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.     

Race-specificity of temperature-sensitive genes for resistance to Puccinia striiformis in Triticum dicoccoides

Summary Twenty-four entries of wild emmer possessing temperature-sensitive genes for resistance to yellow rust were studied in the seedling stage, at two temperature-profiles, with 15 pathogenic races from 11 countries in South America, Africa, Asia and Europe. It was shown that the majority of the resistance genes in these wild emmer entries were race-specific. In most of these entries a more resistant reaction was displayed at the higher temperature-profile; however in three entries a shift in reaction towards resistance was observed with certain races but towards susceptibility with some of the other races, suggesting that two different kinds of temperature-sensitive genes were involved in each of these entries. The similarity of temperature-sensitive genes occurring in wild emmer and in cultivated wheat is discussed.     

Components of adult plant resistance to leaf rust in wheat

Summary Winter wheat genotypes were tested for resistance in the field by assessing the percentage sporulating leaf area after infection with wheat leaf rust. The disease level in the first field trial was too low for selection. In the second field trial a low sporulating leaf area was found on several genotypes showing a susceptible infection type. These genotypes possibly have partial resistance. Six genotypes possibly possess adult plant resistance, as they showed a resistant infection type and a low sporulating leaf area.The latency period, infection frequency and uredosorus size of sixteen genotypes were determined in the greenhouse after infection with two races of leaf rust at two temperature regimes. The temperature × genotype interaction, found for latency period and infection frequency, was mostly influenced by the cultivars Cerco, Tundra and Miller. Adult plant resistance was postulated for four genotypes whereas another four appeared to have partial resistance.Only one of the sixteen genotypes (Apexal) possessed adult plant resistance and two genotypes (Arminda and Cappelle Desprez) showed partial resistance in the field as well as in the greenhouse.     

Molecular mapping and detection of the yellow rust resistance gene Yr26 in wheat transferred from Triticum turgidum L. using microsatellite markers

Yellow rust (stripe rust), caused by Puccinia striiformis Westend f. sp. tritici, is one of the most devastating diseases of wheat throughout the world. Wheat-Haynaldia villosa 6AL.6VS translocation lines R43, R55, R64 and R77, derived from the cross of three species, carry resistance to both yellow rust and powdery mildew. An F₂ population was established by crossing R55 with the susceptible cultivar Yumai 18. The yellow rust resistance in R55 was controlled by a single dominant gene, which segregated independently of the powdery mildew resistance gene Pm21 located in the chromosome 6VS segment, indicating that the yellow rust resistance gene and Pm21 are unlikely to be carried by the same alien segment. This yellow rust resistance gene was considered to beYr26, originally thought to be also located in chromosome arm 6VS. Bulked Segregation Analysis and microsatellite primer screens of the population F₂ of Yumai 18 × R55 identified three chromosome 1B microsatellite locus markers, Xgwm11, Xgwm18 and Xgwm413, closely linked to Yr26. Yr26 was placed 1.9 cM distal of Xgwm11/Xgwml8, which in turn were 3.2 cM from Xgwm413. The respective LOD values were 21 and 36.5. Therefore, Yr26 was located in the short arm of chromosome 1B. The origin and distribution of Yr26 was investigated by pedigree, inheritance of resistance and molecular marker analysis. The results indicated that Yr26 came from Triticum turgidum L. Three other 6AL.6VS translocation lines, R43, R64 and R77, also carried Yr26. These PCR-based microsatellite markers were shown to be very effective for the detection of the Yr26 gene in segregating populations and therefore can be applied in wheat breeding. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of genes for resistance to Puccinia striiformis f. sp. hordei in 18 barley genotypes

Barley genotypes Hor 1428, Hor 2926, Hor 3209, BBA 2890, Abyssinian 14, Grannelose Zweizeilige, and Stauffers Obersulzer are resistant to all races of Puccinia striiformis f. sp. hordei so far detected in the U.S.A. Heils Franken, Cambrinus, Astrix, Emir, Hiproly, Varunda, Trumpf,Mazurka, Bigo, BBA 2890, and I 5 are resistant to some races and susceptible to others. Previous studies showed that Hor 1428, Hor 2926, Hor 3209, Abyssinian 14, Stauffers Obersulzer, I 5, Heils Franken, Emir, Astrix, Hiproly, Varunda, and Trumpf each have two genes, and BBA 2890, Grannelose Zweizeilige, Cambrinus, Mazurka, Bigo, and BBA 809 each have a single genefor resistance. To determine the genes in specific genotypes and their relationships, all possible crosses were made among the 18 genotypes. Seedlings of parents and F₂ progeny were tested under controlledconditions for resistance to selected races that were avirulent on both parents. Based on segregation within the individual crosses to selected races, at least 26 of 30 genes detected in the 18 genotypes were different. Allelic and linkage relationships of some of the genes were determined. The genetic information should be useful for understanding the host-pathogen interactions and for control of stripe rust using resistance.     

