Characterisation and origin of rust and powdery mildew resistance genes in VPM1 wheat

Summary The expression of rust resistances conferred by closely linked genes derived from VPM1 varied with environmental conditions and with genetic backgrounds. Under low light and low temperature conditions seedlings carrying Yr17 showed susceptible responses. Stem rust and leaf rust resistance genes Sr38 and Lr37 tended to confer more resistance at 17±2° C than at normal temperatures above > 20° C. These studies supported the hypothesis that Yr17, Lr37 and Sr38 were derived from Aegilops ventricosa, whereas Pm4b was probably derived from T. persicum. Studies on certain addition lines and parental stocks indicated that wheat cytoplasm may enhance the expression of Sr38.     

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.     

Monosomic analyses of stripe rust and leaf rust resistance genes in winter wheat varieties Lüquyu and Yantar

Summary A set of 21 monosomics of Novosadska Rana-1 was used to locate the rust resistance genes of Lüqiyu, a stripe rust resistant line developed by BAU and Yantar, a leaf rust resistant wheat introduced from Bulgaria. The resistance of the former to p. striiformis race C25 was conditioned by a dominant gene located on chromosome 2B, whereas that of the latter to P. recondita race CL3 was controlled by two complementary dominant genes located on chromosomes 5A and 1D, respectively. The relationship of the stripe rust resistance gene in Lüqiyu to Yr5, Yr7 or Yr _Suwon' all located on chromosome 2B is unknown. The two complementary leaf rust resistance factors in Yantar appear to be new.     

Cytogenetic studies in wheat XVII. Monosomic analysis and linkage relationships of gene Yr15 for resistance to stripe rust

Summary The highly effective stripe rust resistance gene, Yr15, derived from Triticum dicoccoides, was located in chromosome 1BS. Yr15 showed linkage of 0.30 (34 cM) with Yr10 and 0.07 with the centromere. Yr15 was preferentially transmitted relative to its alternate allele.     

Genes for resistance to stripe rust in four spring wheat varieties

Summary Four spring wheat (Triticum aestivum L.) varieties differing in origin and reaction in the seedling stage to pathotype CDL-6 (extant in California) were intercrossed and examined in greenhouse conditions in F₁, F₂, and F₃ generations. Digenic and transgressive segregation was found in all crosses. The four varieties each had infection types (1 immune, 9 susceptible) and putative resistance genes as follows: Anza, IT 7, YrA; Glennson 81, IT 2, Yr9; Yecora Rojo, IT 6, YrC; and Ollanta, IT 4–6, YrL. Anza was classified as susceptible, Yecora Rojo and Ollanta as intermediate in seedling resistance, and Glennson 81 as resistant in the seedling stage.     

Cytogenetical studies in wheat XIX. Location and linkage studies on gene Yr27 for resistance to stripe (yellow) rust

The stripe (yellow) rust resistance gene Yr27 was located in wheat (Triticum aestivum L.) chromosome 2B and shown to be closely linked to the leaf (brown) rust resistance genes Lr13 and Lr23 in the proximal region of the short arm. Gene Yr27 was genetically independent of Lr16, which is distally located in the same arm. While Yr27 was often difficult to score in segregating seedling populations, it is apparently quite effective in conferring resistance to avirulent cultures under field conditions. The occurrence of Yr27 in Mexican wheat germplasm and the current over-dependence on Yr27 for crop protection in Asia are discussed.     

Variation of characters in near-isogenic lines of wheat with added genes for leaf rust resistance

Summary Using the cultivar Arina as the recurrent parent, six backcrosses were made with two donor lines carrying the leaf rust resistance genes Lr1 and Lr9, respectively. Selection for leaf rust resistance occurred at the seedling stage in the greenhouse; the first plants transferred to the field were BC₆F₄s. Frequency distribution of the 332 Lr1/7 × Arina and the 335 Lr9/7 × Arina lines showed continuous variation for yellow rust resistance and heading date in these leaf rust near-isogenic lines (NILs). Similar results were also obtained for plant height, for resistance to powdery mildew and glume blotch, as well as for baking quality characters in another set of more advanced NILs. The available information on the behaviour of one of the parents of cultivar Arina led to the conclusion that the expressed yellow rust resistance is quantitative and might possibly be durable.     

