Characterization of two new <Emphasis Type="Italic">Puccinia graminis</Emphasis> f. sp. <Emphasis Type="Italic">tritici</Emphasis> 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.     

Genes for wheat stem rust resistance postulated in German cultivars and their efficacy in seedling and adult‐plant field tests

Stem rust of wheat (caused by Puccinia graminis f.sp. tritici) gained high international attention in the last two decades, but does not occur regularly in Germany. Motivated by a regional epidemic in 2013, we analysed 15 spring and 82 winter wheat cultivars registered in Germany for their resistance to stem rust at the seedling stage and tested 79 of these winter wheat cultivars at the adult‐plant stage. A total of five seedling stem rust resistance genes were postulated: Sr38 occurred most frequently (n = 29), followed by Sr31 (n = 11) and Sr24 (n = 8). Sr7a and Sr8a occurred only in two spring wheat genotypes each. Four cultivars had effective seedling resistance to all races evaluated that could only be explained by postulating additional resistance genes (‘Hyland’, ‘Pilgrim PZO’, ‘Tybalt’) or unidentified gene(s) (‘Memory’). The three winter wheat cultivars (‘Hyland’ ‘Memory’ and ‘Pilgrim PZO’) were also highly resistant at the adult‐plant stage; ‘Tybalt’ was not tested. Resistance genes Sr24 and Sr31 highly protected winter wheat cultivars from stem rust at the adult‐plant stage in the field. Disease responses of cultivars carrying Sr38 varied. Mean field stem rust severity of cultivars without postulated seedling resistance genes ranged from 2.71% to 41.51%, nine of which were significantly less diseased than the most susceptible cultivar. This suggests adult‐plant resistance to stem rust may be present in German wheat cultivars.     

Evaluation of seedling and adult plant resistance to stem rust in European wheat cultivars

A total of 105 European wheat cultivars were assessed for seedling and adult plant resistance (APR) to stem rust using an array of Australian isolates of Puccinia graminis f. sp. tritici. Twenty-seven cultivars were susceptible at both seedling and adult plant growth stages. Twelve catalogued seedling stem rust resistance genes (Sr7b, Sr8a, Sr8b, Sr9b, Sr9g, Sr11, Sr15, Sr17, Sr29, Sr31, Sr36 and Sr38) were detected in the remaining cultivars, and 13 cultivars carried additional seedling resistance genes that could not be postulated with the isolates used. Low levels of APR to stem rust were found in the cultivars Artaban, Forno, Mec, Mercia, Pandas and Vlada. Although the genetic identity of this APR was not determined, it was clear that the only designated stem rust APR gene Sr2 was not present in any of the cultivars tested based on the absence of the linked traits seedling chlorosis and pseudo black chaff. One of these cultivars, Forno, is believed to carry the leaf rust APR gene Lr34, previously reported to be associated with improved resistance to stem rust. A detailed genetic characterisation of the APRs in these cultivars will be needed to understand their modes of inheritance and relationships with catalogued stem rust resistance genes. Such knowledge may help in developing cultivars with effective gene combinations that confer higher levels of protection.     

The linkage between the stem rust resistance gene Sr2 and pseudo-black chaff in wheat can be broken

Recessively inherited gene Sr2 has provided the basis of durable resistance to stem rust (caused by Puccinia graminis tritici) in wheat (Triticum aestivum L.) worldwide. The associated earhead and stem melanism or ‘pseudo‐black chaff’ is generally used as a marker for this gene. Sr2 has been postulated in many wheat cultivars of India including ‘Lok 1’, based on associated pseudo‐black chaff in adult plants, and leaf chlorosis in seedlings. However, dominant inheritance of the resistance factor operating in ‘Lok 1’, and a 13 : 3 (resistant : susceptible) F₂ segregation in the ‘Sr2‐line’ (‘Chinese Spring’⁶ × ‘Hope’ 3B) × ‘Lok 1’ cross confirmed that Sr2 was absent in ‘Lok 1’. Susceptible plants with a pseudo‐black chaff phenotype were observed in F₂ populations of ‘Agra Local’ (susceptible) × ‘Lok 1’, and the ‘Sr2‐line’ × ‘Lok 1’ crosses. Most of the F₃ families derived from the susceptible F₂ segregants with pseudo‐black chaff phenotypes were true breeding for the expression of pseudo‐black chaff with susceptibility to stem rust. Thus, linkage of pseudo‐black chaff with Sr2 in wheat can be broken, and hence, caution may be exercised in using pseudo‐black chaff as a marker for selecting Sr2 in breeding programmes.     