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.     

Evolution of virulence patterns in yellow rust races and its implications for Ereeding for resistance in wheat in Kenya

Summary Virulence patterns of yellow rust isolates collected in Kenya between 1986–1989 were compared with earlier results. The number of virulence factors per race and the range in virulence factors both increased considerably. Before 1976 races carried on average 4.5 to 5.0 virulence factors, whereas the races after 1986 had a mean of 6.5 virulence factors. The range in the number of virulence factors increased from some seven to eight in the first period to 12 in the second out of the 17 evaluated. In the period 1986–1989 another three virulence factors (2, 9 and A) were assessed. All three occurred at a high frequency.Virulence neutralizing the resistance genes Yr2, Yr2+, Yr6, Yr6+, Yr7, Yr7+, Yr8, Yr9, Yr9+ and those in the cultivars Anza (A), Strubes Dickkopf (SD) and Suwon92/Omar (SU) occurred at a high frequency, while virulence for Yr3V, Yr4+, Yr5, CV and SP (resistance in Carstens V and Spaldings Prolific resp.) were not found. The remaining three virulence factors for Yr1, 10 and 3N were rare.In the past ten years the resistance of most released cultivars became ineffective in less than six years. They were shown to carry race-specific major resistance genes such as Yr7+, Yr9+, SD and A. However, in the field, the resistance of the cultivars was not completely neutralized. A residual resistance, ranging from moderate to fairly high, was observed in all cultivars in which the major gene resistances were neutralized by corresponding virulence genes.Other wheat cultivars such as Africa Mayo, Kenya Kudu, Enkoy, Kenya Leopard, Bounty, Frontatch, Bonny and Kenya Plume appeared to keep their resistance over a condiserable period of time. They are considered to be durably resistant to the Kenyan yellow rust populations. This form of resistance, together with the residual resistance, can be recommended for use in breeding programmes.     

Wheat mutants showing altered adult plant disease resistance

Wheat mutants, selected on an altered resistance phenotype to Puccinia striiformis f.sp. tritici, the causal agent of yellow rust, were assessed in greenhouse tests to determine the growth stage at which the mutant phenotype was expressed and which components of yellow rust resistance were altered. Four of the mutants showed reduced levels of yellow rust infection, while three exhibited increased levels of infection. The infection characters altered were those seen after the establishment of hyphal growth, i.e. days after inoculation to sporulation, the percentage of inoculated leaf tissue producing sporulating colonies and the number of spores produced per cm² of inoculated leaf tissue. The altered resistance phenotype was developmentally regulated in all of the mutants. The changes seen in the infection characters differed for each mutant, suggesting that different genes may have been altered. An altered resistance phenotype to brown rust and/or powdery mildew was also seen for some of the yellow rust mutants. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of slow rusting to stem rust in wheat

Summary The inheritance of the slow rusting character was studied on F₅ progenies from seven spring wheat cultivars (Triticum aestivum) crossed in all possible combinations without reciprocals. The cultivars and their progenies were evaluated for slow rusting in 1974 and 1975 in epidemics of Puccinia graminis f. sp. tritici, races 15 and 151, and traces of other races. Slow rusting varied significantly among the parents and among the F₅ progeny of each cross. Transgressive segregation occurred in each cross, i.e. some progeny rusted more slowly than the parents and some faster. In crosses with both Idaed 59 and Kenya 58 the progeny distributions were skewed towards slow rust development but the distributions in the other crosses were normal. The genetic control of slow rusting was predominantly additive, and narrow sense heritability was approximately 80 percent. The number of segregating genes having an effect on slow rusting was estimated to be 2 to 12 pairs depending on the cross. Correlation between slow rusting and maturity was usually negative but in most crosses the relationship was small.Contribution No. 9624 from the Agricultural Experiment Station, University of Minnesota, St. Paul, Minnesota 55108.     

