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.     

Effects of inoculum density and temperature on three components of leaf rust resistance controlled by Lr34 in wheat

Summary Different inoculum densities had negligible effects on latent period, uredinium density and uredinium size measured on flag leaves of adult RL6058 (Thatcher^*6/PI58548[Lr34]) plants kept at low (17.5°C) post-infection temperatures in a glasshouse. In a qualitative assessment of rust severity at higher (24.6°C) temperatures, all three components of resistance indicated a susceptible flag leaf response on RL6058. In the latter environment, precise estimations of receptivity to different inoculum densities showed that adult RL6058 plants supported significantly less pustules than the leaf rust-susceptible cultivar Thatcher. In tillering plants, statistically equal numbers of uredinia developed on RL6058 and Thatcher in all paired temperature-inoculum density combinations. Growth stage-related susceptibility, and higher temperatures conducive to a shorter latent period and larger uredinia, could result in high terminal severities of Puccinia recondita f. sp. tritici on wheat genotypes containing Lr34. The reduction in receptivity associated with this gene may contribute, however, to delayed disease increase on cultivars or lines with monogenic Lr34 resistance.     

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 role of Lr34 in imparting durable resistance to wheat leaf rust through gene interaction

Summary The leaf rust responses of wheat lines carrying the complementary genes Lr27 and Lr31 and the same genes in a Chinese Spring background which contains Lr34, indicate that Lr34 interacts with the complementary genes to give enhanced levels of field resistance to leaf rust. Lr34, particularly in combination with other genes, is considered to be an important gene for imparting a high degree of durable resistance to leaf rust. Its similarity to Sr2, an adult plant gene for resistance to stem rust and its association with adult plant resistances to stem and stripe rusts are discussed.     

Effect of Lr34 Resistance on Leaf Rust Development, Grain Yield and Baking Quality in Wheat

The protection provided by the leaf rust resistance gene Lr34 against Puccinia recondita f. sp. tritici was studied in the field over two seasons. In leaf-rust inoculated and fungicide-sprayed control plots, yield of RL6058, the ‘Thatcher’ backcross line with Lr34, was compared to that of the susceptible cultivar ‘Thatcher’. Leaf rust severity remained low on RL6058 in both seasons, but was high on ‘Thatcher’. The latent period of wheat leaf rust isolate 3SA132 in flag leaves of RL6058 was 256 h longer than in ‘Thatcher’. The uredinium density on ‘Thatcher’ was 14.4/cm², compared to 3.7/cm² flag leaf surface on RL6058. Leaf rust infection of ‘Thatcher’ reduced the total grain yield per plot by 25.4% and 1,000 kernel mass by 15.6%. Leaf rust caused little or no damage on RL6058 and rusted plots outyielded the control plots by 0.3 %. Seed weight of RL6058 was reduced by 0.7%. Compared to previous greenhouse studies, the adult-plant resistance conferred by Lr34 is more clearly expressed in the field. Evaluation of milling and baking quality characteristics revealed that compared to ‘Thatcher’, RL6058 had a higher flour protein content, but that its milling, dough development and baking properties were inferior.     

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.     

Adult plant leaf rust resistance from 111 wheat (Triticum aestivum L.) cultivars

Adult plant resistance against Indian leaf rust race 77 and five of its highly virulent variants have been identified from 111 bread wheat cultivars originating from 12 countries. The adult plant resistance of only 16 of these cultivars is due to hypersensitive seedling or adult plant resistance genes. All others expressed nonhypersensitive type of resistance characteristic of the genes Lr34 and Lr46.Forty five of the 111 cultivars showed tip necrosis on flag leaves, a trait linked to the gene Lr34. Therefore, the nonhypersensilive type of resistance of these 45 cultivars is attributed to Lr34. The nonhypersensitive resistance of the remaining cultivars is likely to be due to the gene(s) different than Lr34. The reaction pattern of these 111 cultivars to six races suggests the presence of at least six to seven new hypersensitive adult plant resistance genes and at least three new hypersensitive seedling resistance genes. The known genes Lr10, Lr23 and Lr26 were detected frequently but these genes did not contribute towards the adult plant resistance of any of the 111 cultivars. Based on the presence of new genes for hypersensitive and nonhypersensitive type of resistance, the 111 cultivars have been classified into 31 diverse resistance groups. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mapping of the leaf rust resistance gene <Emphasis Type="Italic">Lr38</Emphasis> on wheat chromosome arm 6DL using SSR markers

