Genetic control of effective juvenile resistance to leaf rust in collection accessions of Triticum aestivum L

Of 153 accessions reported to be resistant to leaf rust (Puccinia recondita Rob. ex. Desm.), only 70 were not affected by a pooled P. recondita population. According to phytopathological tests (inoculation with test clones), 14 accessions contained the Lr19 gene; 36, the Lr24 gene; 1, the Lr41 gene; and 19 presumably had the Lr9 gene. The presence of these resistance genes was confirmed by hybrid analysis for 26 accessions. Of 28 accessions reported to carry new effective resistance genes other than the known genes, 23 were affected by the P. recondita population. In four of the other five accessions, resistance proved to be controlled by known genes. Possible causes of false identification of new effective leaf rust resistance genes in wheat are discussed.     

Mapping novel sources of leaf rust and stripe rust resistance introgressed from Triticum dicoccoides in cultivated tetraploid wheat background

Wild emmer wheat, Triticum dicoccoides, the progenitor of modern tetraploid and hexaploid wheats, is an important resource for new variability for disease resistance genes. T. dicoccoides accession pau4656 showed resistance against prevailing leaf rust and stripe rust races in India and was used for developing stable introgression lines (IL) in T. durum cv Bijaga yellow and named as IL pau16068. F₅ Recombinant inbred lines (F₅ RILs) were developed by crossing IL pau16068 with T. durum cultivar PBW114 and RIL population was screened against highly virulent Pt and Pst pathotypes at the seedling and adult plant stages. Inheritance analyses revealed that population segregated for two genes for all stage resistance (ASR) against leaf rust, one ASR gene against stripe rust and three adult plant resistance (APR) genes for stripe rust resistance. For mapping these genes a set of 483 SSR marker was used for bulked segregant analysis. The markers showing diagnostic polymorphism in the resistant and susceptible bulks were amplified on all RILs. Single marker analysis placed all stage leaf rust resistance genes on chromosome 6A and 2A linked to the SSR markers Xwmc256 and Wpaus268, respectively. Likewise one all stage stripe rust resistance gene were mapped on long arm of chromosome 6A linked to markers 6AL-5833645 and 6AL-5824654 and two APR genes mapped on chromosomes 2A and 2B close to the SSR marker Wpaus268 and Xbarc70, respectively. The current study identified valuable leaf rust and stripe rust resistance genes effective against multiple rust races for deployment in the wheat breeding programme.

     

Microsatellite DNA polymorphism divergence in Triticum dicoccoides accessions highly resistant to yellow rust

 Stripe rust (yellow rust), caused by Puccinia striiformis f. sp. tritici, is one of the most important diseases of wheat throughout the world. Wild emmer wheat, Triticum dicoccoides, the progenitor of cultivated wheat, was found to be a valuable source for novel stripe-rust-resistance genes. The objective of the present study was to estimate the extent of genetic diversity among the wild emmer wheat accessions, previously identified as highly resistant to stripe rust, in order to select suitable parents for genetic-mapping studies. Twenty three wheat microsatellite (WMS) markers were used to detect DNA polymorphism among 21 accessions of T. dicoccoides, which included 19 resistant and two susceptible accessions originating mainly from the center of origin and diversity in the Upper Galilee and Hermon Mountain in northern Israel. In addition, two Triticum durum and one Triticum aestivum lines were also included in the analysis. The 23 WMS markers used were located on 23 chromosome arms, representing all 14 chromosomes of genomes A and B of wheat, and revealed a total of 230 alleles. The number of alleles ranged from 5 to 18, with an average of ten alleles per WMS. Genetic dissimilarity values between genotypes, calculated by the WMSderived data, were used to produce a dendrogram of the relationships among accessions using the unweighted pair-group method with arithmetic averages (UPGMA). The results showed that all of the wild emmer wheat accessions could be distinguished. Most of the resulting groups were strongly related to the ecogeographical origin of the accessions, indicating that the genetic diversity of T. dicoccoides is correlated with geographic distribution. The three major groups were the Rosh Pinna group (north of the Sea of Galilee), the Mount Hermon group (north of the Golan Heights) and Mount Kena’an group (Upper Galilee). The genetic similarity (GS) of the 21 T. dicoccoides accessions based on WMS results averaged 0.31. As expected, the T. durum and T. aestivum lines were grouped separately from the T. dicoccoides accessions. The results obtained suggest that a relatively small number of microsatellites can be used for the estimation of genetic diversity in wild material of T. dicoccoides. These results will be useful in the identification of suitable parents for the development of mapping populations for tagging yellow-rust resistance genes derived from T. dicoccoides. Furthermore, future work could test the adaptive evolutionary significance of microsatellites in natural populations of wild emmer wheat. Received: 8 August 1997 / Accepted: 25 August 1997     

