Track to Combat Wheat Stem Rust ( Puccinia graminis f. sp. tritici ) Races: Pathogenicity Spectrum, Tempo-Spatial Dynamics, and Impacts on Irrigated Wheat in Ethiopia under Climate Ch

1. Abstract Wheat stem rust (Puccinia graminis f. sp. tritici) is a major global threat to wheat production, driven by rapid shifts in virulence and race diversity. This study aimed to identify physiological races of stem rust pathogens in Ethiopia’s irrigated wheat-growing areas. Surveys and race analyses were conducted during the 2020/21–2023/24 large-scale wheat expansion periods, alongside a systemic review of rust dynamics from 2012 to 2022. Results revealed significant shifts in stem rust races, with an increasing dominance of virulent races over the last decade. TKTTF dominated from 2012 to 2016, succeeded by TTTTF in 2017, and TKKTF in 2019–2020. By 2021–2022, TTKTT and TTKTF races were prevalent, affecting 90% of wheat fields. From 2020 to 2024, five major races were identified: TTKTT, TTKTF, TTTTF, TKKTF, and TKTTF, with TTKTT, a virulent Ug99 mutant, emerging as the most dominant. This race exhibits a 95% virulence spectrum, overcoming resistance genes such as Sr24, widely used in commercial cultivars. The study highlights the urgent need to prioritize resistance breeding against virulent races, particularly in irrigated wheat regions, which serve as hotspots for pathogen evolution. Strategic rust intervention and improved screening are essential for protecting both irrigated and rain-fed wheat crops, ensuring sustainable wheat production in Ethiopia and the globe under climate change. 1. Abstract Wheat stem rust (Puccinia graminis f. tritici) is a significant concern for farmers and agricultural communities around the world. It is essential to monitor the changes in virulence among these infections, as understanding these shifts can help prevent unexpected epidemics that impact livelihoods and food security. Objective This study seeks to compassionately uncover the physiological races of stem rust pathogens present in irrigated wheat-growing regions of Ethiopia, recognizing the vulnerability of local farmers to these challenges. Materials and Methods We conducted thorough surveys and surveillance, accompanied by detailed physiological and genetic race analyses. The insights gained from this research will be instrumental in guiding the large-scale demonstration and expansion of irrigated wheat from 2020/21 to 2023/24. Furthermore, we conducted comprehensive reviews of the dynamics of stem and yellow rust races affecting wheat production in Ethiopia over the past decade, from 2012 to 2022. Results and Discussion Our findings reveal that changes in stem rust pathogen races, with a wider pathogenicity spectrum, are prevalent in both rain-fed and irrigated wheat-growing areas of Ethiopia. The rise of virulent races in affected fields has understandably raised concerns among farmers, with frequencies reaching up to 50%. Over the last ten years, the dynamics of wheat rust races illustrate a troubling shift toward more virulent strains. For example, the TKTTF race, virulent to resistance gene 85, was predominant from 2012 to 2016 but was subsequently replaced in 2017 by the more aggressive TTTTF race, which is harmful to gene 90. In 2018, we observed that TTTTF, along with TKTTF and TKKTF (virulent to gene 80), became equally common, creating additional challenges for our farming communities. The TKKTF race held dominance in 2019 and 2020, while from 2021 to 2022, TTKTT (virulent to gene 95) and TTKTF (virulent to gene 85) became more widespread, affecting around 90% of wheat fields.During the years 2021/22 to 2023/24, we carefully collected and analyzed 51 field rust samples to understand the situation better. We identified seven Pgt pathotypes in irrigated wheat in the 2021/22 season: TTKTT, TTKTF, TKKTF, TTRTF, TTTTF, TKTTF, and TKPTF. The emergence of the TTKTT race, a pathogenic Ug99 mutant, particularly in regions like Jimma, Buno Bedale, West Arsi, and East Shoa, is particularly concerning for the local farmers who depend on stable crops. In 2020/21, five stem rust races TTKTT, TTKTF, TTTTF, and TKTTF were identified, with TTKTF and TTKTT showing dominance in geographic distribution. Many popular varieties are now susceptible to these races, adding to the anxiety of farmers who rely on these crops for their livelihoods. The TTKTT race has been particularly alarming, with a 49% prevalence in Ethiopia for the first time, and it has shown varying reactions ranging from susceptible to moderately susceptible among different wheat types. Importantly, the resistance gene Sr24, which many farmers have relied upon in their cultivars, has unfortunately been rendered ineffective against this race. Conclusion To develop durable resistance, it is crucial to introduce a transgene cassette containing five resistance genes into bread wheat as a single locus within the wheat production area. Integrating robust resistance genes from wild grass relatives with modern scientific breakthroughs is essential. Consequently, breeding programs should focus on identifying additional sources of resistance to counter the more virulent races of the pathogen. Special attention must be given to irrigated wheat production, which harbors virulent and genetically diverse races, threatening the belg and main season wheat crops, as well as the green bridge. The background data collected from this study will aid in strategic rust intervention, screening, and guidance for resistance breeding among wheat breeders and seed technology multiplication units in the area addressing the climate change.