Z. A. Pretorius, Department of Plant Sciences, University of the Free State, Bloemfontein 9300, South Africa;
L. J. Szabo, USDA-ARS, Cereal Disease Laboratory, University of Minnesota, St. Paul 55108;
W. H. P. Boshoff, Pannar, Bainsvlei 9338, South Africa; and
L. Herselman and
B. Visser, Department of Plant Sciences, University of the Free State, Bloemfontein 9300, South Africa
Seven races have been described in the Ug99 race group of Puccinia graminis f. sp. tritici (2). Ug99-related races previously recorded in South Africa are TTKSF, TTKSP, and PTKST (4). In December 2010, severe stem rust infection of the winter wheat cv. Matlabas was observed for the first time in South Africa. Race analysis using the 20 North American (NA) stem rust differential lines and letter code system classified the race as TTKSF. In comparative infection studies in a greenhouse, cv. Matlabas seedlings were susceptible (infection type [IT] 4) to isolate UVPgt61/1 (TTKSF+) collected from Afrikaskop in the eastern Free State, whereas the cultivar was resistant (IT 1 to 2) to stem rust isolates 2013 (TTKSF), UVPgt55 (TTKSF), UVPgt59 (TTKSP), and UVPgt60 (PTKST). Isolate 2013 represents the original collection of race TTKSF in South Africa (1). In addition to the NA differentials, no variation in the IT range of seedlings of lines with Sr7a, 8b, 12, 13, 14, 16, 18, 19, 22, 25, 26, 27, 28, 29, 32, 33, 34, 35, 39, 41, 42, 43, 44, Em, R, Tt2, and Satu was observed between UVPgt61/1 and UVPgt55. With the exception of cv. Matlabas, ITs of 106 South African cultivars likewise did not differentiate UVPgt61/1 and UVPgt55. Seedling IT studies were conducted at least twice. Microsatellite analysis (4) showed that all single pustule isolates established from the original Matlabas isolate formed part of the Ug99 group. When characterized with selected single nucleotide polymorphisms (SNPs), all single pustule isolates shared an identical genotype that differed from UVPgt55 (TTKSF), a foreign introduction into South Africa (1,3). SNP genotype analysis suggests that UVPgt61/1 is genetically dissimilar to UVPgt55, as is Zim1009, another TTKSF+ isolate that was collected from Birchenough in Zimbabwe. Studies are underway to determine the identity of the defeated Sr gene in Matlabas and the cultivar has been added to the South African stem rust differential set. TTKSF+ is the eighth race detected in the Ug99 group. Since no other cultivars or advanced lines were found to carry the Matlabas gene, it is unlikely that race TTKSF+ will threaten wheat production in South Africa. However, the occurrence of a new Ug99-related race emphasizes the variability within this internationally important group.
References: (1) W. H. P. Boshoff et al. Plant Dis. 86:922, 2002. (2) R. F. Park et al. Euphytica 179:109, 2011. (3) B. Visser et al. Mol. Plant Pathol. 10:213, 2009. (4) B. Visser et al. Euphytica 179:119, 2011.