Linkage disequilibrium and association analysis of stripe rust resistance in wild emmer wheat (Triticum turgidum ssp. dicoccoides) population in Israel

Key Message

Rapid LD decay in wild emmer population from Israel allows high-resolution association mapping. Known and putative new stripe rust resistance genes were found.

Abstract

Genome-wide association mapping (GWAM) is becoming an important tool for the discovery and mapping of loci underlying trait variation in crops, but in the wild relatives of crops the use of GWAM has been limited. Critical factors for the use of GWAM are the levels of linkage disequilibrium (LD) and genetic diversity in mapped populations, particularly in those of self-pollinating species. Here, we report LD estimation in a population of 128 accessions of self-pollinating wild emmer, Triticum turgidum ssp. dicoccoides, the progenitor of cultivated wheat, collected in Israel. LD decayed fast along wild emmer chromosomes and reached the background level within 1 cM. We employed GWAM for the discovery and mapping of genes for resistance to three isolates of Puccinia striiformis, the causative agent of wheat stripe rust. The wild emmer population was genotyped with the wheat iSelect assay including 8643 gene-associated SNP markers (wheat 9K Infinium) of which 2,278 were polymorphic. The significance of association between stripe rust resistance and each of the polymorphic SNP was tested using mixed linear model implemented in EMMA software. The model produced satisfactory results and uncovered four significant associations on chromosome arms 1BS, 1BL and 3AL. The locus on 1BS was located in a region known to contain stripe rust resistance genes. These results show that GWAM is an effective strategy for gene discovery and mapping in wild emmer that will accelerate the utilization of this genetic resource in wheat breeding.     

Genetic Characterization of a Fusarium Head Blight Resistance QTL from Triticum turgidum ssp. dicoccoides

Plant Molecular Biology Reporter - Qfhs.ndsu-3AS in wild emmer wheat (Triticum turgidum L. ssp. dicoccoides) is a major quantitative trait locus associated with type II resistance to Fusarium head...     

Ecogeographical distribution of HMW glutenin alleles in populations of the wild tetraploid wheat Triticum turgidum var. dicoccoides

Summary Polymorphism of high molecular weight (HMW) glutenin subunits in 466 accessions of the wild tetraploid wheat Triticum turgidum var. dicoccoides in Israel was characterized with regard to the ecogeographical distribution of the HMW glutenin alleles, both between and within 22 populations, and along transects in a single population. While some populations were monomorphic for all the HMW glutenin loci, namely, Glu-A1-1, Glu-A1-2, Glu-B1-1 and Glu-B1-2, others contained up to four alleles per locus. Intrapopulation variability could be predicted by the geographical distribution: marginal populations tended to be more uniform than those at the center of distribution. The various HMW glutenin alleles tended to be clustered, both at a regional level and within a single population along transects of collection. It is suggested that this clustering is due to selection pressures acting both at a regional and at a microenvironmental level. This was confirmed by the significant correlations found between the MW of subunits encoded by Glu-A1-1 and the populations' altitude, average temperature and rainfall. The possible selective values of seed storage proteins are discussed.     

