Looking ahead: Races and resistances to stem rust in European wheat and triticale

Stem rust in wheat caused by Puccinia graminis f.sp. tritici is an upcoming disease in Europe. Although practically absent in the last five decades, epidemics in Europe have been reported in increasing numbers, e.g., 2013 in Germany, since 2016 in Sicily, Italy, 2017 in southern Sweden and 2022 in the United Kingdom. In the RustWatch project, a total of 373 stem rust samples have been collected during 2019-2022 across 19 European countries from Spain to Latvia. Several researchers expect a higher occurrence of stem rust in Europe due to global warming (Prank et al., 2019; Miedaner & Juroszek, 2021). Here, we present data on the prevailing stem rust races in Germany and Austria and information on resistances in European wheat and triticale diversity panels and segregating populations. In Germany and Austria, six distinct genetic groups (clades) have been identified from 56 samples collected during 2017-21 and analysed by twenty SSR (simple sequence repeat) markers. Two clades (Clades III_B, IV_F) are exotic incursions from East Africa and West Asia (pre 2016), clade IV_B is from Ethopia (2013-14, ‘Digalu’), the others are from internal spread within the EU. These are the same clades known from other European countries (Patpour et al., 2022). Additionally, 79 races were identified from the alternative host barberry (Berberidaceae) (Rodriguez-Algaba et al., 2022). The results highlight the high mobility of stem rust spores within Eurasia and a fundamental role of barberry in generating new genetic variation. Resistances in European wheat are scarce. In the RustWatch panel with 263 varieties tested at Berlin-Dahlem and Sicily, Italy, in four years, only 14% of the varieties were resistant (score 1-4 on the 1- 9 scale). The correlation between both locations/countries was high across three years (r = 0.752, p < 0.001) although the experiment in Germany was inoculated with German races and the experiment in Italy was naturally infected. In another diversity panel of 280 winter wheat varieties inoculated by a mixture of German stem rust isolates 14% were highly resistant (0-5% infected stem area) and a further 18% resistant (5-10%) tested across six environments. Molecular analyses of this diversity panel by a 25k iSelect SNP (single-nucleotide polymorphism) chip revealed only three already known major resistance loci for stem rust resistance: Sr38 on chromosome 2A, Sr31 on chromosome 1B, and Sr24 on chromosome 3D. All three genes were still effective although none of them provided full resistance (Fig. 1). These three resistance genes were also detected in different combination in biparental populations with the resistance donors ‘Memory’, ‘LG Character’, ‘KWS Montana’ and two breeding lines. Seven minor QTL have also been additionally found, each explaining 0.6% to 12% of the genotypic variation. A triticale diversity panel of 565 varieties and two biparental populations with 182 and 162 progenies, respectively, was inoculated with a mixture of wheat stem rust isolates. The resistance situation was totally different: the majority of progenies was resistant in all three populations. In the diversity panel five loci have been detected that explained 3.6% to 8.4% of genotypic variation. Four of these loci had already high allele frequencies explaining the high resistance level in triticale. The locus on 3RL segregated also in the DH population Tulus × Massimo explaining 50.1% of genotypic variation and resulting in full resistance. This clearly illustrates that QTL mapping is better suited for the genetic characterization of individual loci. The Cando × Triticon population was monomorphic for the same resistance allele, therefore being fully resistant to stem rust. In summary, stem rust resistance in European winter wheat is rare as shown by the analysis of two diversity panels with a total of 543 varieties. Only three major genes have been detected in the winter wheat panel with emphasis on German varieties, that are already known from international sortiments. Sr24, Sr31, and Sr38 are still effective in Germany, however, in Spain already an isolate from the barberry area has been detected with a combined virulence for all three genes (Patpour et al., 2022). Also in Germany, two isolates with each of two combinations of the respective virulences have been detected recently. In future, more efforts should be made by wheat breeders to broaden the basis of stem rust resistance, since it must be assumed that the number and extent of epidemics will increase caused by climate change. Triticale seems to be still very resistant to stem rust.