Studies On Wheat Resistance To UG99

Abstract

Wheat stem rust caused by the fungus <italic>Puccinia graminis</italic> f. sp. <italic>tritici</italic> (<italic>Pgt</italic>) has historically been a devastating disease of wheat. In 1998, a race of <italic>Pgt</italic> was detected in Uganda that is virulent to the majority of wheat cultivars being grown. This new race, commonly referred to as Ug99, was characterized as race TTKSK based upon the North American stem rust differentials. Race TTKSK has recently spread throughout eastern and southern Africa, and into the Middle East. Data suggest that Ug99 can alternatively be described as <italic>Pgt</italic> race PTKSK, based upon demonstrated avirulence to stem rust resistance gene <italic>Sr21</italic>. Accessions of wild relatives of wheat, <italic>Triticum monococcum</italic>, <italic>T</italic>. <italic>urartu</italic>, and <italic>Aegilops tauschii</italic> were screened with <italic>Pgt</italic> race TTKSK. Crosses among resistant and susceptible accessions of <italic>T</italic>. <italic>monococcum</italic> led to the characterization of two new stem rust resistance genes effective to <italic>Pgt</italic>-TTKSK. Preliminary screening of 700 accessions of spring wheat led to the identification of 88 accessions with resistance to Ug99. Examination of the genetics of resistance in one of these accessions, SD 1691, identified <italic>Sr28</italic> as conferring a high level of resistance. Molecular markers linked to <italic>Sr28</italic> were identified on chromosome arm 2BL. Examination of the genetics of resistance in Gabo 56 indicated the presence of a single dominant gene. This gene was temporarily designated as <italic>SrGabo56</italic> and mapped on chromosome arm 2BL. Segregation of resistance in the progeny of the cross between SD 1691 and Gabo 56 indicated that <italic>Sr28</italic> and <italic>SrGabo56</italic> are linked. Characterization of the genetics of resistance to <italic>Pgt</italic>-TTKSK in synthetic wheat TA4152-37, resulted in the identification of <italic>Sr13</italic> in TA4152-37. Molecular markers linked to <italic>Sr13</italic> in hexaploid wheat were identified on chromosome arm 6AL. Overall, 3 new stem rust resistance genes effective to <italic>Pgt</italic>-TTKSK were identified. Molecular markers linked to <italic>SrGabo56</italic>, <italic>Sr13</italic>, and <italic>Sr28</italic> were described. The identification of markers linked to multiple stem rust resistance genes will facilitate the combination of these genes in breeding lines. The identification of the new sources of resistance to <italic>Pgt</italic>-TTKSK will provide plant breeders with additional tools to protect wheat from this dangerous race.