ABSTRACT
Phenotypic screening of germplasm for race-specific resistance genes is often the first recourse when new races develop, even if the newly identified genes are not expected to be durable. While the primary gene pool is the preferred source of this resistant germplasm due to ease of introgression, wheat breeders are fortunate that there are several species in the secondary and tertiary gene pools that have contributed key resistance genes. A few examples include: Wsm1 for Wheat Streak Mosaic Virus (Gill et al., 1995); Bdv2 for Barley Yellow Dwarf Virus (Sharma et al., 1997; Ayala-Navarrete et al., 2013); Pch1 for strawbreaker foot rot caused by Pseudocercosporella herpotrichoides (Fron) Deighton [teleomorph Tapesia yallundae] (Wallwork & Spooner, McMillian et al., 1986). More than 29 genes for stem rust resistance have been identified and introgressed into durum or common wheat (reviewed by Singh et al., 2015). Because of the vast reservoir of resistance genes contained in these extended gene pools, this is a source of germplasm that wheat researchers cannot afford to ignore in the future. New genetic resources (Friebe et al.,
Figure 1 Process of identifying and deploying useful resistance. Screening for race-specific resistance can often be accomplished in relatively high-throughput screens of seedlings. Identifying race-nonspecific resistance is more difficult as this type of resistance may be expressed only in adult plants (e.g. wheat rusts), and highly virulent races must be used to avoid masking by race-specific genes.