Molecular characterization of a novel powdery mildew resistance gene Pm30 in wheat originating from wild emmer

Powdery mildew caused by Erysiphe graminis f. sp. tritici is one of the most important wheat diseases in many regions of theworld. A powdery mildew resistance gene, originating from wild emmerwheat (Triticum dicoccoides) accession `C20', from Rosh Pinna, Israel,was successfully transferred to hexaploid wheat through crossing andbackcrossing. Genetic analysis indicated that a single dominant genecontrols the powdery mildew resistance at the seedling stage. SegregatingBC₁F₂ progenies of the cross 87-1/C20//2*8866 wereused for bulked segregant analysis (BSA). The PCR approach was used togenerate polymorphic DNA fragments between the resistant and susceptibleDNA pools by use of 10-mer random primers, STS primers, and wheatmicrosatellite primers. Three markers, Xgwm159/430,Xgwm159/460, and Xgwm159/500, were found to be linked tothe resistance gene. After evaluating the polymorphic markers in twosegregating populations, the distance between the markers and the mildewresistance gene was estimated to be 5–6 cM. By means of ChineseSpring nullisomic-tetrasomics and ditelosomics, the polymorphic markersand the resistance gene were assigned to chromosome arm 5BS and werephysically mapped on the gene rich regions of fragment length (FL) 0.41–0.43 by Chinese Spring deletion lines. As no powdery mildew resistancegene has been reported on chromosome arm 5BS, the mildew resistancegene originating from C20 should be a new gene and is designated Pm30.     

Genes for resistance to stripe rust in four spring wheat varieties

Summary The stripe rust resistant spring wheat (Triticum aestivum L.) varieties Anza, Glennson 81, Ollanta, and Yecora Rojo gave 1,2,2, and 2-gene segregations, respectively, in hybrids with susceptible Jupateco 73 when inoculated in field conditions at Davis, California USA with Puccinia striiformis West. pathotype CDL-6 and rated at post-heading stage. Intercrosses of these varieties, Anza/Yecora Rojo was not studied, permitted the following conclusions about the genes expressed in adult plants: Anza, one recessive gene; Glennson 81, two dominant genes; Ollanta, two genes, at least one is dominant; and Yecora Rojo; one dominant and one recessive gene, one of which is common with Ollanta. The resistance genes in these varieties, which expressed resistance in the seedling stage, were believed to be effective at the adult stage. Thus, seven resistance genes were identified in the four varieties. The genotypes were designated for the purposes of this study as follows: Anza, YrA YrH; Glennson 81, Yr9, YrJ, Ollanta YrL YrD; and Yecora Rojo, YrC YrD. It was recommended that these and other Yr genes be used as multiple gene complexes to increase durability of resistance to P. striiformis, an organism known to evolve virulence rapidly in field conditions. The demonstrated durability of Anza in California may be a result of its combination of resistance alleles at two loci.     

Isolation of wheat mutants with increased resistance to powdery mildew from small induced variant populations

Small variant wheat populations created by induced mutagenesis (n = 69) or adventitious regeneration (n = 66) were intensively screened for an altered response (compared to the parent variety ‘Guardian’) to the causal pathogen of powdery mildew in wheat, Blumeria graminis f. sp. tritici. Intensive field screening following natural infection of replicated plots of wheat lines over two years revealed a total of 13 mutants exhibiting significantly greater resistance than ‘Guardian’: eight from induced mutagenesis (11.6%) of the M2 population and five from adventitious regeneration (7.6%). Complete resistance was identified in two lines, (one (M66) developed following induced mutagenesis, and the other (SC240) by adventitious regeneration). The complete resistance in the induced mutant was stable over two generations and was associated with a high frequency of leaf flecking, and consequently a low grain yield. Resistance in SC240 proved to be unstable; SC240 exhibited complete resistance to powdery mildew in the SC₂ and SC₃ generations, but only 20% of the SC₄ plants were completely resistant, while the remainder were indistinguishable in mildew response to ‘Guardian’. The mildew response of all the SC₅ generation of SC240 was not significantly different from ‘Guardian’. Yield analysis of the thirteen mutants with increased resistance in the presence of powdery mildew indicated that eleven exhibitedgrain yields at least as high as that of ‘Guardian’, while the mutant M19 exhibited a yield significantly higher than that of ‘Guardian’. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Molecular mapping of powdery mildew resistance genes in wheat: A review

Powdery mildew, caused by Blumeria graminis f. sp. tritici (syn. Erysiphe graminis f. sp. tritici), is one of the most important diseases of common wheat (Triticum aestivum L.) worldwide. Molecular mapping and cloning of genes for resistance to powdery mildew in hexaploid wheat will facilitate the study of molecular mechanisms underlying resistance to powdery mildew diseases and help understand the structure and function of powdery mildew resistance genes, and permit marker-assisted selection in breeding programs. So far, 48 genes/alleles for resistance to powdery mildew at 32 loci have been identified and located on 16 different chromosomes, of which 21 resistance genes/alleles have been tagged by restriction fragment length polymorphisms (RFLPs), random-amplified polymorphic DNAs (RAPDs), amplified fragment length polymorphisms (AFLPs), sequence characterized amplified regions (SCARs), sequence-tagged sites (STS) or simple sequence repeats (SSRs). Several quantitative trait loci (QTLs) for adult plant resistance (APR) to powdery mildew have been associated with molecular markers. The detailed information on chromosomal location and molecular mapping of these genes has been reviewed. Isolation of powdery mildew resistance genes and development of valid molecular markers for pyramiding resistance genes in breeding programs is also discussed.     

