Cereals and Pseudocereals
Wheat – Resistance to Stem Rust Race Ug99
THE GLOBAL EFFORT TO IDENTIFY RESISTANCE TO STEM RUST RACE GROUP UG99 (TTKSK) IN WHEAT AND BARLEY
Patrick F. Byrne and Gordon Cisar
Department of Soil and Crop Sciences, Colorado State University, 307 University Ave, Fort Collins, Colorado 80523.
Borlaug Global Rust Initiative (retired), Cornell University, B75 Mann Library, Ithaca, New York 14853.
Corresponding author<EMAIL_ADDRESS>OUTLINE
1. SUMMARY
The appearance of a new virulent race of stem rust of wheat in Uganda in the 1998–1999 growing season set off alarms for this threat to wheat production worldwide. The stem rust resistance gene Sr31 from rye germplasm had been deployed in numerous cultivars around the world due to perceived stable, durable resistance. Wheat cultivars were not protected from the Ug99 race by this single gene, thus placing global wheat production at risk under a potentially devastating epidemic. The global response, coordinated by the Borlaug Global Rust Initiative, has improved monitoring of the pathogen’s spread and evolution, evaluated many tens of thousands of wheat germplasm lines, identified new sources of resistance and the causative genes, and rapidly incorporated resistance into new cultivars.
The goals were to:
- Monitor global population dynamics of the stem rust pathogen
- Improve phenotyping capability for response to stem rust infection in east Africa
- Identify novel sources of resistance to race group Ug99 in germplasm of wheat and its wild relatives
- Map the responsible resistance genes
- Deploy resistance in durable combinations in improved cultivars
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2. PROBLEMS ADDRESSED
Stem rust caused by the fungus Puccinia graminis f. sp. tritici (Pgt) is a destructive disease that has plagued wheat (Triticum aestivum) growing regions since ancient times. Severe infestations impede the flow of nutrients to developing seeds, thus causing reduced grain fill and shriveled grains (Bhavani et al., 2019). In addition, weakened stems are more susceptible to lodging. Yield losses up to 100% in susceptible cultivars may occur (Olivera et al., 2015). Stem rust epidemics occurred on various continents throughout the 20th century (Bhavani et al., 2019). A diagram of the complex life cycle of Pgt is provided in Fig. 2 of Fetch et al. (2021).
Since the early 20th century, plant breeders have incorporated stem rust resistance into wheat cultivars. Typically, the resistance has been provided by single major resistance genes (R genes), which tend to be race-specific and are identified at the seedling stage. Often, they have been effective for several years before being overcome by evolution of the pathogen (Bhavani et al., 2019; Fetch et al., 2021; McIntosh et al., 1995). The preferred, more durable form of resistance is controlled by multiple, minor-effect genes, known as adult plant resistance (APR) or all-stage resistance genes, which are generally