The reaction of x Tritordeum and its Triticum spp. and Hordeum chilense parents to rust diseases

Summary Hexaploid and octoploid tritordeums and their parents Hordeum chilense and Triticum spp. were screened for resistance to isolates of wheat and barley yellow and brown rusts. All H. chilense lines were highly resistant to both wheat and barley brown rust, few lines were susceptible to wheat yellow rust while susceptibility to barley yellow rust was common. In general the resistance of tritordeum is predominantly contributed by the wheat parent and apparently the genes for resistance in H. chilense are inhibited in their expression by the presence of the wheat genome.Abbreviations WYR wheat yellow rust - WBR wheat brown rust - BYR barley yellow rust - BBR barley brown rust     

Resistance in spelt wheat to yellow rust

Summary Seven spelt wheat accessions of different origin were hybridized with the susceptible bread wheat cultivar Taichung 29 in order to study the genetics of their resistance to yellow rust (Puccinia striiformis Westend. f. sp. tritici). One Iranian and five European accessions were found to carry Yr5 of Triticum aestivum ssp. spelta var. album, whereas a factor for resistance in the Iranian accession 415 was confirmed to be genetically distinct from Yr5. The alleles for resistance in each of the accessions studied showed a monogenic dominant mode of inheritance. Twenty-eight spelt wheat accessions, including those studied for their resistance to yellow rust, were subjected to polyacrylamide-gel-electrophoresis to study variation for gliadin storage protein patterns. Thirteen distinct patterns were revealed, implying the presence of duplicates within the studied spelt wheat collection.     

Enhanced leaf rust resistance in wheat conditioned by resistance gene pairs with Lr13

Summary Leaf rust resistance gene Lr13 is present in many North American hard red spring wheat cultivars that have shown durable resistance to leaf rust. Fifteen pair-wise combinations of Lr13 and seedling leaf rust resistance genes were developed by intercrossing near isogenic Thatcher lines. In both seedling and adult plant tests, homozygous paired combinations of specific resistance genes with Lr13 had enhanced resistance relative to either parent to rust isolates that had intermediate avirulent infection types to the additional genes. In field tests, homozygous lines were more resistant than either parent if the additional leaf rust gene conditioned an effective level of resistance when present singly.     

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.     

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.     

Suppression/expression of resistance to stripe rust in synthetic hexaploid wheat (Triticum turgidum×T. tauschii)

Seventy-four hexaploid wheats, synthesized by either crossing resistantTriticum turgidum L. var.durum with susceptible/intermediateT. tauschii or susceptible/intermediateT. turgidum with resistantT. tauschii, and their parents were evaluated as seedlings in the greenhouse and as adult-plants at two field locations in Mexico for resistance to pathotype 14E14 of stripe (or yellow) rust (caused byPuccinia striiformis Westend). The seedlings of different synthetic hexaploids showed high phenotypic diversity for resistance. However, the resistance level of only 15 of the 74 synthetic hexaploid wheats were similar to the low infection types of the respective donor parents. The remaining synthetic wheats displayed either intermediate or high infection types. A similar result was also obtained in field tests, where only 18 synthetic hexaploids were resistant as adult-plants. In general, genotypes with seedling resistance were also resistant as adult-plants. A few synthetic hexaploids, which displayed intermediate or susceptible infection types as seedlings were resistant as adult-plants, indicating that additional genes for adult-plant resistance were also present. The fact that resistance of some donor parents was not expressed, or only partially expressed, in a synthetic hexaploid background suggests the presence of suppressor genes in the both the A or B, and D genomes ofT. turgidum andT. tauschii, respectively. The resistance of a donor parent was expressed in a synthetic hexaploid only if the corresponding suppressor was absent in the second parent. Moreover, the suppressors appeared to be resistance gene specific.     

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.     

Practical breeding for durable resistance to rust diseases in self-pollinating cereals

Summary The rust pathogens of cereals exist as populations of races that differ in their ability to attack various varieties. Varieties that are resistant when first released often become susceptible later due to the spread of previously undetected races but the time taken for this to occur in very variable. It often occurs so rapidly as to curtail the commercial use of otherwise satisfactory varieties.Some varieties, however, are widely grown for many years and remain adequately resistant to the prevalent rust diseases. They may aptly be described as having durable resistance. This durable or long-lasting resistance can be detected without any assumptions about, or detailed knowledge of, whether durability depends on any particular mechanisms of resistance, on various degrees of racespecificity or on many or few genes. Cappelle-Desprez is given as an example of a wheat variety with durable resistance to yellow rust.The most powerful test for the detection of durable resistance occurs when a variety is widely grown commercially for several years. A much weaker test is obtained by growing varieties in small disease nursery plots even when the test is repeated for several years. Usually, resistance which is durable is also partial or incomplete. Often, however, partial resistance of wheat to yellow rust has not been durable. Thus the observation that resistance is partial is not, of itself, a satisfactory criterion for the detection of durable resistance.It is suggested that the most obvious sources of durable resistance for use in breeding programmes are varieties which have been widely grown and have displayed this character. The transfer of such resistance during breeding may be achieved if the creation or incorporation of higher levels of resistance that have not been tested for durability is avoided. It should then be possible to derive resistance from the durably. resistant parent. Methods of achieving this are discussed.     

