First Report of a New Wheat Leaf Rust (Puccinia triticina) Race with Virulence for Lr12, 13, and 37 in South Africa.

A new race of Puccinia triticina was collected from common wheat (Triticum aestivum) in the Eastern and Western Cape provinces during the annual rust survey in 2009. Six single-pustule isolates from a field collection, which were shown to be a new race in preliminary analyses, were inoculated onto seedlings of 16 Thatcher (Tc) near-isogenic differential lines (1) and other tester lines with known Lr genes. Standard procedures for inoculation, incubation, and rust evaluation were followed (4) and all infection studies were repeated. The low infection type of Lr18 was confirmed at 18°C. All six isolates were avirulent (infection types [ITs] 0; to 2) to Lr1, 2a, 2c, 9, 11, 16, 18, and 24 and virulent (ITs 3 to 4) to Lr3, 3ka, 10, 14a, 17, 26, 30, B, and Tc (control). The new race, named 3SA145 according to the ARC-Small Grain Institute notation, corresponds to race CCPS in the North American system (1). On the basis of seedling ITs of the extended Lr gene set, 3SA145 was avirulent (ITs 0; to 22+) to Lr2b, 19, 21, 23, 25, 28, 29, 32, 36 (E84081), 38, 45, 47 (KS90H450), 50 (KS96WGRC36), 51 (R05), and 52 and virulent to Lr3bg, 15, 20 (Thew), 27+31 (Gatcher), and 33. Lines containing the adult plant resistance (APR) genes Lr12 (RL6011, IT 3++), Lr13 (CT263, IT 3), Lr22b (Tc, IT 4), and Lr37 (RL6081, IT 3) were susceptible in the adult stage to 3SA145, whereas lines with the APR genes Lr22a (RL6044, IT ;1), Lr34 (RL6058, IT Z1), and Lr35 (RL6082, IT ;1) were resistant in controlled infection studies in a greenhouse. A control, the common race (3SA133), was virulent only on Tc adult plants. In seedlings, 3SA133 was avirulent to Lr15, 17, 26, and 27+31, but unlike 3SA145, it was virulent to Lr1, 2c, 11, 18, 24, and 28. Races 3SA133 and 3SA145 did not differ in their virulence to the remaining seedling genes. Virulence to Lr37 has been reported in several countries, including Australia, Canada, Uruguay, and the United States (1,2). Prior to the detection of 3SA145, adult plants of RL6081 were resistant to all wheat leaf rust races in South Africa. In 2009, however, RL6081 showed severity levels of up to 30S at certain Western Cape trap plot sites. Of 124 South African bread wheat cultivars and advanced breeding lines tested at the seedling stage, 3SA145 was virulent to 48, whereas 3SA133 was virulent to 36 entries. A further six entries were heterogeneous in their reaction to 3SA145. In adult plant infection studies of 48 South African spring wheats in a greenhouse, 19 were susceptible (flag leaf IT ≥3) and 22 were resistant to 3SA145. Seven entries showed a Z3 flag leaf IT indicating adult plant resistance. According to a simple sequence repeat (SSR) study using 17 primer-pair combinations described by Szabo and Kolmer (3), 3SA145 showed 30% homology with the dominant South African races. Although virulence to Lr12 and Lr13 has been known in different leaf rust races in South Africa, to our knowledge, this is the first report of combined virulence to Lr12, 13, and 37. The SSR data and unique avirulence/virulence profile suggest that 3SA145 may be an exotic introduction to South Africa. References: (1) J. A. Kolmer et al. Plant Dis. 89:1201, 2005. (2) B. McCallum and P. Seto-Goh. Can. J. Plant Pathol. 31:80, 2009. (3) L. Szabo and J. Kolmer. Mol. Ecol. Notes 7:708, 2007. (4) T. Terefe et al. S. Afr. J. Plant Soil 26:51, 2009.