Leaf rust caused by the fungus Puccinia triticina is one of the most important diseases of wheat (Triticum aestivum) worldwide. The use of resistant wheat cultivars is considered the most economical and environment-friendly approach in controlling the disease. The Lr38 gene, introgressed from Agropyron intermedium, confers a stable seedling and adult plant resistance against multiple isolates tested in Europe. In the present study, 94 F₂ plants resulting from a cross made between the resistant Thatcher-derived near-isogenic line (NIL) RL6097, and the susceptible Ethiopian wheat cultivar Kubsa were used to map the Thatcher Lr38 locus in wheat using simple sequence repeat (SSR) markers. Out of 54 markers tested, 15 SSRs were polymorphic between the two parents and subsequently genotyped in the population. The P. triticina isolate DZ7-24 (race FGJTJ), discriminating Lr38 resistant and susceptible plants, was used to inoculate seedlings of the two parents and the segregating population. The SSR markers Xwmc773 and Xbarc273 flanked the Lr38 locus at a distance of 6.1 and 7.9 cM, respectively, to the proximal end of wheat chromosome arm 6DL. The SSR markers Xcfd5 and Xcfd60 both flanked the locus at a distance of 22.1 cM to the distal end of 6DL. In future, these SSR markers can be used by wheat breeders and pathologists for marker assisted selection (MAS) of Lr38-mediated leaf rust resistance in wheat.     

Genetics of leaf rust resistance in three Americano landrace-derived wheat cultivars from Uruguay

A genetic analysis of the landrace‐derived wheat accessions Americano 25e, Americano 26n, and Americano 44d, from Uruguay was conducted to identify the leaf rust resistance genes present in these early wheat cultivars. The three cultivars were crossed with the leaf rust susceptible cultivar ‘Thatcher’ and approximately 80 backcross (BC1) F₂ families were derived for each cross. The BC1F₂ families and selected BC1F₄ lines were tested for seedling and adult plant leaf rust resistance with selected isolates of leaf rust, Puccinia triticina. The segregation and infection type data indicated that Americano 25e had seedling resistance genes Lr3, Lr16, an additional unidentified seedling gene, and one adult plant resistance gene that was neither Lr12 nor Lr13, and did not phenotypically resemble Lr34. Americano 26n was postulated to have genes Lr11, Lr12, Lr13, and Lr14a. Americano 44d appeared to have two possibly unique adult plant leaf rust resistance genes.     

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 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.     

Characterization of stem rust resistant derivatives of wheat cultivar Amigo

Summary Originally developed for resistance to greenbug derived from Insave rye, Amigo wheat carries two genes for resistance to stem rust. One of these genes is associated with a rye chromosome 1RS segment carrying the Sec-1 protein marker and presumably greenbug resistance. The second gene which is genetically linked to leaf rust resistance is associated with an Agropyron-derived segment. Rust tests in Canada confirmed that these genes were Sr24 and Lr24. In contrast to Agent and certain 3D/Ag derivatives from Dr. E.R. Sears, the Amigo source of Sr24/Lr24 freely recombined with white seed colour during backcrossing.     

The influence of leaf rust resistance genes Lr29, Lr34, Lr35 and Lr37 on breadmaking quality in wheat

Leaf rust, caused by Puccinia triticina, is considered one of the most important diseases of wheat. In South Africa the genes Lr29, Lr34, Lr35 and Lr37 confer effective resistance to leaf rust, qualifying them for use in cultivar improvement. To study possible secondary effects of these genes, a collection of BC6 lines containing each of the genes singly, was evaluated for breadmaking quality. The recurrent parent Karee, and Thatcher NILs used as resistance donors in the primary crosses, as well as Thatcher, were included as checks. The presence of Lr29, Lr34, Lr35 and Lr37 caused a significant increase in flour protein and water absorption. For most of the other characteristics the NILs performed statistically similar to the recurrent parent. Some sib lines performed significantly better than others, emphasising the value of selecting for improved quality among backcross lines. This revised version was published online in August 2006 with corrections to the Cover Date.     