Postulation of leaf (brown) rust resistance genes in 70 wheat cultivars grown in the United Kingdom

Multi-pathotype tests on 70 U.K. wheat cultivars permitted postulation of eight known seedling genes for resistance to Puccinia recondita f. sp.tritici either singly or in combinations. The most commonly detected gene was Lr13 (present in approximately 57% of cultivars), followed by Lr26 (22%), Lr37 (20%), Lr10 (17%), Lr17b (LrH) (10%), Lr1 (7%), Lr3a (6%) and Lr20(4%). This information permitted assessments of adult plant resistance (APR) in some cultivars, in field nurseries inoculated with pathotypes of P. recondita f. sp. tritici of known pathogenicities for characterized seedling resistance genes. APR was identified in eleven cultivars, including Avalon and Maris Ranger, which lacked detectable seedling resistance genes. The results provided a better understanding of specific resistances in the cultivars tested than was available from previous reports. This revised version was published online in July 2006 with corrections to the Cover Date.     

The introduction into bread wheat of a major gene for resistance to powdery mildew from wild emmer wheat

Summary A new source of resistance to wheat powdery mildew caused by Erysiphe graminis has been transferred to hexaploid bread wheat, Triticum aestivum, from the wild tetraploid wheat, Triticum dicoccoides. The donor was crossed to bread wheat and the pentaploid progeny was then self-pollinated. Plants having a near stable hexaploid chromosome complement were selected in the F₃ progeny and topcrossing and backcrossing of these to a second wheat cultivar to improve the phenotype was undertaken. Monosomic analysis of early backcross lines showed the transferred gene to be located on chromosome 4A. The gene has been designated Pm16.     

Leaf rust and stripe rust resistance genes Lr54 and Yr37 transferred to wheat from Aegilops kotschyi

The tendency of unpaired meiotic chromosomes to undergo centric misdivision was exploited to translocate leaf rust and stripe rust resistance genes from an Aegilops kotschyi addition chromosome to a group 2 chromosome of wheat. Monosomic and telosomic analyses showed that the translocation occurred to wheat chromosome arm 2DL. The introgressed region did not pair with the corresponding wheat 2DL telosome during meiosis suggesting that a whole arm may have been transferred. Female transmission of the resistance was about 55% whereas male transmission was strongly preferential (96%). The symbols Lr54 and Yr37 are proposed to designate the new resistance genes.     

Chromosome location of genes for resistance to powdery mildew in common wheat (Triticum aestivum L.) 1. Mlk and other alleles at the Pm3 locus

Summary Several wheat cultivars/lines were inoculated with isolates of Erysiphe graminis tritici to identify new genes/alleles for resistance. The wheats were tested with 13 isolates that had been characterized from responses on differential lines with known resistance genes. Gene Mlk which occurs in cultivars Kolibri, Syros, Ralle and several other European common wheats was found to be an allele at the Pm3 locus and is now designated Pm3d. The mildew resistance in an old Australian wheat, W150, is conferred by a single gene also allelic to Pm3 and now designated Pm3e. The near-isogenic line Michigan Amber/8^*Cc possesses another allele now designated Pm3f. A Syrian land variety of common wheat shows mildew resistance that is conditioned by the combination of genes Pm1 and Pm3a. Finally, two accessions of Triticum aestivum ssp. sphaerococcum appeared to possess the Pm3c allele.     

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 oat line Pc54 as a source of resistance to crown rust,stem rust and powdery mildew in Europe

Summary The oat line Pc54 was found to be resistant to powdery mildew under both field and glasshouse conditions. The ratio of resistant to susceptible F₂ and F₂ progeny of a cross between a selection from the Pc54 line (Cc7422) and a susceptible cultivar (Selma) showed that, in addition to carrying the crown rust resistance gene Pc54 and the pg15 gene for stem rust resistance, the mildew resistance of the Pc54 line was conditioned by a single incompletely dominant gene along with additional factors which modified the expression of resistance. Previous results, that there was no linkage between genes Pc54 and Pg15, were confirmed. In addition, there was no evidence of linkage between the mildew resistance gene and gene Pc54. Evaluation of selections from within the Pc54 line showed that the expression of both stem rust and mildew resistance was modified by, or linked to, plant height. The effectiveness of genes Pc54 and Pg15, as measured by virulence frequencies, in central and eastern Europe is described.     

Genetic and environmental variability in wheat characteristics reported to be involved in morphological resistance to wheat stem rust

Summary Four morphological characteristics of wheat peduncles were evaluated for genetic and environmental variability. The thickness of the epidermis and the amount and distribution of chlorenchyma tissue in the peduncle were measured. The genetic variability was estimated to be large, whereas the environmental variability was small. It was also concluded that cultivars could be screened for morphological characteristics of the peduncle that contributed to non-specific resistance to stem rust with a minimum sample size of about 10 plants per replication from a single location.Paper No 9328, Scientific Journal Series, Minnesota Agricultural Experiment Station. University of Minnesota. St. Paul. MN 55108.