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.