Race Specific Resistance to Ug99 From Aegilops Species with C,U and M Genomes and Strategy for Durable Resistance to Stem Rust in Wheat.

Ug99 - a new virulent mutant of wheat stem rust pathogen (Puccinia graminis f. sp. tritici), first detected in Uganda in 1998 and having virulence on widely deployed stem rust resistance gene Sr31 (TTKSK), has been recognized as a major threat to world wheat production. Efforts in the form of international initiatives are on to identify sources of resistance. At the Punjab Agricultural university (PAU), screening of about 1,000 accessions of wild Aegilops species over years suggested that the species with S, C, U and M  genome(s) are excellent sources of resistance to major wheat diseases, including rusts. Though attempt is being made to identify and utilize race specific resistance to Ug99 from S genome species (USDA coordinated Action Plan), considerable attention has not been given to C, U and M genome species which have shown impressive multiple resistance. In fact, in an attempt to transfer leaf rust resistance from Aegilops ovata (UUMM) into bread wheat led to transfer of stripe rust resistance as well. Similarly, transfer of leaf rust resistance from Aegilops triuncialis (UUCC) into bread wheat, led to transfer of resistance to other diseases like Karnal bunt (Neovossia indica) and cereal cyst nematode (Heterodera avenae). Probably these are linked genes that have evolved as copies of the same gene but gained different functional specificities (with respect to recognition of pathogen) during course of evolution. Therefore, these derivatives and the donor accessions may possess stem rust resistance as well. So it may be worthwhile to test these derivatives and concentrate on screening of C,U and M genome species in East Africa (where Ug99 pathotypes are  present naturally) through collaboration/ an international initiative to identify plausible multiple resistance/resistance to Ug99.

Testing of 24 bread wheat lines at two locations of the Haramaya University, Ethiopia during 2004 (stem rust pathotypes with virulence on Sr31 were already prevalent there) identified five lines with seedling susceptibility but low Area under Disease Progress curve and low rate of rust development under field conditions (slow rusting resistance). It is widely advocated in literature that race specific resistance to cereal rusts supported by a background non-race specific resistance of this kind would make the resistance durable. Also, in countries like India, where rust over-summers  in southern hills (Nilgiri Hills), deployment of  a number of race specific resistance genes at the focus of infection of stem rust would make the pathogen diverse. This could allow to exploit stabilizing selection in the pathogen population which is blown to central and southern India where bulk of the wheat is cultivated during the main season. Stabilization of pathogen population would help to achieve durable resistance in the plains.