stem rust

Puccinia graminis f. sp. tritici (Pgt) race TKTTF was reported as the dominant race in the wheat stem rust epidemics in Ethiopia during 2014–15 (Olivera et al. 2015). The race and variants hereof have also been recorded elsewhere in Africa, the Middle East, and Europe (www.wheatrust.org/stem-rust-tools-maps-and-charts/race-frequency-map). Here, we report the presence of additional virulence to Sr25 in the TKTTF population, a resistance gene transferred to several Australian and CIMMYT wheat genotypes. At the seedling stage, Sr25 confers infection type (IT) 2 or lower for isolates in the Ug99 race group and up to IT 2+ toward race TKTTF (Newcomb et al. 2016; Olivera et al. 2015). Our results are based on Pgt isolates of the TKTTF race from Ethiopia (2012, 2013, 2015), Egypt (2014), Azerbaijan (2014), Iran (2009, 2011, 2014), Iraq (2014), Lebanon, Sudan, and Turkey (2012), Denmark and Germany (2013), and Sweden (2014). Race typing was carried out at the Global Rust Reference Center according to Jin et al. (2008), except that we scored IT on both leaf 1 and 2; additional single pustule isolates of each sample were raised and stored in liquid nitrogen (–196°C). Sr25 response was assayed using seedling leaves and stems of adult plants of Misr1 (Oasis/Skauz//4*BCN/3/2*Pastor) and Agatha/9*LMPG (Sr25 carriers) along with two reference lines, Triumph 64 (SrTmp) and NA101/MqSr7a (Sr7a), and Morocco as a control. Seedling ITs were scored 17 days post-inoculation at 18 ± 2°C using a 0 to 4 scale (McIntosh et al. 1995). Isolates showing ITs of 33+ to 4 on Misr1, Agatha/9*LMPG, and susceptible check were considered Sr25 virulent, and clearly different from ITs conferred by Sr25 avirulent isolates. Results were confirmed for each isolate by race typing additional single-pustule isolates derived from cultivars Misr1 and/or Agatha, along with avirulent reference isolates. Virulence for Sr25 was observed in race TKTTF isolates from Azerbaijan, Egypt, Ethiopia, Iran, Iraq, and Sweden, collected in 2014 or 2015, but not in any sample collected earlier than 2014. The results were confirmed on adult plants of Misr1 and Agatha/9*LMPG by Sr25 virulent and avirulent isolates of TKTTF, TTKSK, and TTKST, respectively. Spore suspensions of ∼0.5 ml at concentration of ∼3 × 10⁵ spores/ml were injected into the stem internodes at Zadoks 45. The adult plant and seedling tests were carried out concurrently using the environmental conditions described above. The plants containing Sr25 were susceptible to the Sr25 virulent isolate and moderately resistant to moderately susceptible to the Sr25-avirulent isolates of TKTTF, TTKSK, and TTKST. The experiments were repeated two times with three replicates, using cv. Morocco as a susceptible check. Emergence of virulence to Sr25 in the race TKTTF is considered significant due to its spread into new areas and the potential loss of a significant source of resistance against Ug99.

Stem rust is one of the most important diseases of wheat in the world. When single stem rust resistance (Sr) genes are deployed in wheat, they are often rapidly overcome by the pathogen. To this end, we initiated a search for novel sources of resistance in diverse wheat relatives and identified the wild goatgrass species Aegilops sharonesis (Sharon goatgrass) as a rich reservoir of resistance to wheat stem rust. The objectives of this study were to discover and map novel Sr genes in Ae. sharonensis and to explore the possibility of identifying new Sr genes by genome-wide association study (GWAS). We developed two biparental populations between resistant and susceptible accessions of Ae. sharonensis and performed QTL and linkage analysis. In an F₆ recombinant inbred line and an F₂ population, two genes were identified that mapped to the short arm of chromosome 1S^sh, designated as Sr-1644-1Sh, and the long arm of chromosome 5S^sh, designated as Sr-1644-5Sh. The gene Sr-1644-1Sh confers a high level of resistance to race TTKSK (a member of the Ug99 race group), while the gene Sr-1644-5Sh conditions strong resistance to TRTTF, another widely virulent race found in Yemen. Additionally, GWAS was conducted on 125 diverse Ae. sharonensis accessions for stem rust resistance. The gene Sr-1644-1Sh was detected by GWAS, while Sr-1644-5Sh was not detected, indicating that the effectiveness of GWAS might be affected by marker density, population structure, low allele frequency and other factors.

In durum wheat (Triticum turgidum subsp. durum), the gene Sr47 derived from Aegilopsspeltoides conditions resistance to race TTKSK (Ug99) of the stem rust pathogen (Puccinia graminis f. sp. tritici). Sr47 is carried on small interstitial translocation chromosomes (Ti2BL-2SL-2BL·2BS) in which the Ae. speltoides chromosome 2S segments are divided into four bins in genetic stocks RWG35, RWG36, and RWG37. Our objective was to physically map molecular markers to bins and to determine if any of the molecular markers would be useful in marker-assisted selection (MAS). Durum cultivar Joppa was used as the recurrent parent to produce three BC₂F₂ populations. Each BC₂F₂ plant was genotyped with markers to detect the segment carrying Sr47, and stem rust testing of BC₂F₃ progeny with race TTKSK confirmed the genotyping. Forty-nine markers from published sources, four new SSR markers, and five new STARP (semi-thermal asymmetric reverse PCR) markers, were evaluated in BC₂F₂ populations for assignment of markers to bins. Sr47 was mapped to bin 3 along with 13 markers. No markers were assigned to bin 1; however, 7 and 13 markers were assigned to bins 2 and 4, respectively. Markers Xrwgs38a, Xmag1729, Xwmc41, Xtnac3119, Xrwgsnp1, and Xrwgsnp4 were found to be useful for MAS of Sr47. However, STARP markers Xrwgsnp1 and Xrwgsnp4 can be used in gel-free systems, and are the preferred markers for high-throughput MAS. The physical mapping data from this study will also be useful for pyramiding Sr47 with other Sr genes on chromosome 2B.

Phenotypic and genotypic evaluation of wheat genetic resources and development of segregating populations are pre-requisites for identifying rust resistance genes. The objectives of this study were to assess adult plant resistance (APR) of selected wheat genotypes to leaf rust and stem rust and to develop segregating populations for resistance breeding. Eight selected Kenyan cultivars with known resistance to stem rust, together with local checks were evaluated for leaf rust and stem rust resistance at seedling stage and also across several environments. Selected diagnostic markers were used to determine the presence of known genes. All eight cultivars were crossed with local checks using a bi-parental mating design. Seedling tests revealed that parents exhibited differential infection types against wheat rust races. Cultivars Paka and Popo consistently showed resistant infection types at seedling stage, while Gem, Romany, Pasa, Fahari, Kudu, Ngiri and Kariega varied for resistant and susceptible infection types depending on the pathogen race used. The control cultivars Morocco and McNair consistently showed susceptible infection types as expected. In the field, all cultivars except for Morocco showed moderate to high levels of resistance, indicating the presence of effective resistance genes. Using diagnostic markers, presence of Lr34 was confirmed in Gem, Fahari, Kudu, Ngiri and Kariega, while Sr2 was present in Gem, Romany, Paka and Kudu. Seedling resistance gene, Sr35, was only detected in cultivar Popo. Overall, the study developed 909 F_6:8 recombinant inbred lines (RILs) as part of the nested mating design and are useful genetic resources for further studies and for mapping wheat rust resistance genes.