Inheritance of leaf rust and stem rust resistance in 'Roblin' wheat.

The Canadian common wheat (Triticum aestivum L.) cultivar 'Roblin' is resistant to both leaf rust (Puccinia recondita Rob. ex. Desm.) and stem rust (Puccinia graminis Pers. f. sp. tritici Eriks. and E. Henn.). To study the genetics of this resistance, 'Roblin' was crossed with 'Thatcher', a leaf rust susceptible cultivar, and RL6071, a stem rust susceptible line. A set of F6 random lines was developed from each cross. The random lines and the parents were grown in a field rust nursery artificially inoculated with a mixture of P. recondita and P. graminis isolates and scored for rust reaction. The same material was tested with specific races of leaf rust and stem rust. These data indicated that 'Roblin' has Lr1, Lr10, Lr13, and Lr34 for resistance to P. recondita and Sr5, Sr9b, Sr11, and possibly Sr7a and Sr12 for resistance to P. graminis. In a 'Thatcher' background, the presence of Lr34 contributes to improve stem rust resistance, which appears also to occur in 'Roblin'.     

Genetic control of effective juvenile resistance to leaf rust in collection samples of Triticum aestivum L.

Of 153 samples reported to be resistant to leaf rust (Puccinia recondita Rob. ex. Desm.), only 70 were not affected by a complex P. recondita population. According to phytopathological tests (inoculation with test clones), 14 samples possessed the Lr19 gene; 36, the Lr24 gene; 1, the Lr41 gene; and 19 presumably had the Lr9 gene. The presence of these genes for resistance was confirmed by hybridological analysis for 26 samples. Of 28 samples reported to carry new effective genes for resistance other than the known genes, 23 were susceptible to the P. recondita population. In four of the other five samples, resistance proved to be controlled by known genes. Possible causes of false identification of new effective genes for leaf rust resistance in wheat are discussed.     

Resistance to wheat leaf rust and stem rust in Triticum tauschii and inheritance in hexaploid wheat of resistance transferred from T. tauschii.

Twelve accessions of Triticum tauschii (Coss.) Schmal. were genetically analyzed for resistance to leaf rust (Puccinia recondita Rob. ex Desm.) and stem rust (Puccinia graminis Pers. f.sp. tritici Eriks. and E. Henn.) of common wheat (Triticum aestivum L.). Four genes conferring seedling resistance to leaf rust, one gene conferring seedling resistance to stem rust, and one gene conferring adult-plant resistance to stem rust were identified. These genes were genetically distinct from genes previously transferred to common wheat from T. tauschii and conferred resistance to a broad spectrum of pathogen races. Two of the four seedling leaf rust resistance genes were not expressed in synthetic hexaploids, produced by combining tetraploid wheat with the resistant T. tauschii accessions, probably owing to the action of one or more intergenomic suppressor loci on the A or B genome. The other two seedling leaf rust resistance genes were expressed at the hexaploid level as effectively as in the source diploids. One gene was mapped to the short arm of chromosome 2D more than 50 cM from the centromere and the other was mapped to chromosome 5D. Suppression of seedling resistance to leaf rust in synthetic hexaploids derived from three accessions of T. tauschii allowed the detection of three different genes conferring adult-plant resistance to a broad spectrum of leaf rust races. The gene for seedling resistance to stem rust was mapped to chromosome ID. The degree of expression of this gene at the hexaploid level was dependent on the genetic background in which it occurred and on environmental conditions. The expression of the adult-plant gene for resistance to stem rust was slightly diminished in hexaploids. The production of synthetic hexaploids was determined to be the most efficient and flexible method for transferring genes from T. tauschii to T. aestivum, but crossing success was determined by the genotypes of both parents. Although more laborious, the direct introgression method of crossing hexaploid wheat with T. tauschii has the advantages of enabling selection for maximum expression of resistance in the background hexaploid genotype and gene transfer into an agronomically superior cultivar.     

