First Report of Race TTRTF of Wheat Stem Rust (Puccinia graminis f. sp. tritici) in Eritrea

Race TTRTF of Puccinia graminis f. sp. tritici was first detected in 2016 in a severe outbreak of wheat stem rust on thousands of hectares of durum and bread wheat in Sicily, Italy (Bhattacharya 2017; Patpour et al. 2018). However, a recent study suggested that TTRTF was already present in Georgia in 2014 (Olivera et al. 2019). During 2016 to 2018, 140 samples of rust-infected wheat and barley were collected from field trials and farmers’ fields in Eritrea and submitted to the Global Rust Reference Center (GRRC), Denmark. Fifty-one urediniospore isolates were derived from 62 samples recovered on the susceptible wheat cultivars Morocco and Line E. Race typing was completed for 40 isolates based on 20 North American wheat lines using the standard race-typing procedures and infection type (IT) criteria determining virulence and avirulence (Jin et al. 2008). All isolates except two, representing 24 sampling sites, were diagnosed as TTRTF, whereas two samples collected from farmers’ fields in Metera and Temajila were identified as TKKTF and TTKSK, respectively. In April 2019, two stem rust samples were collected from wheat in Fars province, Iran. After submission to GRRC, two single-pustule isolates per sample were raised and race typed as described above. All isolates were determined to be race TTRTF, previously reported without data documentation (Roohparvar and Omrani 2018). This is the first report of race TTRTF in Eritrea (2016) and southern Iran (2019). Six isolates of TTRTF from Eritrea and two from Iran were tested for additional virulence using 24 supplementary lines and cultivars containing Sr7a, Sr13a, Sr22, Sr25, Sr26, Sr27, Sr31, Sr32, Sr33, Sr35, Sr37, Sr39, Sr40, Sr44, Sr45, Sr47, SrSatu, Sr1RSᴬᵐⁱᵍᵒ, Sr50, Siouxland (Sr24+31), Sisson (Sr31+36), and Triumph 64 (SrTmp). Durum wheat Iumillo (Sr9g,12,+) and Leeds (Sr9e,13b,+) were also included in the experiment. The results were consistent for all isolates across two to three independent experiments. In summary, TTRTF had virulence for 23 Sr genes (IT 3 or higher): Sr5, Sr6, Sr7a, Sr7b, Sr8a, Sr9a, Sr9b, Sr9d, Sr9e, Sr9g, Sr10, Sr11, Sr13b, Sr17, Sr21, Sr35, Sr36, Sr37, Sr38, Sr44, Sr45, SrTmp, and SrMcN. Fed*3/Gabo*51BL.1RS-1-1 (Sr50) showed IT 3-3, which is significantly higher than for typical avirulent isolates. Simple sequence repeat genotyping of TTRTF isolates confirmed that they belonged to clade III-B defined by Olivera et al. (2019). The occurrence of TTRTF in Eritrea and Iran in 2016 and 2019, respectively, extends the known distribution of this race (sampling year in parenthesis): Italy (2016), Georgia (2014), Hungary (2017) (Olivera et al. 2019), and Egypt (2015 to 2016) (Esmail and Szabo 2018). TTRTF is considered a serious threat to wheat productivity in affected areas due to a broad virulence spectrum, including virulence to Sr13b from durum wheat and higher than usual IT for Sr50. Detection of TTRTF in different regions since 2014 stresses the high potential of onward spread to new areas and imminent risks for new epidemic outbreaks of wheat stem rust in many countries, because they are highly dependent on wheat as main source for food and feed. There is an urgent need to sustain wheat rust surveillance efforts globally and to identify effective sources of resistance to stem rust in cereal germplasm.