ABSTRACT
Stripe or yellow rust, caused by the fungal pathogen P. striiformis Westend. f. sp. tritici Eriks, is reportedly adapted to cooler climates (2-15°C) and can be as destructive as stem rust under epidemic conditions. Yield losses up to 50% may occur due to shrivelled grain and damaged tillers (Roelfs et al., 1992). Randhawa et al. (2012) report that the incidence and severity of stripe rust increased dramatically in North America during 1999 because of the occurrence of new P. striiformis isolates showing significant adaptation to warmer temperatures and enhanced aggressiveness. Whereas durum wheat has been generally and globally highly resistant to stripe rust, it is risky to assume that the crop is reliably and durably protected from this pathogen. A relative increase in incidence of stripe rust was observed during 2015 in the CIMMYT breeding germplasm in Mexico (some 10% of advanced lines showing some susceptibility as opposed to less than 2% previously) and in North Africa, Spain and Italy (Ammar, pers. comm.). While there is no evidence that these observations are related to the appearance of durum-specific virulence (as in the case of
leaf rust, see below), it indicates that, under highly favourable environmental conditions, the newly predominant races affecting bread wheat are starting to become more of a problem on durum wheat. Whatever the cause, it is important that the durum wheat community worldwide starts addressing the potential stripe rust problem in a preventative manner. Most of the numerous studies on the genetics of stripe rust resistance focused on hexaploid with very few addressing durum wheat (Bansal et al., 2014). Among 67 officially named Yr genes, only three, Yr53, Yr64 and Yr65, were detected in durum wheat (Xu et al., 2013; Cheng et al., 2014). Bansal et al. (2014) detected QTL for stripe rust on chromosomes 2A, 3B and 5B of a bi-parental cross. Xue Lin et al. (unpublished) identified genomic regions for stripe rust resistance in durum wheat on 5A and 7B by association mapping and linkage mapping, with the 7B region explaining the greatest proportion of the variation. Interestingly, the location of the 7B resistance is coincident with Qlr.ubo-7B.2 and the location of a meta-QTL (Soriano and Royo, 2015), which suggests that this region is a genetic hotspot for rust resistance in durum wheat.