Characterization of the durable resistance to yellow rust in old winter wheat cultivars in the Netherlands

Summary Winter wheat cultivars released in the Netherlands before 1930 carried durable resistance to yellow rust. Cultivars released in the period between 1930 and 1950 often were durably resistant while recent cultivars infrequently showed durable resistance. This durable resistance was not difficult to transfer to new cultivars. Twenty nine older cultivars with durable resistance and eight recent non-durably resistant cultivars were tested in the seedling stage and in the adult plant stage against 12 West-European yellow rust races and against some non-European races in the seedling stage only. The adult plant tests were carried out in race nursery tests in the Flevopolder. Per race nursery all 37 cultivars, planted in hills of about 20 plants on both sides of the highly susceptible cv. Michigan Amber, were exposed to one race.The infection type of each cultivar-race combination was scored on 0 to 9 scale once in the seedling stage and twice in the adult plant stage. In the race nurseries the percentage leaf area affected was evaluated three times to be used to calculate the area under the disease progress curve (AUDPC). This AUDPC multiplied with the mean infection type in the field gave the susceptibility index (SI).The infection types were classified into resistant (R), intermediate (I) or susceptible (S) when the infection types were 0 to 3, 4 to 6 or 7 to 9, respectively. Four categories of resistance were discerned on the basis of the three infection type scores: 1) RRR, overall resistance; complete or near-complete resistant at all stages. 2) SRR, adult plant resistance, complete- or near-complete resistant at the adult plant stage only. 3) SRS and SSR, temperature sensitive resistance, the resistance changed from the one evaluation data to the other. 4) SSS and an SI lower than that of Michigan Amber, partial resistance.The frequencies of overall, adult plant and temperature sensitive resistance were 1.4, 52 and 54% in the older cultivars and 40, 62 and 22% in the recent ones, respectively. Among the older cultivars all had a fair to high level of partial resistance, the SI being on average only 20% of that of Michigan Amber, while most cultivars also seemed to carry temperature sensitive resistance. The partial resistance of the recent cultivars was of a much lower level with a mean SI compared to that of Michigan Amber of 61%. Partial resistance was highly correlated (r = –0.94) with the mean resistance scores from the Dutch Recommended Cultivars Lists. It was concluded that partial resistance and temperature sensitive resistance were the major components of the durable resistance in the older cultivars.     

Yr15 — a new gene for resistance to Puccinia striiformis in Triticum dicoccoides sel. G-25

Summary In a comparative study of reaction patterns and by analysis of segregation ratios in cross progenies, Triticum dicoccoides Koern. sel. G-25 was shown to possess a yet unknown gene for resistance to yellow rust. It is suggested to assign provisionally the symbol Yr15 to this gene.     

Resistance to stripe rust in Triticum turgidum,T. tauschii and their synthetic hexaploids

Resistance to stripe rust (caused by Puccinia striiformis Westend.) of 34 Triticum turgidum L. var.durum, 278 T. tauschii, and 267 synthetic hexaploid wheats (T. turgidum x T. tauschii) was evaluated at the seedling stage in the greenhouse and at the adult-plant stage at two field locations. Mexican pathotype 14E14 was used in all studies. Seedling resistance, expressed as low infection type, was present in all three species. One hundred and twenty-eight (46%) accessions of T. tauschii, 8 (23%) of T. turgidum and 31 (12%) of synthetic hexaploid wheats were highly resistant as seedlings. In the field tests, resistance was evaluated by estimating area under disease progress curve (AUDPC). Synthetic hexaploid wheats showed a wide range of variability for disease responses in both greenhouse and field tests, indicating the presence of a number of genes for resistance. In general, genotypes with seedling resistance were also found to be resistant as adult plants. Genotypes, which were susceptible or intermediate as seedlings but resistant as adult plants, were present in both T. turgidum and the synthetic hexaploids. Resistances from either T. turgidum or T. tauschii or both were identified in the synthetic hexaploids in this study. These new sources of resistance could be incorporated into cultivated hexaploid wheats to increase the existing gene pool of resistance to stripe rust.