High-density mapping and marker development for the powdery mildew resistance gene PmAS846 derived from wild emmer wheat (Triticum turgidum var. dicoccoides)

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is an important foliar disease of wheat worldwide. The dominant powdery mildew resistance gene PmAS846 was transferred to the hexaploid wheat lines N9134 and N9738 from wild emmer wheat (Triticum dicoccoides) in 1995, and it is still one of the most effective resistance genes in China. A high resolution genetic map for PmAS846 locus was constructed using two F₂ populations and corresponding F_2:3 families developed from the crosses of N9134/Shaanyou 225 and N9738/Huixianhong. Synteny between wheat and Brachypodium distachyon and rice was used to develop closely linked molecular markers to reduce the genetic interval around PmAS846. Twenty-six expressed sequence tag-derived markers were mapped to the PmAS846 locus. Five markers co-segregated with PmAS846 in the F₂ population of N9134/Shaanyou 225. PmAS846 was physically located to wheat chromosome 5BL bin 0.75–0.76 within a gene-rich region. The markers order is conserved between wheat and Brachypodium distachyon, but rearrangements are present in rice. Two markers, BJ261635 and CJ840011 flanked PmAS846 and narrowed PmAS846 to a region that is collinear with 197 and 112 kb genomic regions on Brachypodium chromosome 4 and rice chromosome 9, respectively. The genes located on the corresponding homologous regions in Brachypodium, rice and barley could be considered for further marker saturation and identification of potential candidate genes for PmAS846. The markers co-segregating with PmAS846 provide a potential target site for positional cloning of PmAS846, and can be used for marker-assisted selection of this gene.     

Transfer of leaf rust and stem rust resistance genes from Triticum triaristatum to durum and bread wheats and their molecular cytogenetic localization.

Six accessions of Triticum triaristatum (Willd) Godr. &Gren. (syn. Aegilops triaristata) (6x, UUMMUnUn), having good resistance to both leaf rust (Puccinia recondita f.sp. tritici Rob. ex Desm) races and stem rust (P. graminis f.sp. tritici Eriks. &Henn.) races, were successfully crossed with both susceptible durum wheats (T. turgidum var. durum L., 2n = 28, AABB) and bread wheats (T. aestivum, 2n = 42, AABBDD). In some crosses, embryo rescue was necessary. The T. triaristatum resistance was expressed in all F1 hybrids. Backcrossing of the F1 hybrids to their wheat parents to produce BC1F1 plants was more difficult (seed set 0-7.14%) than to produce F1 hybrids (seed set 12.50-78.33%). The low female fertility of the F1 hybrids was due to low chromosome pairing. Only gametes with complete or nearly complete genomes from the F1 hybrids were viable. In BC2F4 populations from the cross MP/Ata2//2*MP, monosomic or disomic addition lines (2n = 21 II + 1 I or 22 II) with resistance to leaf rust race 15 (IT 1) were selected. In BC2F2 populations from the crosses CS/Ata4//2*MP and MP/Ata4//2*MP, monosomic or disomic addition lines with resistance to either leaf rust race 15 or stem rust race 15B-1 (both IT 1) were selected. Rust tests and cytology on the progeny of the disomic addition lines confirmed that the genes for rust resistance were located on the added T. triaristatum chromosomes. The homoeologous groups of the T. triaristatum chromosomes in the addition lines from the crosses MP/Ata2//2*MP, CS/Ata4//2*MP, and MP/Ata4//2*MP were determined to be 5, 2, and 7, respectively, through the detecting of RFLPs among genomes using a set of homoeologous group specific wheat cDNA probes. The addition lines with resistance to leaf rust race 15 from the crosses MP/Ata2//2*MP and CS/Ata4//2*MP were resistant to another nine races of leaf rust and the addition line with resistance to stem rust race 15B-1 from the cross MP/Ata4//2*MP was resistant to another nine races of stem rust as were their T. triaristatum parents. Since such genes provide resistance against a wide spectrum of rust races they should be very valuable in wheat breeding for rust resistance.     