Induced mutagenesis for the development of multiline cultivars in bread wheat

Summary Variation for resistance toPuccinia graminis f.sp.tritici, P. recondita f.sp.tritici andP. striiformis was induced in theTriticum aestivum cultivar Lalbahadur using nitrosomethyl urea. Variations were isolated from the M₂ population in the post-seedling stage in the field when infected with a mixture of races of each of the three rusts. Plants exhibiting simultaneous resistance to stem rust, leaf rust and yellow rust were indentified. Repeated screening in the subsequent generations confirmed the resistance of the mutant lines that are morphologically similar to the parental cultivar. The rust resistance of 20 mutant lines was also confirmed at the seedling stage using individual races of stem rust and leaf rust. The different patterns observed in the mutant lines tested against a wide range of races show that these lines can be used as components of a multiline. The patterns of variation compared with those of the known genes for resistance against the Indian races of the pathogens suggest that the mutations for rust resistance are due to factor different from those already known in bread wheat, providing a broadened genetic base for future breeding programmes.     

Analysis of durable resistance to stem rust in barley

Summary Since the mid-1940's, barley cultivars grown in the northern Great Plains of the USA and Canada have been resistant to stem rust caused byPuccinia graminis f. sp.tritici. This durable resistance is largely conferred by a single gene,Rpg1, derived from a single plant selection of the cultivar Wisconsin 37 and an unimproved Swiss cultivar. At the seedling stage, barley genotypes withRpg1 generally exhibit low mesothetic reactions at 16–20° C and slightly higher mesothetic reactions at 24–28° C to many stem rust pathotypes. This resistance is manifested by a low level of rust infection and mostly incompatible type uredia on adult plants.Rpg1 reacts in a pathotype-specific manner since some genotypes ofP. g. f. sp.tritici are virulent on cultivars carrying this gene in the field. Several factors may have contributed to the longevity of stem rust resistance in barley, a) since barley is planted early and matures early, it can sometimes escape damage from stem rust inoculum carried from the south; b) one or more minor genes may augment the level of resistance already provided byRpg1; c) the cultivation of resistant wheat cultivars and eradication of barberry have reduced the effective population size and number of potential new pathotypes ofP. g. f. sp.tritici, respectively; and d) virulent pathotypes ofP. g. f. sp.tritici andP. g. f. sp.secalis have not become established. This situation changed in 1989 when a virulent pathotype (Pgt-QCC) ofP. g. f. sp.tritici became widely distributed over the Great Plains. However,Rpg1 may still confer some degree of resistance to pathotype QCC because stem rust severities have been low to moderate and yield losses light on barley cultivars carrying the gene during the last four seasons (1989–1992). Several sources of incomplete resistance to pathotype QCC have been identified in barley. To facilitate the transfer of resistance genes from these sources into advanced breeding lines, molecular marker assisted selection is being employed.     

