Races of the wheat stem rust pathogen in Ethiopia during 2014-2022

Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a persistent threat to Ethiopian wheat production, posing risks to food security. This study evaluated the virulence and genotypic diversity of the Pgt population following the stem rust epidemic in 2013, and the vulnerability of Ethiopian bread wheat cultivars to the newly identified Pgt races. We analyzed 639 stem rust samples collected between 2014 and 2022 in Ethiopia and identified 21 Pgt races from 722 single-pustule isolates. The Ethiopian Pgt population has undergone significant temporal shifts driven by exotic race introductions and possibly by selection pressure. Race TKTTF (clades IV-A and -B) dominated in 2014-2016 seasons but was largely replaced by races TTRTF (clade III-B) and TKKTF (clade IV-F) from 2017 onward. Since 2020, races TTKTT (clade I) and TTKTF (clade IV-F) were detected with increasing frequencies, emerging alongside TKKTF as the predominant races by 2022. All major races from genetic clades III and IV detected in Ethiopia exhibit a similar, broad geographic distribution with a likely origin in the Caucasus region. All five variants in the Ug99 race group (clade I) observed in Ethiopia were previously detected in Kenya, suggesting a potential incursion from this bordering country. Races BKBJC and BKGKC with distinct avirulence and genotypic profiles were derived from a sample collected near the alternate host Berberis holstii, suggesting that these avirulent isolates likely originated through sexual recombination. Seedling assays of 101 Ethiopian bread wheat cultivars against six prevailing races showed varying resistance, with only 36% and 48% resistant to race TTKTT and TTKSK, respectively. Although resistance genes deployed in Ethiopian cultivars do not appear to have exerted significant selection pressure favoring races TTRTF and TKKTF, the increased deployment of Sr24 in recent years may have contributed to the establishment and dissemination of race TTKTT. Our findings emphasize the need for breeding wheat cultivars with pyramided effective resistance genes and continuous pathogen surveillance to preempt future stem rust epidemics in Ethiopia and beyond.