Function and evolution of allelic variations of Sr13 conferring resistance to stem rust in tetraploid wheat (Triticum turgidum L.)

The resistance gene Sr13 is one of the most important genes in durum wheat for controlling stem rust caused by Puccinia graminis f. sp. tritici (Pgt). The Sr13 functional gene CNL13 has haplotypes R1, R2 and R3. The R1/R3 and R2 haplotypes were originally designated as alleles Sr13a and Sr13b, respectively. To detect additional Sr13 alleles, we developed Kompetitive allele specific PCR (KASP™) marker KASPSr13 and four semi-thermal asymmetric reverse PCR markers, rwgsnp37–rwgsnp40, based on the CNL13 sequence. These markers were shown to detect R1, R2 and R3 haplotypes in a panel of diverse tetraploid wheat accessions. We also observed the presence of Sr13 in durum line CAT-A1, although it lacked any of the known haplotypes. Sequence analysis revealed that CNL13 of CAT-A1 differed from the susceptible haplotype S1 by a single nucleotide (C2200T) in the leucine-rich repeat region and differed from the other three R haplotypes by one or two additional nucleotides, confirming that CAT-A1 carries a new (R4) haplotype. Stem rust tests on the monogenic, transgenic and mutant lines showed that R1 differed from R3 in its susceptibility to races TCMJC and THTSC, whereas R4 differed from all other haplotypes for susceptibility to TTKSK, TPPKC and TCCJC. Based on these differences, we designate the R1, R3 and R4 haplotypes as alleles Sr13a, Sr13c and Sr13d, respectively. This study indicates that Sr13d may be the primitive functional allele originating from the S1 haplotype via a point mutation, with the other three R alleles probably being derived from Sr13d through one or two additional point mutations.     

Storage-protein variation in wild emmer (Triticum turgidum ssp.dicoccoides) from Jordan and Turkey.

Genetic diversity in the seed storage-proteins encoded at theGlu-A1,Glu-B1 andGli-B1/Glu-B3 loci was studied electrophoretically in 315 individuals belonging to nine populations ofT. dicoccoides from Jordan and three from Turkey. The inter- and intra-population distribution of seed storage-protein alleles at the considered loci and its link with geographical factors were investigated. Population differentiation in seed storage-proteins was in some cases very high with very weak correlations with geographic distance. Greater gene differentiation was found within and between populations which were geographically very close in Jordan than between those from Jordan and Turkey. However the distribution of alleles appeared to be non random. Samples collected from populations at locations over 900 m above sea level were less polymorphic than those collected at lower altitudes (500–700 m), whereas the relative genetic differentiation between populations was greater between those collected at higher altitudes. Seed storage-protein differentiation was significantly correlated with the altitude of the collecting sites. Although it is difficult to point out the selective pressure of altitude per se, altitude can reflect an integration of several environmental parameters. The possible adaptive value of seed storage-proteins is discussed.     

High-temperature adult-plant (HTAP) stripe rust resistance gene Yr36 from Triticum turgidum ssp. dicoccoides is closely linked to the grain protein content locus Gpc-B1

Several new races of the stripe rust pathogen have become frequent throughout the wheat growing regions of the United States since 2000. These new races are virulent to most of the wheat seedling resistance genes limiting the resistance sources that can be used to combat this pathogen. High-temperature adult-plant (HTAP) stripe rust resistance has proven to be more durable than seedling resistance due to its non-race-specific nature, but its use is limited by the lack of mapping information. We report here the identification of a new HTAP resistance gene from Triticum turgidum ssp. dicoccoides (DIC) designated as Yr36. Lines carrying this gene were susceptible to almost all the stripe rust pathogen races tested at the seedling stage but showed adult-plant resistance to the prevalent races in California when tested at high diurnal temperatures. Isogenic lines for this gene were developed by six backcross generations. Field tests in two locations showed increased levels of field resistance to stripe rust and increased yields in isogenic lines carrying the Yr36 gene compared to those without the gene. Recombinant substitution lines of chromosome 6B from DIC in the isogenic background of durum cv. Langdon were used to map the Yr36 gene on the short arm of chromosome 6B completely linked to Xbarc101, and within a 2-cM interval defined by PCR-based markers Xucw71 and Xbarc136. Flanking locus Xucw71 is also closely linked to the grain protein content locus Gpc-B1 (0.3-cM). Marker-assisted selection strategies are presented to improve stripe rust resistance and simultaneously select for high or low Gpc-B1 alleles.     