Development of SCAR markers for identification of stem rust resistance gene Sr31 in the homozygous or heterozygous condition in bread wheat

The stem rust resistance gene Sr31, transferred from rye (Secale cereale) into wheat (Triticum aestivum L.) imparts resistance to all the virulent pathotypes of stem rust (Puccinia graminis f. sp. tritici) found in India. Wheat genotypes including carriers and non‐carriers of the Sr31 gene were analysed using arbitrary primed polymerase chain reaction (AP‐PCR). AP‐PCR markers viz. SS30.2_580(H) associated with the Sr31 gene and SS26.1₁₁₀₀ associated with the allele for susceptibility were identified. Linkage between the markers and phenotypes was confirmed by analysing an F₂ population obtained from a cross between a resistant and a susceptible genotype. The markers were tightly linked to the respective alleles. Both the AP‐PCR markers were converted into sequence characterized amplified region (SCAR) markers, viz. SCSS30.2₅₇₆ and SCSS26.1₁₁₀₀ respectively. The markers were validated in two more segregating populations and 49 wheat genotypes. Using both markers it was possible to distinguish the homozygous from the heterozygous carriers of the Sr31 gene in the F₂ generation. The markers developed in this study can be used for pyramiding of the Sr31 gene with other rust resistance genes and in marker‐assisted selection.     

Genomic selection for durable stem rust resistance in wheat

Inheritance of stem rust (caused by Puccinia graminis f. sp. tritici) resistance in wheat can be either qualitative or quantitative. While quantitative disease resistance is believed to be more durable, it is more difficult to evaluate if it is expressed only in mature plants, i.e. adult plant resistance (APR). Marker-assisted selection (MAS) methods for APR would be useful; however, the multigenic nature of APR impedes the use of MAS efforts that aim to pyramid only a few target genes. A promising alternative is genomic selection (GS), which utilizes genome-wide marker coverage to predict genotypic values for quantitative traits. In turn, GS can reduce the selection cycle length of a breeding program for traits like APR that could take several seasons to generate reliable phenotypes. In this paper, we describe the GS process for use in crop improvement, both specifically for APR and in general. We also propose a GS–based wheat breeding scheme for quantitative resistance to stem rust that, when compared to current breeding schemes, can reduce cycle time by up to twofold and facilitates pyramiding of major genes with APR genes. Thus, GS could be an important tool for achieving the Borlaug Global Rust Initiative’s (BGRI) goal of developing durable stem rust resistance in wheat.     

A test of supposed mutants for stem rust resistance in wheat

Summary Eight stem rust (Puccinia graminis tritici Eriks. and Henn.) resistant lines (designated TICENA lines) that had been selected by Veiga et al. (1981) following gamma radiation of BH-1146 wheat (Triticum aestivum L.) were studied. Six of the lines were resistant to race 15B-1 of stem rust and susceptible to race 56, and proved to carry the gene Sr7a. TICENA 4 carries two unidentified genes, each giving resistance to one of the two races. TICENA 10 carries Sr6, Sr7a and an unidentified gene giving resistance to race 56 but not 15B-1. The results raise doubts about the supposed origin of the lines as mutants.     

Role of <Emphasis Type="Italic">Berberis</Emphasis> spp. as alternate hosts in generating new races of <Emphasis Type="Italic">Puccinia graminis</Emphasis> and <Emphasis Type="Italic">P. striiformis</Emphasis>

The common barberry and several other Berberis spp. serve as the alternate hosts to two important rust pathogens of small grains and grasses, Puccinia graminis and P. striiformis. Barberry eradication has been practiced for centuries as a means to control stem rust. Diverse virulence variations have been observed in populations of P. graminis f. sp. tritici that were associated with susceptible barberries in North America. Barberry likely has played a role in generating new races of P. striiformis f. sp. tritici in some regions in the world. Several North American stem rust races, namely races 56, 15B and QCC, initially originated from barberry, were subsequently responsible for generating large-scale epidemics. Thus, sexual cycles on Berberis spp. may generate virulence combinations that could have serious consequences to cereal crop production.     

