C. M. Bender

Identification of adult plant resistance to stripe rust in the wheat cultivar Cappelle-Desprez

Following the appearance of stripe rust in South Africa in 1996, efforts have been made to identify new sources of durable resistance. The French cultivar Cappelle-Desprez has long been considered a source of durable, adult plant resistance (APR) to stripe rust. As Cappelle-Desprez contains the seedling resistance genes Yr3a and Yr4a, wheat lines were developed from which Yr3a and Yr4a had been removed, while selecting for Cappelle-Desprez derived APR effective against South African pathotypes of the stripe rust fungus, Puccinia striiformis f. sp. tritici. Line Yr16DH70, adapted to South African wheat growing conditions, was selected and crossed to the stripe rust susceptible cultivar Palmiet to develop a segregating recombinant inbred line mapping population. A major effect QTL, QYr.ufs-2A was identified on the short arm of chromosome 2A derived from Cappelle-Desprez, along with three QTL of smaller effect, QYr.ufs-2D, QYr.ufs-5B and QYr.ufs-6D. QYr.ufs-2D was located within a region on the short arm of chromosome 2D believed to be the location of the stripe rust resistance gene Yr16. An additional minor effect QTL, QYr.ufs-4B, was identified in the cv. Palmiet. An examination of individual RILs carrying single or combinations of each QTL indicated significant resistance effects when QYr.ufs-2A was combined with the three minor QTL from Cappelle-Desprez, and between QYr.ufs-2D and QYr.ufs-5B.

Characterization of two new Puccinia graminis f. sp. tritici races within the Ug99 lineage in South Africa

Two new races of the wheat (Triticum aestivum L.) stem rust pathogen, representing the fifth and sixth variants described within the Ug99 lineage, were detected in South Africa. Races TTKSP and PTKST (North American notation) were detected in 2007 and 2009, respectively. Except for Sr24 virulence, race TTKSP is phenotypically identical to TTKSF, a commonly detected race of Puccinia graminis f. sp. tritici (Pgt) in South Africa. PTKST is similar to TTKSP except that it produces a lower infection type on the Sr21 differential and has virulence for Sr31. Simple sequence repeat (SSR) analysis confirmed the genetic relationship amongst TTKSF, TTKSP, PTKST and TTKSK (Ug99). TTKSK, PTKST and TTKSF grouped together with 99% similarity, while sharing 88% genetic resemblance with TTKSP. These four races in turn shared only 31% similarity with other South African races. It is proposed that both TTKSP and PTKST represent exotic introductions of Pgt to South Africa.

Components of rust resistance in lentil

Four components of resistance to Uromycesviciae-fabae, namely, latent period (LP), infection efficiency (IE), pustule size (PS) and spore production (SP) were evaluated on lentil (Lensculinaris) in glasshouse experiments. Four cultivars, Gudo, R-186, FLIP-87-66L and FLIP-89-60L, with different levels of resistance, and the susceptible check EL-142, were included in this study. The cultivars were also compared for area under the disease progress curve (AUDPC), area under the pustule density curve (APDC), apparent infection rate (rG), disease severity (DS), pustule density (PD), LP and PS under field conditions. Gudo and R-186 had significantly smaller and fewer pustules, lower spore yield and longer latent period than EL-142. FLIP-87-66L was intermediate for infection efficiency and pustule size. In addition, significant differences were found between cultivars for AUDPC, APDC, DS and rG. Estimates of AUDPC, APDC, DS and rG were reduced in Gudo, R-186 and FLIP-87-66L compared with the susceptible check EL-142. FLIP-89-60L also showed low AUDPC, APDC and DS. Some of the components obtained in the field were significantly correlated with each other (r = 0.92–0.99, P ≤ 0.05) and those measured in the glasshouse. Most of the components studied in the glasshouse were significantly (P≤ 0.05) correlated with AUDPC and DS. Data indicated the existence of incomplete [partial (PR)] resistance in the test cultivars, and the possibility of using IE, LP, SP and PS as selection criteria in the evaluation of PR to rust in lentil. Since there was an interdependence of the components, selection based on more than one component should help obtain lines with higher levels of PR. The AUDPC, DS and rG could also be used for selecting lines with PR in the field.

