Detection of Virulence to Resistance Gene Sr24 Within Race TTKS of Puccinia graminis f. sp. tritici

Abstract

The stem rust resistance gene Sr24 is effective against most races of Puccinia graminis f. sp. tritici, including race TTKS (syn. Ug99), and is used widely in commercial wheat cultivars worldwide. In 2006, susceptible infection responses were observed on wheat lines and cultivars carrying Sr24 in a field stem rust screening nursery at Njoro, Kenya. We derived 28 single-pustule isolates from stem rust samples collected from the 2006 Njoro nursery. The isolates were evaluated for virulence on 16 North American stem rust differential lines; on wheat lines carrying Sr24, Sr31, Sr38, and SrMcN; and on a wheat cultivar with a combination of Sr24 and Sr31. All isolates were identified as race TTKS with additional virulence on Sr31 and Sr38. These isolates were divided into two groups: group A (seven isolates and the two control isolates), producing a low infection type, and group B (21 isolates), producing a high infection type on Sr24, respectively. Isolates of group B represented a new variant of race TTKS with virulence to Sr24. Eighteen simple sequence repeat (SSR) markers were used to examine the genetic relationship between these two groups of isolates in race TTKS and five North American races (MCCF, QCCQ, RCRS, RTHS, and TPMK) that are representative of distinct lineage groups. All isolates of race TTKS shared an identical SSR genotype and were clearly different from North American races. The virulence and SSR data indicated that the new variant of race TTKS with Sr24 virulence likely has arisen via mutation within the TTKS genetic lineage. We propose to revise the North American stem rust nomenclature system by the addition of four genes (Sr24, Sr31, Sr38, and SrMcN) as the fifth set. This revision recognizes the virulence on Sr31 and differentiates isolates within race TTKS into two separate races: TTKSK and TTKST, with avirulence and virulence on Sr24, respectively. The occurrence of race TTKST with combined virulence on Sr24 and Sr31 has substantially increased the vulnerability of wheat to stem rust worldwide.

Status and strategies in breeding for rust resistance in wheat

“…The race, named as TTKS (Ug99), is virulent on majority of mega wheat varieties and can cause 100% yield losses whereas, up to 80% yield losses have been reported in Kenya. A new variant of this stem rust race has been found in Kenya since 2006, which is virulent on Sr24 [28,29]. Now a day fungicides are being used to control stem rust in Kenya [30].…”

Discovery and characterization of two new stem rust resistance genes in Aegilops sharonensis

“…To date, 11 races in the Ug99 race group have been identified and characterised by their diverse virulence spectra on defined Sr genes (Newcomb et al 2016; Singh et al 2015). Many of these Sr genes have been overcome by one or more of these races, including Sr24 (Jin et al 2008), Sr31 (Pretorius et al 2000), Sr36 (Jin et al 2009) and SrTmp (Newcomb et al 2016). Recently, a severe, localized stem rust epidemic developed on the variety Digalu in southern Ethiopia during November–December 2013.…”

Identification and mapping in spring wheat of genetic factors controlling stem rust resistance and the study of their epistatic interactions across multiple environments

“…Globally, stem rust has become a concern with the emergence of Ug99 or TTKS, a race designated after the discovery of virulence to stem rust resistance gene Sr31 in wheat nurseries in Uganda in 1999 (Pretorius et al 2000). Recently, two more important stem rust resistance genes Sr24 (Jin et al 2008) and Sr36 (Jin et al 2009) became ineffective against the TTKS race lineage of stem rust in Kenya. Ug99 and its variants are rapidly evolving making them capable of causing devastating damage to susceptible wheat cultivars in affected regions.…”