Stripe rust resistance and genes in Chinese wheat cultivars and breeding lines

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most important diseases on wheat in China. To assess resistance in wheat cultivars and breeding lines in China, 330 leading cultivars and 164 advanced breeding lines were evaluated with stripe rust. In the greenhouse tests, seedlings of the entries were inoculated separately with several Pst pathotypes. In the field tests, the entries were evaluated for stripe rust resistance in Yangling, Shaanxi Province artificially inoculated and in Tianshui, Gansu Province under natural infection of Pst. The oversummering/wintering and spring epidemic zones of resistance genes were postulated using molecular markers for Yr5, Yr9, Yr10, Yr15, Yr17, Yr18, and Yr26, in combination with resistance spectra. Out of the 494 wheat entries, 16 (3.24 %) entries had all-stage resistance (ASR) in all race tests, 99 (20.04 %) had adult-plant resistance (APR), 28 (5.67 %) were considered to have slow-rusting (SR), and 351 (71.05 %) were susceptible to one or more races in both seedling and adult-plant stages. Advanced breeding lines had a higher percentage (37.2 %) of resistant entries (The sum of ASR, APR and SR) than leading cultivars (24.85 %). Among the epidemic regions, southern Gansu had a higher percentage of resistant entries than any other regions. Based on stripe rust reactions and molecular markers, two cultivars were found to possibly have Yr5 while no entries have Yr10 or Yr15. Resistance genes Yr9, Yr17, Yr18, and Yr26 were found in 134 (29.4 %), 45 (9.1 %), 10 (2 %), and 15 (3 %) entries, respectively.     

Genetic mapping of seedling and adult plant stem rust resistance in two European winter wheat cultivars

A recombinant inbred line (RIL) population derived from the cross Arina/Forno was field tested for 2 years against Puccinia graminis f. sp. tritici under artificially created epidemic conditions. Both parents showed intermediate adult plant stem rust responses and the RIL population showed continuous variation for this trait. Composite interval mapping identified genomic regions controlling low stem rust response on chromosomes 5B and 7D consistently across all experiments. These genomic regions were named QSr.Sun-5BL and QSr.Sun-7DS and explained on an average 12% and 26% of the phenotypic variation in adult plant stem rust response, respectively. QSr.Sun-5BL mapped close to Xglk0354 and was contributed by Arina. The Lr34-linked markers csLV34 and swm10 were closely associated with QSr.Sun-7DS suggesting the involvement of Lr34 in controlling adult plant stem rust response of cultivar Forno. Additional minor and inconsistent QTLs explaining variation in adult plant stem rust response were identified on chromosome arms 1AS and 7BL. The QTL located on chromosome 7BL corresponded to the stem rust resistance gene Sr17 carried by cultivar Forno. A seedling stem rust resistance gene carried by Arina, SrAn1, was ineffective under field conditions and was mapped on the long arm of chromosome 2A. Genotypes carrying combinations of QSr.Sun-5BL and QSr.Sun-7DS based on positive alleles of the respective closest marker loci Xglk0354 and XcsLV34 or Xswm10 exhibited a lower response than either parent indicating an additive effect of these genes. Transfer of these genes into cultivars carrying Sr2 would provide a more effective and durable resistance against the stem rust pathogen. Markers csLV34 and/or swm10 could be used in marker assisted selection of QSr.Sun-7DS in breeding programs.     

Genes for resistance to wheat powdery mildew in derivatives of Triticum timopheevi and T. carthlicum

Summary The winter wheat line TP 114 derived from CI 12633, a Triticum timopheevi derivative, has two unlinked dominant genes conditioning resistance to the powdery mildew fungus (Erysiphe graminis f. sp. tritici). One of the genes is identical to gene Pm2 (Ml _u ). The other gene differs from the eleven Pm and/or Ml designated genes; a temporary designation, Ml _f ,is proposed for this gene. Gene Ml _f is closely associated with a gene conditioning resistance to the stem rust fungus (Puccinia graminis f. sp. tritici), probably gene Sr9c.The winter wheat line TP 229 derived from Triticum carthlicum has one dominant mildew resistance gene identical to gene Ml _e in Weihenstephaner M 1.     