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

Summary A set of 105 European wheat cultivars, comprising 68 cultivars with known seedling resistance genes and 37 cultivars that had not been tested previously, was tested for resistance to selected Australian pathotypes of P. triticina in seedling greenhouse tests and adult plant field tests. Only 4% of the cultivars were susceptible at all growth stages. Twelve cultivars lacked detectable seedling resistance to leaf rust, and among the remaining cultivars, 10 designated genes were present either singly or in combination. Lr13 was the most frequently detected gene, present in 67 cultivars, followed by the rye-derived gene Lr26, present in 19 cultivars. Other genes present were Lr1, Lr3a, Lr3ka, Lr10, Lr14a, Lr17b, Lr20 and Lr37. There was evidence for unidentified seedling resistance in addition to known resistance genes in 11 cultivars. Field tests with known pathotypes of P. triticina demonstrated that 57% of the cultivars carried adult plant resistance (APR) to P. triticina. The genetic identity of the APR is largely unknown. Genetic studies on selected cultivars with unidentified seedling resistances as well as all of those identified to carry APR are required to determine the number and inheritance of the genes involved, to determine their relationships with previously designated rust resistance genes, and to assess their potential value in breeding for resistance to leaf rust.     

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.     

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.     

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.     

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.     

Genes Lr48 and Lr49 for hypersensitive adult plant leaf rust resistance in wheat (Triticum aestivum L.)

The genetic bases of leaf rust resistance in wheat (Triticum aestivum L.) line CSP44, selected from the Australian cultivar Condor, and Indian cultivar VL404, were studied. The reaction patterns of CSP44 and VL404 against Indian races 12, 77, 77-1, 77-2, 77-3, 77-4, 77-5 and 108 were different from reaction patterns shown by near-isogenic lines with known adult plant resistance (APR) genes, viz. Lr12, Lr13, Lr22b and Lr34. Although the reaction patterns of CSP44 and VL404 were similar to the near-isogenic line Tc+Lr22a, tests of allelism indicated absence of Lr22a in both CSP44 and VL404. On the basis of genetic studies, their resistances in field tests against race 77-5, the most virulent race from the Indian sub-continent, were each ascribed to two genes. One of the two genes in each wheat was identified to be the non-hypersensitive APR gene Lr34. The second APR genes in CSP44 and VL404 gave hypersensitive reaction types and were recessive and dominant, respectively. The gene in CSP44 was designated Lr48and the gene in VL404, Lr49. This revised version was published online in August 2006 with corrections to the Cover Date.     

An analysis of genes for resistance against Indian stem rust races in two bread wheat cultivars

Summary The genetic constitution of two bread wheat accessions from the International Spring Wheat Rust Nurseries (E 5883 and E 6032) has been studied for reaction to four Indian races of stem rust. Analysis of E 5883 has revealed that for each of the races 15C, 21 and 40 a single dominant gene operates for resistance. The dominant gene against race 15C was identified as Sr6. The dominant genes for resistance against races 21 and 40 were found to be different from the genes described so far. Resistance against race 122 is controlled by a single recessive gene producing characteristically a 2 type of reaction. This gene was identified as Sr8.The resistance of E 6032 against each of the races 15C, 21 and 40 is controlled by two genes, one dominant and one recessive, which act independently. Dominant genes effective against 15C, 21 and 40 were conclusively identified as Sr6, Sr5 and Sr9b, respectively. From the correlated behaviour against races 15C and 40 as well as from the phenotypes of the resistance reactions rhe same recessive gene, undescribed so far, operates against the two races. The second recessive gene operating against race 21 was also observed to be different from those so far designated. E 6032 was, however, found to be susceptible to races 122.The presence of Sr6 both in E 5883 and E 6032 against race 15C was further confirmed through F₂ and F₃ segregation data.