Microsatellite analysis of selected Puccinia triticina races in South Africa

Microsatellite analysis of ten South African races of Puccinia triticina, the causal organism of leaf rust of wheat (Triticum aestivum), divided the population into two groups sharing 32% genetic similarity. In the first group, race 3SA145 (Agricultural Research Council notation) clustered with 3SA122 and 3SA125, two races that have not been detected recently. This grouping was supported by avirulence for Lr1 in all three races. The second group included the current dominant race 3SA133, that shared 42% genetic homology with the remaining six races. Five of these, including 3SA144, formed a sub-group with 96% genetic homology amongst them. It was concluded that 3SA144, detected on triticale in 2005 and virulent for Lr32, probably represents a mutation from an existing race, whereas 3SA145 most likely was an exotic introduction. Race 3SA145 was first detected in 2009 and differed from known races in terms of combined virulence for the adult-plant resistance genes Lr12, Lr13 and Lr37. Microsatellite analysis complemented traditional phenotypic characterization of P. triticina and provided insight on genetic relationships amongst older and more recently detected races in South Africa.

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.

Development of a Greenhouse Screening Method for Adult Plant Response in Wheat to Stem Rust

Screening for adult plant resistance in wheat to stem rust, caused by Puccinia graminis f. sp. tritici, is generally conducted in field plots. Although such evaluations are successful if managed properly, field ratings are time consuming, expensive, weather dependent, and open to inoculum of unwanted races or other confounding diseases. The objective of this study was to develop a dependable system of screening the response of adult plants to stem rust under greenhouse conditions. A comparison of inoculation methods and incubation environments showed that plants inoculated with urediniospores suspended in water, followed by a 24 h dew period in a plastic chamber constructed in a greenhouse, gave the most consistent results. Measurements of response type, stem rust severity, and frequency in follow-up experiments indicated that the most reliable infection was obtained when plants sprayed with 1.25 mg urediniospores per ml water were incubated in the plastic chamber. Using the optimized protocol, a Kariega × Avocet S doubled haploid population was inoculated with two P. graminis f. sp. tritici races. Depending on the race, composite interval mapping showed flag leaf infection type to be significantly influenced by regions on chromosomes 6A, 6D, and 7D. Stem rust severity and reaction type mapped to chromosomes 6D and/or 6A. The Lr34/Yr18/Sr57 gene derived from Kariega on chromosome 7D affected the rust response on flag leaves but not on stems of greenhouse-grown plants. This study showed that phenotyping and genetic analysis of especially major effect stem rust resistance in adult wheat plants is possible and reproducible under controlled conditions in a greenhouse.

Phenotypic and genotypic description of Puccinia graminis f. sp. tritici race 2SA55 in South Africa

Race 2SA55 of the wheat stem rust pathogen, Puccinia graminis f. sp. tritici, was described for the first time during 2000 in South Africa. This race is of particular interest as it was the first local report of increased virulence towards barley cultivars by wheat stem rust. Using three original accessions of 2SA55 from the rust collection of the University of the Free State, nine single pustule isolates were established. Phenotyping of isolates to characterise 2SA55 according to the North American nomenclature system revealed variation in virulence for Sr9g on the tester lines Kubanka and Acme, which was not detected in the initial description of 2SA55. Seven isolates coded as BNGSC, which except for virulence on Sr9b resembled the initial avirulence/virulence description for 2SA55. The remaining two isolates with Sr9g virulence coded as BPGSC. The seedling infection types for the two 2SA55 derived races BNGSC and BPGSC on the Stakman differential set as well as on an extended set of Sr lines and 144 South African wheat cultivars and advanced breeding lines revealed no further distinction in reaction between them. Microsatellite analysis indicated that while race BN/PGSC shares phenotypic similarity with several of the South African non-Ug99 races, it was genetically distinct.