The inheritance of stem rust and leaf rust resistance in some Ethiopian wheat collections

Summary Common and durum wheat populations obtained from Sweden and originally collected in Ethiopia were screened for resistance to steum rust and leaf rust. Resistant selections of common wheat were crossed and backcrossed with either stem rust susceptible RL6071, or leaf rust susceptible Thatcher. Genetic studies, based largely on tests of backcross F₂ families, showed that four of the selections had in common a recessive gene SrA. Plants with this gene were resistant (1⁺ infection type) to all stem rust races tested. This gene was neither Sr26 nor Sr29. The resistance of other selections, based on tests with an array of rust isolates, was due to various combinations of Sr6, 8a, 9a, 9d, 9c, 11, 13, 30, and 36. One of the selections had linked genes, Lr19/Sr25. Another selection had a dominant gene for resistance (;1 infection type) to all the races of leaf rust. With the possible exception of this gene for leaf rust resistance and SrA, no obviously new resistance was found.     

Identification of recombinants carrying stripe rust resistance gene Yr57 and adult plant stem rust resistance gene Sr2 through marker‐assisted selection

Many stem rust resistance genes have been formally named in wheat. Adult plant stem rust resistance gene Sr2 was mapped in the short‐arm of chromosome 3B. Stripe rust resistance gene Yr57, identified in Aus91463, was mapped about 5 cM away from Sr2 based on its linkage with Sr2‐linked marker gwm533. The objective of this study was to combine Sr2 and Yr57 in a single genotype. A mapping population containing 107 recombinant inbred lines was developed from a cross between Aus91463‐Yr57 and Hartog‐Sr2. This population was tested at the seedling stage in the glasshouse for variation in stripe rust response, and high temperature induced Sr2‐linked seedling chlorosis. The RIL population was screened for Sr2‐linked pseudo black chaff phenotype at the adult plant stage in field. Five recombinants carrying Sr2 and Yr57 in coupling were detected using phenotypic and marker data. Four recombinants also carried leaf rust resistance gene Lr23 from Aus91463. These recombinants are being used as triple rust resistance source in the Australian Cereal Rust Control Program.     

Identification and chromosomal locations of novel genes for resistance to powdery mildew and stripe rust in a wheat line 101-3

Wheat powdery mildew and stripe rust, caused by Blumeria graminis f.sp.tritici (syn. Erysiphe graminis f.sp.tritici) and Puccinia striiformis Westend., respectively, are two important fungal diseases of wheat in many regions in the world that cause significant annual yield losses. In the present study, a dominant powdery mildew and a dominant stripe rust resistance gene in wheat line 101-3 which derived from the progenies of the wide cross between common wheat and Dasypyrum villosum Candary L., was located on chromosome 6B and 1B, respectively, by monosomic analyses. The two genes are different from known resistance genes on chromosome 6B for powdery mildew and 1B for stripe rusts, suggesting that the two genes might be novel resistance genes for powdery mildew and stripe rust, respectively. It is uncertain whether the two genes are allelic or lined with other resistance genes located on chromosome 6B for powdery mildew and 1B for stripe rust. Further allelism tests are necessary to determine the relationships between the resistance gene and other genes located on chromosome 6B for powdery mildew and 1B for stripe rust through molecular markers.     

Lack of durability of resistance to cereal rusts in wheat when selection is for complete resistance

Twenty-one bread-wheat entries were selected after careful screening for complete or near-complete resistance to yellow rust (Puccinia striiformis), stem rust (P. graminis), and leaf rust (P. recondita). In 1987, the 21 entries were intercrossed in a near-half diallel scheme. The resulting 190 F₂ populations were advanced to F₇ under selection for complete resistance to the three rusts and for good agronomic types. In 1992 the 21 parents and 140 selected F₇ lines were assessed for their resistance to the three rusts. Of the 21 parents, 12 showed a breakdown of yellow rust resistance, five a breakdown of stem rust resistance and two a breakdown of leaf rust resistance. In addition, several of the 140 selected F₇ lines, all still resistant in F₆, had become susceptible to one or more of the rusts. It appears that a progression towards more complex races, especially of yellow rust, is inevitable for the wheat-cereal rust patho-systems when the selection is for complete or near-complete resistance.     

