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

Authors: Olivera Firpo, Pablo; GUTU, KITESSA - Ethiopian Institute Of Agricultural Research; ALEMAYEHU, YOSEPH - International Maize & Wheat Improvement Center (CIMMYT); TOLA, JEMAL - Ethiopian Institute Of Agricultural Research; HODSON, DAVID - International Maize & Wheat Improvement Center (CIMMYT); Carter, Melissa; GREATENS, NICHOLAS - Oak Ridge Institute For Science And Education (ORISE); VEREGGE, MOLLY - University Of Minnesota; BADEBO, AYELE - International Maize & Wheat Improvement Center (CIMMYT); BACHA, NETSANET - Ethiopian Institute Of Agricultural Research; Rouse, Matthew; Luster, Douglas; SZABO, LEZ - Retired ARS Employee; Jin, Yue

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: Wheat stem rust, caused by the fungus Puccinia graminis f. sp. tritici (Pgt), remains a serious threat to what production worldwide due to its rapid movement by air currents and travelers. The stem rust race Ug99, identified in East Africa, was virulent on more than 75% of the US wheat cultivars grown at the time. Thus, it is critical that we protect the US wheat crop by identifying any new stem rust races and their virulence on wheat varieties being grown. Following a major outbreak in 2013, we analyzed 639 stem rust samples collected across Ethiopia from 2014 to 2022, identifying 21 different pathogen races and tracking their changes over time. Changes in dominant races, mostly incursions from neighboring regions, revealed an increasing presence of highly virulent strains, such as those in the Ug99 group and other major genetic groups. Evaluating 101 Ethiopian wheat cultivars showed that many were vulnerable to these virulent races, in many cases due to the presence of a single resistance gene. The widespread use of genes like Sr24 has possibly favored the spread of races in the Ug99 group. This research highlights the need for continued monitoring to detect the evolution of virulence in the pathogen population and to breed cultivars with multiple resistance genes to protect wheat from future stem rust epidemics.

Technical Abstract: 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. A total of 21 Pgt races were identified 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 early seasons (2014-2016) 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 becoming the predominant races with TKKTF in 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.