Molecular mapping of stem and leaf rust resistance in wheat

Stem rust caused by Puccinia graminis f. sp. tritici Eriks and Henn and leaf rust caused by Puccinia triticina Rob. ex Desm. are major constraints to wheat production worldwide. In the present study, F₄-derived SSD population, developed from a cross between Australian cultivars ‘Schomburgk’ and ‘Yarralinka’, was used to identify molecular markers linked to rust resistance genes Lr3a and Sr22. A total of 1,330 RAPD and 100 ISSR primers and 33 SSR primer pairs selected ob the basis of chromosomal locations of these genes were used. The ISSR marker UBC 840₅₄₀ was found to be linked with Lr3a in repulsion at a distance of 6.0 cM. Markers cfa2019 and cfa2123 flanked Sr22 at a distance of 5.9 cM (distal) and 6.0 cM (proximal), respectively. The use of these markers in combination would predict the presence or absence of Sr22 in breeding populations. A previously identified PCR-based diagnostic marker STS638 linked to Lr20 was validated in this population. This marker showed a recombination value of 7.1 cM with Lr20.     

Genetic and physical characterization of theLR1 leaf rust resistance locus in wheat (Triticum aestivum L.)

The objective of this study was to characterize the leaf rust resistance locusLr1 in wheat. Restriction fragment length polymorphism (RELP) analysis was performed on the resistant lineLr1/6^*Thatcher and the susceptible varieties Thatcher and Frisal, as well as on the segregating F₂ populations. Seventeen out of 37 RFLP probes mapping to group 5 chromosomes showed polymorphism betweenLr1/6^*Thatcher and Frisal, whereas 11 probes were polymorphic between the near-isogenic lines (NILs)Lr1/6^*Thatcher and Thatcher. Three of these probes were linked to the resistance gene in the segregating F₂ populations. One probe (pTAG621) showed very tight linkage toLr1 and mapped to a single-copy region on chromosome 5D. The map location of pTAG621 at the end of the long arm of chromosome 5D was confirmed by the absence of the band in the nulli-tetrasomic line N5DT5B of Chinese Spring and a set of deletion lines of Chinese Spring lacking the distal part of 5DL. Twenty-seven breeding lines containing theLr1 resistance gene in different genetic backgrounds showed the same band asLr1/6^*Thatcher when hybridized with pTAG621. The RFLP marker was converted to a sequence-tagged-site marker using polymerase chain reaction (PCR) amplification. Sequencing of the specific fragment amplified from both NILs revealed point mutations as well as small insertion/deletion events. These were used to design primers that allowed amplification of a specific product only from the resistant lineLr1/6^*Thatcher. This STS, specific for theLr1 resistance gene, will allow efficient selection for the disease resistance gene in wheat breeding programmes. In addition, the identification of a D-genome-specific probe tightly linked toLr1 should ultimately provide the basis for positional cloning of the gene.     

