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

Abstract

A new race of Puccinia triticina was collected from common wheat {Triticum aestivum) in the Eastem 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 Lrl8 was confirmed at I8°C. All six isolates were avirulent (infection types [ITs] 0; to 2) to Lrl, 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), 5/ (R05), and 52 and virulent to Lr3bg, 15, 20 (Thew), 27+31 (Gatcher), and 33. Lines containing the adult plant resistance (APR) genes Lrl2 (RL601I, IT 3++), Lrl3 (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 Zl), and Lr35 (RL6O82, 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 Lrl5, 17, 26, and 27+31, but unlike 3SA145, it was virulent to Lrl, 2c, II, 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 vimlent 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 Z,ry2 and Lrl3 has been known in different leaf rust races in South Africa, to our knowledge, this is the first report of combined virulence to Lrl2, 13, and 37. The SSR data and unique avirulence/virulence profile...

Microsatellite analysis of selected Puccinia triticina races in South Africa

“…1). The occurrence of virulence for Lr37 in 3SA145 (Terefe et al 2011) follows similar reports in countries such as Canada (McCallum and Seto-Goh 2009), Australia and France (Huerta-Espino et al 2011), andUruguay (German et al 2007) where the gene has become ineffective. In France the cultivation of wheat cultivars containing Lr37 contributed to the rapid appearance of Lr37 virulence (Huerta-Espino et al 2011).…”

“…Races 3SA125 and 3SA133 both attack Lr22b but are avirulent for Lr12 and Lr13, and 3SA122 is virulent for Lr12 and Lr22b, but avirulent for Lr13. Virulence for Lr37 has only been found in 3SA145, a race which also combines virulence for Lr12, Lr13 and Lr22b (Terefe et al 2011). No virulence has been detected in South Africa for Lr22a, Lr34 and Lr35 (Pretorius, unpublished data).…”

“…McIntosh (1992) included migration, mutation, recombination, selection and chance as the major evolutionary forces influencing cereal rust populations. Although all of these, except sexual recombination, are applicable to wheat leaf rust in South Africa, the local Pt population is small and relatively stable (Pretorius et al 2007) with only two new races detected in the field since 1988 (Pretorius and Bender 2010;Terefe et al 2011).…”

“…Race 3SA144 with virulence for Lr32 was collected from triticale in the Western Cape in 2005 (Pretorius and Bender 2010). In 2009 virulence for Lr37 was detected for the first time in South Africa when 3SA145 was collected from the Western and Eastern Cape provinces (Terefe et al 2011). This race is virulent for the adult-plant resistance genes Lr12, Lr13, Lr22b and Lr37.…”

Molecular and genetic study of wheat rusts

“…However, the use of these resistant cultivars must be carefully managed because if they are introduced and used too widely (effectively creating a monoculture) the rusts will quickly develop new virulences due to the very high selective pressures being placed upon them (McDonald and Linde, 2002) as can be seen with the emergence of race TTKS in East Africa in 1999 (Pretorius et al, 2000; Expert Panel on the Stem Rust outbreak in East Africa, 2005) and its rapid spread and acquisition of new virulences (Terefe et al, 2011;Pretorius et al, 2012). Breeding a new resistant cultivar requires extremely long timeframes; sometimes above of fifteen years can pass between the initial cross made for a cultivar and the first release of commercial seed.…”