Objective 8: Discovering New Sources of Rust Resistance in Wild Wheat and Wild Barley

Ug99 and its derivatives possess virulence combinations that are capable of overcoming most of the 50+ major stem rust resistance genes available in wheat, including those most commonly used in major varieties throughout the world. Of the few major genes that remain effective against Ug99 in wheat, most were introduced from related species by inter-specific hybridizations. For durum wheat, an estimated 70% of CIMMYT and ICARDA, and 50% of North American advanced lines and varieties are susceptible to Ug99. Additional virulences specific to durum wheat are found in Ethiopia, rendering that crop’s vulnerability to stem rust even higher. Stem rust resistance in cultivated barley is very limited in general. Thus, identifying new sources of resistance to Ug99 is a top priority. Most of the phenotyping of wheat (common and durum) and barley conducted to date has focused on advanced breeding lines, varieties, and special genetic stocks carrying known genes. This Objective expands the search for resistance into the wealth of genetic diversity in wild relatives preserved in gene banks. Work to initiate or expand efforts to introgress resistance genes discovered in diploid wild relatives will also be initiated or expanded.

Because the majority of Ug99-effective major genes are derived from relatives of common and durum wheat, it is logical to extend the search for new resistances to those gene pools. Preliminary experiments at the Cereal Disease Laboratory have shown that resistance to Ug99 in Aegilops speltoides, Triticum timopheevi and Ae. sharonensis is common and that many accessions exhibited near-immune reactions. Resistance to Ug99 also was observed in Hordeum vulgare subsp. spontaneum. Identifying resistance in close relatives of these crop species will likely produce novel genes. The goal of this Objective is to discover new sources of resistance to Ug99 in wild relatives of common wheat, durum wheat, and barley. This will be accomplished by targeted, systematic phenotyping of wild relative accessions held in the gene banks.

This Objective will therefore include seedling assay phenotyping of gene bank accessions of several wild relatives of wheat, including Triticum monococcum, T. dicoccoides, T. dicoccum, T. carthlicum, T. polonicum, T. timopheevii, Aegilops speltoides, Ae. sharonensis, and Ae. tauschii. Accessions of wild relatives of barley, including Hordeum vulgare subsp. spontaneum and H. bulbosum will also be subjected to seedling tests. Initial screening of wild relatives will be done with Ug99. Resistant accessions will be tested for seedling reaction to other races as the first step in establishing the uniqueness of any major gene resistances. Work to transfer (introgress) putatively new resistance genes from the wild diploids will be initiated or expanded. Adult-plant screening of wild tetraploid species from ICARDA’s gene bank will also be conducted using the Critical Facility site in Ethiopia.

Outputs (deliverables) of this Objective will be the discovery of accessions of wild relatives of wheat and barley carrying putatively novel resistance genes and of wheat and barley lines carrying the newly discovered genes (from diploid species). All data will be registered with the originating gene bank and made available on the Project Website. The identity and nature of accessions exhibiting resistance will be pro-actively disseminated to the research community.