High molecular weight glutenin subunit variation in Triticum turgidum var. dicoccum

Summary Variation in high molecular weight (HMW) glutenin subunit composition among 167 accessions of dicoccum wheat (Triticum turgidum L. var. dicoccum Schrank) of diverse origins was investigated using one-dimensional sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). A total of 20 alleles were identified, and 9 of them were found to be different from those previously detected by Payne and Lawrence (1983 b) in hexaploid wheat (Triticum aestivum L.). The newly discovered alleles enhance the genetic variability available to improve the industrial quality of wheats and some of them may facilitate basic research on the relationship of industrial quality with HMW glutenin subunit number. The novel variants include a GLU-A1 encoded subunit which has higher molecular mass than any other so far described in tetraploid and hexaploid wheats, and a null GLU-B1 allele. Dicoccums containing neither GLU-A1- nor GLU-B1-encoded subunits were also identified. A comparison of the mean number of HMW glutenin subunits contained in various primitive and modern domesticated wheats of different ploidy levels and the identification of wheats containing no HMW glutenin subunits suggest that the occurrence of null GLU-1 alleles in these species depends on chance rather on an inherent tendency on the part of modern polyploid wheats to suppress the activity of redundant GLU-1 genes.     

Genetic analysis of durable leaf rust resistance in winter wheat

Quantitative resistance that delays the epidemic development of leaf rust in wheat is an important source for durable resistance breeding. The Swiss winter wheat variety ’Forno’ shows a high level of quantitative resistance against leaf rust. This resistance has been effective for more than 10 years and can therefore be considered to be durable. In order to map quantitative trait loci (QTL) for durable leaf rust resistance we analysed 204 F₅ recombinant inbred lines (RILs) of the cross between the winter wheat ’Forno’ and the winter spelt ’Oberkulmer’ for their level of leaf rust resistance (LR) and leaf tip necrosis (LTN) in four different environments. Both traits showed a continuous distribution and were significantly correlated (r=−0.5). Across environments we detected 8 QTL for leaf rust resistance (6 inherited from ’Forno’) and 10 QTL for the quantitative expression of LTN (6 inherited from ’Forno’). Of the 6 QTL responsible for the durable leaf rust resistance of ’Forno’, 1 major QTL coincided with a thaumatin locus on 7BL explaining 35% of the phenotypic variance. Four QTL for LR coincided with QTL for LTN. At these loci the alleles of ’Forno’ increased the level of resistance as well as the extent of LTN, indicating pleiotropy. Received: 1 July 1999 / Accepted: 30 July 1999     

Pathogen race determines the type of resistance response in the stripe rust –Triticum dicoccoides pathosystem

Wild relatives of crop plants may serve as a promising source for screening for new disease resistance genes that can be utilized in breeding programs. Triticum dicoccoides, the wild progenitor of most cultivated wheats, was shown to harbor many resistance genes against the major diseases attacking cultivated wheat. Stripe rust is a devastating fungal disease that attacks wheat in many regions of the world. New races of Puccinia striiformis Westend. f. sp. tritici, the causative agent of stripe rust, have overcome most of the known Yr resistance genes in wheat. Therefore, there is a need to search for new resistance genes in the T. dicoccoides gene pool. A set of 120 T. dicoccoides accessions, collected from 13 populations representing different habitats in Israel and vicinity, was tested for resistance to three prevalent stripe rust races (38E134, 6E16 and 6E0). Of these 120 accessions, 14, 8 and 12% were resistant to races 38E134, 6E16 and 6E0, respectively, while 57, 2 and 4% were moderately resistant to these races, respectively. A unique resistance was found in the population of Mt Hermon where >80% of the accessions showed resistance to all races. Distribution of infection types (ITs) of race 38E134 showed a normal distribution that can fit a quantitative pattern of response, while the distributions of ITs of races 6E16 and 6E0 had excess of extreme values and therefore showing a qualitative pattern of response. anova testing the main factor effects and interaction showed significant effects of population, race and their interaction on IT. Significant positive correlations were obtained between the resistance to races 6E16 and 6E0 and humidity variables of the collections sites, while resistance to race 38E134 was positively correlated with temperature variables. These results show that the pathogen race can determine the type of resistance response, qualitative or quantitative, in the stripe rust—T. dicoccoides pathosystem. The obtained results also reveal that the distribution of resistance to different pathogen races can be affected by different climatic factors.