30°38'29.18''E), inoculated with race TTKSP, near Greytown in KwaZulu-Natal, South Africa. Inoculation of urediniospores of the field collection (isolate UVPgt60) onto seedlings of line Federation4*/Kavkaz confirmed virulence for Sr31. In three independent, replicated, and comparative seedling tests, eight single-pustule isolates of UVPgt60 all typed to race PTKST following the NA race nomenclature. These isolates produced compatible infection types (ITs) (3+ to 4) on the Sr31 testers Gamtoos, Sr31/6*LMPG, Federation4*/Kavkaz, Kavkaz, and Clement, whereas isolate UVPgt59 (TTKSP) was avirulent (ITs ;1 to 1) on these genotypes. In addition to Sr31 virulence, the new race differed from TTKSP by producing a lower IT (2 to 2++) on Cns_T. mono_ deriv. , the accepted entry for Sr21 in the NA differential set. The UVPgt60 isolates were clearly avirulent on Einkorn (Sr21) (IT ;1=), a response that also differed from those produced by BPGSC, TTKSF, and TTKSP (IT 2). With the exception of Sr21, UVPgt60 isolates had a virulence pattern similar to race TTKST (1), notably the virulence combination for Sr24 and Sr31. Isolate UVPgt60.6 was randomly selected for testing on additional Sr genes and South African wheat cultivars and breeding lines. Similar to the race identification experiments seedling tests were duplicated and compared with reactions produced by TTKSP and other races. Greenhouse temperatures for all seedling tests ranged between 18 and 25°C. On the basis of primary leaf responses, PTKST is avirulent (ITs 0; to 2++) for Sr13, 14, 21, 22, 25, 26, 27, 29, 32, 33, 35, 36, 37, 39, 42, 43, 44, Em, Tmp, and Satu and virulent (ITs 3 to 4) for Sr5, 6, 7b, 8a, 8b, 9a, 9b, 9d, 9e, 9g, 10, 11, 16, 17, 24, 30, 31, 34, 38, 41, and McN. From 103 South African wheat cultivars and lines tested as seedlings, 59 and 47 were susceptible (IT ≥ 3) to races PTKST and TTKSP, respectively. Simple-sequence repeat analysis (4) with selected primer pairs showed that PTKST clusters with isolates belonging to the Ug99 lineage. Subsequent to the collection made at Greytown, stem rust sampled in December 2009 from naturally infected breeders' lines at Cedara (29°32'19.59''S, 30°16'03.50''E), KwaZulu-Natal, revealed five isolates with a virulence profile similar to PTKST. On the basis of current evidence it appears that PTKST may be an introduction to South Africa rather than a single-step mutation from local stem rust races. References: (1) Y. Jin et al. Plant Dis. 92:923, 2008. (2) K. Nazari et al. Plant Dis. 93:317, 2009. (3) R. P. Singh et al. Adv. Agron. 98:271, 2008. (4) B. Visser et al. Mol. Plant Pathol. 10:213, 2009.