Wheat (Triticum aestivum L.) stem rust caused by Puccinia graminis f. sp. tritici (Pgt) was generally insignificant in Europe from the 1960s until 2016, where a new race (TTRTF) caused damage on huge areas of durum wheat and bread wheat in Sicily (Bhattacharya, 2017). During the following five years, TTRTF was detected in eight additional countries in south- and central Europe (Patpour et al. 2022). In July 2021, seven wheat stem rust samples collected from spring wheat and one from barley in Svalov (55° 54' 10.8″ N, 13° 6' 54″ E) and Alnarp (55° 39' 39.6″ N, 13° 4' 40.8″ E), Sweden. Both cereal fields had a total disease incidence of 50% or higher. The samples were sent to the Global Rust Reference Center (GRRC, Denmark). Urediniospores of each sample were recovered on two susceptible cultivars, Line E and Morocco, which were used as susceptible controls in all experiments. Single pustular isolates were extracted and race typing was generally repeated two to three times based on Patpour et al. (2022) using 20 North American stem rust differential lines. Seedling infection types (IT) were scored on the first and second leaf 17 days post-inoculation using a 0-4 scale (Stakman et al. 1962; McIntosh et al. 1995). Isolates conferring 'low' ITs (i.e., 0, 0; 1, 1+, 2, and 2+), or combinations thereof, were considered 'avirulent' (incompatible), whereas ITs of 3-, 3, 3+, and 4 were considered 'high' (i.e., compatible, 'virulent'). Race nomenclature was based on a modified letter code proposed by Jin et al. (2008). We conducted DNA extraction and molecular genotyping using seventeen Simple Sequence Repeat (SSR) primer pairs derived from Stoxen (2012) and applied at large scale by Patpour et al. (2022). Based on the results from pathotyping and genotyping, two samples from wheat showed Pgt race TKTTF (Clade IV-B), three from wheat TKKTF (Clade IV- F), and three samples from wheat and barley were identified as TTRTF (Clade III-B). This is the first report of race TTRTF in northern Europe, particularly, Sweden, which significantly extends the known distribution of this race. The TTRTF race is a serious threat to wheat productivity and evaluation of resistance of commercial European wheat varieties to the TTRTF race confirmed that 70% of the cultivars were susceptible (Patpour et al. 2022). Therefore, if the conditions are suitable for the establishment and development of stem rust, the disease can cause significant damage to the wheat crop in these countries. Susceptibility of European wheat varieties stress an urgent need to initiate new breeding efforts to identify effective sources of resistance to wheat stem rust in breeding programs. Funding: The work is funded by the European Union's Horizon 2020 research and innovation programme under grant agreement No 773311 (RustWatch). References: Bhattacharya, S. 2017. Nature 542, 145-146. Doi: 10.1038/nature.2017.21424 Jin, Y., et al. 2008. Plant Dis. 92:923. 10.1094/PDIS-92-6-0923 McIntosh, R. A. et al. 1995. Catalogue of wheat symbols for wheat. 1993, Beijing, China, 1333-1451. Patpour, M., et al. 2022. Front. Plant Sci., 02 June 2022 | https://doi.org/10.3389/fpls.2022.882440. Stakman, E. C., et al. 1962. ARS E-617. USDA, Washington, DC. USA. Stoxen, S. 2012. Master's Thesis, University of Minnesota, Minneapolis, MN USA.
Keywords: Black rust; Clade III-B; Race TTRTF; Sicily race; Wheat commercial varieties.