Development of a PCR assay and marker-assisted transfer of leaf rust resistance gene <Emphasis Type="Italic">Lr58</Emphasis> into adapted winter wheats

Leaf rust resistance gene Lr58 derived from Aegilops triuncialis L. was transferred to the hard red winter wheat (HRWW) cultivars Jagger and Overley by standard backcrossing and marker-assisted selection (MAS). A co-dominant PCR-based sequence tagged site (STS) marker was developed based on the sequence information of the RFLP marker (XksuH16) diagnostically detecting the alien segment in T2BS·2BL-2^tL(0.95). STS marker Xncw-Lr58-1 was used to select backcross F₁ plants with rust resistance. The co-dominant marker polymorphism detected by primer pair NCW-Lr58-1 efficiently identified the homozygous BC₃F₂ plants with rust resistance gene Lr58. The STS marker Xncw-Lr58-1 showed consistent diagnostic polymorphism between the resistant source and the wheat cultivars selected by the US Wheat Coordinated Agricultural Project. The utility and compatibility of the STS marker in MAS programs involving robust genotyping platforms was demonstrated in both agarose-based and capillary-based platforms. Screening backcross derivatives carrying Lr58 with various rust races at seedling stage suggested the transferred rust resistance in adapted winter wheats is stable in both cultivar backgrounds. Lr58 in adapted winter wheat backgrounds could be used in combination with other resistance genes in wheat rust resistance breeding.     

Identification of QTL associated with durable adult plant resistance to stem rust race Ug99 in wheat cultivar ‘Pavon 76’

Stem rust of wheat, caused by Puccinia graminis f. sp. tritici, was under control worldwide for over 30 years by utilizing genetic resistance. The emergence of stem rust in 1998 in eastern Africa in form of race Ug99 and its evolving variants with virulence to many resistance genes were recognized as potential threats to wheat production. In this study we identified genomic regions contributing to putatively durable, adult plant resistance (APR) to wheat stem rust. A recombinant inbred line (RIL) population of 298 lines was previously developed at CIMMYT from a cross between ‘Avocet S’ and ‘Pavon 76’. Pavon 76 has been described to carry APR to stem rust. Avocet S carries the race-specific resistance gene Sr26. A subset of RILs without Sr26 segregated for APR to stem rust race Ug99 when evaluated in Kenya for three years. Single year and joint year analysis by inclusive composite interval mapping using 450 DArT markers identified five quantitative trait loci (QTL) that contributed to the resistance of wheat to stem rust race Ug99. Three of these, including QSr.cim-3B, which probably represents the Sr2 gene, were contributed by Pavon 76 whereas the remaining two QTL were contributed by Avocet S. QSr.cim-3B, or putatively Sr2, on chromosome arm 3BS explained 32 % of the phenotypic variation while the additional QTL in Pavon contributed 24 and 20 %, respectively. Two QTL from Avocet S explained 8 and 6 % of phenotypic variance, respectively. A combination of APR QTL from the two parents resulted in transgressive segregants expressing higher levels of resistance than Pavon 76. Our results indicate that it is possible to accumulate several minor resistance genes each with a small to intermediate effect resulting in a variety that exhibits negligible disease levels even under high stem rust pressure.     

Development and characterization of a new 1BL.1RS translocation line with resistance to stripe rust and powdery mildew of wheat

The 1BL.1RS wheat-rye translocation from Petkus rye has contributed substantially to the world wheat production. However, following the breakdown of disease resistance genes in 1RS, its importance for wheat improvement decreased. We have developed a new 1BL.1RS line, R14, by means of crossing rye inbred line L155, selected from Petkus rye to several wheat cultivars. One new gene each, for stripe rust and powdery mildew resistance, located on 1RS of the line R14, are tentatively named YrCn17 and PmCn17. YrCn17 and PmCn17 confer resistance to Puccinia striiformis f. sp. tritici pathotypes that are virulent on Yr9, and Blumeria graminis f. sp. tritici pathotypes virulent on Pm8. These two new resistances, YrCn17 and PmCn17, are now available for wheat improvement programs. The present study indicates that rye cultivars may carry yet untapped variations as potential sources of resistance.