ABSTRACT

The diligent use of inoculated leaf and stem rust nurseries has also contributed immeasurably to our development of rust-resistant germplasm. The Ug99 race of stem rust is virulent on 84% of spring wheat germplasm (Jin and Singh, 2006). However, several lines and cultivars from our breeding programme, including ‘Tom’ (Anderson et al., 2012) and ‘Linkert’ (released by the University of Minnesota in 2013), have been identified as being moderately resistant when exposed to epidemic conditions in African stem rust nurseries (M. Rouse, personal communication; Singh et al., 2015). How can these two cultivars, both of which are susceptible at the seedling stage to the Ug99 family of races, be moderately resistant at the adult plant stage? The answer lies in their pedigrees. Both cultivars can trace a portion of their parentage back to ‘Chris’ (Heiner and Johnston, 1967) and ‘Thatcher’ (Clark, 1935) that are also moderately resistant. The spring wheat region of the US has not experienced a significant stem rust epidemic since the 1950s (Roelfs, 1978) and the levels of naturally occurring inoculum from this fungus have been too low to provide effective selection environments. Apparently, our

Figure 3 Breeding process to introgress race-specific stem rust resistance genes into elite germplasm. A minimum of two genes are recommended to improve durability. Each process (i.e. B-D) can be completed in one to two years, for a total process time as short as three years. The developed germplasm (D) can be released as a variety, if necessary, or recycled as a parent into the breeding programme.