Y. Jin and
L. J. Szabo, United States Department of Agriculture--Agricultural Research Service, Cereal Disease Laboratory, University of Minnesota, St. Paul 55108;
M. N. Rouse, Department of Plant Pathology, University of Minnesota, St. Paul 55108;
T. Fetch, Jr., Cereal Research Centre, Agriculture and Agri-Food Canada, Winnipeg, MB, R3T 2M9, Canada;
Z. A. Pretorius, Department of Plant Sciences, University of the Free State, Bloemfontein 9300, South Africa; and
R. Wanyera and
P. Njau, Kenyan Agricultural Research Institute, National Plant Breeding Research Center, P.O. Njoro, Kenya
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Accepted for publication 10 December 2008.
The stem rust resistance gene Sr36 confers a near-immune resistance reaction to many races of Puccinia graminis f. sp. tritici and is highly effective against race TTKSK (syn. Ug99), which possesses unusually broad virulence combinations. Because this gene is widely used in United States soft winter wheat germplasm and cultivars, it has been considered to be an important source of resistance to TTKSK. In 2007, moderately susceptible infection responses were observed on wheat lines and cultivars carrying Sr36 in a field screening nursery for stem rust at Njoro, Kenya. We derived 18 single-pustule isolates from stem rust samples collected from the 2007 Njoro nursery. The isolates were evaluated for virulence on 20 North American stem rust differential lines and on wheat lines and cultivars carrying Sr36, Sr31+Sr36, and Sr24+Sr31. Of the 18 isolates, 10 produced infection types 3+ to 4 on line W2691SrTt-1 (monogenic for Sr36) and other lines that carry Sr36 and belonged to a new virulence phenotype that was not detected in previous years. These isolates were identified as race TTTSK. The remaining eight isolates were identified as races TTKSK (five isolates) and TTKST (three isolates), with avirulence and virulence, respectively, to Sr24. Thirteen simple sequence repeat (SSR) markers were used to examine the genetic relationships among the three races in the TTKS lineage. All isolates in the lineage shared an identical SSR genotype and were clearly different from North American races. In all, 16 wheat cultivars and 60 elite breeding lines, postulated to possess Sr36, were susceptible to race TTTSK. The occurrence of race TTTSK with combined virulence on Sr31 and Sr36 has further broadened the virulence spectrum of the TTKS lineage and rendered an important source of resistance ineffective.
The American Phytopathological Society, 2009