Breeding oat for resistance to the crown rust pathogen Puccinia coronata f. sp. avenae: achievements and prospects.

Crown rust, caused by Puccinia coronata f. sp. avenae (Pca), is a significant impediment to global oat production. Some 98 alleles at 92 loci conferring resistance to Pca in Avena have been designated; however, allelic relationships and chromosomal locations of many of these are unknown. Long-term monitoring of Pca in Australia, North America and elsewhere has shown that it is highly variable even in the absence of sexual recombination, likely due to large pathogen populations that cycle between wild oat communities and oat crops. Efforts to develop cultivars with genetic resistance to Pca began in the 1950s. Based almost solely on all all-stage resistance, this has had temporary benefits but very limited success. The inability to eradicate wild oats, and their common occurrence in many oat growing regions, means that future strategies to control Pca must be based on the assumption of a large and variable prevailing pathogen population with high evolutionary potential, even if cultivars with durable resistance are deployed and grown widely. The presence of minor gene, additive APR to Pca in hexaploid oat germplasm opens the possibility of pyramiding several such genes to give high levels of resistance. The recent availability of reference genomes for diploid and hexaploid oat will undoubtedly accelerate efforts to discover, characterise and develop high throughput diagnostic markers to introgress and pyramid resistance to Pca in high yielding adapted oat germplasm.

Discovery and fine mapping of Rph28: a new gene conferring resistance to Puccinia hordei from wild barley.

KEY MESSAGE: A new gene Rph28 conferring resistance to barley leaf rust was discovered and fine-mapped on chromosome 5H from wild barley. Leaf rust is a highly destructive disease of barley caused by the fungal pathogen Puccinia hordei. Genetic resistance is considered to be the most effective, economical and eco-friendly approach to minimize losses caused by this disease. A study was undertaken to characterize and fine map a seedling resistance gene identified in a Hordeum vulgare ssp. spontaneum-derived barley line, HEB-04-101, that is broadly effective against a diverse set of Australian P. hordei pathotypes. Genetic analysis of an F3 population derived from a cross between HEB-04-101 and the H. vulgare cultivar Flagship (seedling susceptible) confirmed the presence of a single dominant gene for resistance in HEB-04-101. Selective genotyping was performed on representative plants from non-segregating homozygous resistant and homozygous susceptible F3 families using the targeted genotyping-by-sequencing (tGBS) assay. Putatively linked SNP markers with complete fixation were identified on the long arm of chromosome 5H spanning a physical interval between 622 and 669 Mb based on the 2017 Morex barley reference genome assembly. Several CAPS (cleaved amplified polymorphic sequences) markers were designed from the pseudomolecule sequence of the Morex assembly (v1.0 and v2.0), and 16 polymorphic markers were able to delineate the RphHEB locus to a 0.05 cM genetic interval spanning 98.6 kb. Based on its effectiveness and wild origin, RphHEB is distinct from all other designated Rph genes located on chromosome 5H and therefore the new locus symbol Rph28 is recommended for RphHEB in accordance with the rules and cataloguing system of barley gene nomenclature.

Characterizing the Genetic Architecture of Nonhost Resistance in Barley Using Pathogenically Diverse Puccinia Isolates.

Barley is an intermediate or near nonhost to many cereal rust pathogens that infect grasses, making it a highly suitable model to understand the evolution and genetic basis of nonhost resistance (NHR) in plants. To characterize the genetic architecture of NHR in barley, we used the Oregon Wolfe Barley doubled haploid and Morex × SusPtrit recombinant inbred line mapping populations. To elicit a wide array of NHR responses, we tested 492 barley accessions and both mapping populations with pathogenically diverse cereal rust isolates representing distinct formae speciales adapted to Avena, Hordeum, Triticum, and Lolium spp.: P. coronata f. sp. avenae (oat crown rust pathogen) and P. coronata f. sp. lolii (ryegrass crown rust pathogen), P. graminis f. sp. avenae (oat stem rust pathogen) and P. graminis f. sp. lolii (the ryegrass stem rust pathogen), and P. striiformis f. sp. tritici (wheat stripe rust pathogen) and P. striiformis f. sp. pseudo-hordei (barley grass stripe rust pathogen). With the exception of P. coronata f. sp. lolii and P. coronata f. sp. avenae, susceptibility and segregation for NHR was observed in the barley accessions and both mapping populations. Quantitative trait loci (QTLs) for NHR were mapped on all seven chromosomes. NHR in barley to the heterologous rusts tested was attributable to a combination of QTLs with either or both overlapping and distinct specificities. Across both mapping populations, broadly effective NHR loci were also identified that likely play a role in host specialization.

Validating Molecular Markers for Barley Leaf Rust Resistance Genes Rph20 and Rph24.

Improving resistance to barley leaf rust (caused by Puccinia hordei) is an important breeding objective in most barley growing regions worldwide. The development and subsequent utilization of high-throughput PCR-based codominant molecular markers remains an effective approach to select genotypes with multiple effective resistance genes, permitting efficient gene deployment and stewardship. The genes Rph20 and Rph24 confer widely effective adult plant resistance (APR) to leaf rust, are common in European and Australian barley germplasm (often in combination), and act interactively to confer high levels of resistance. Here we report on the development and validation of codominant insertion-deletion (indel) based PCR markers that are highly predictive for the resistance alleles Rph20.ai and Rph24.an (both referred to as Rph20 and Rph24).

Isolate Specificity and Polygenic Inheritance of Resistance in Barley to Diverse Heterologous Puccinia striiformis Isolates.

Barley is a host to Puccinia striiformis f. sp. hordei, and is an intermediate or near nonhost to the formae speciales adapted to wheat (P. striiformis f. sp. tritici) and to barley grass (P. striiformis f. sp. pseudo-hordei). The genetic basis of resistance to these forms of P. striiformis is not well understood. Accordingly, a recombinant inbred line (RIL) population was developed using a P. striiformis-susceptible accession (Biosaline-19) and the immune cultivar Pompadour. We investigated the genetic basis of resistance to four diverse P. striiformis isolates (P. striiformis f. sp. pseudo-hordei, and P. striiformis f. sp. tritici pathotypes 104 E137 A-, 134 E16 A+, and 64 E0 A-). and determined that the immunity in Pompadour at the seedling stage to the different P. striiformis isolates was due to quantitative trait loci (QTL) on chromosomes 1H, 3H, 5H, and 7H with both overlapping and distinct specificities. Further histological analysis confirmed the presence of isolate specificity. The RILs were also assessed in the field for resistance to P. striiformis f. sp. pseudo-hordei, P. striiformis f. sp. hordei, and the leaf rust pathogen (P. hordei) to identify pleiotropic QTL loci effective at the adult plant stage and determine whether the leaf rust resistance in Pompadour (Rph20) was also effective to P. striiformis. RILs that were seedling susceptible to P. striiformis f. sp. pseudo-hordei were resistant in the field, implicating the involvement of adult plant resistance (APR). Additional QTLs were identified on chromosome 7H at the same genetic position as Rph23 (APR to leaf rust), suggesting either pleiotropic resistance or the presence of a stripe rust resistance gene closely linked to or allelic with Rph23. Unlike many pleiotropic APR genes identified and isolated in wheat, our data suggest that the Rph20 locus does not confer resistance to the P. striiformis isolates used in this study (P. striiformis f. sp. hordei [χ2 (independence) = 2.47 P > 0.12] and P. striiformis f. sp. pseudo-hordei [χ2 (independence) = 0.42 P > 0.60]).

Investigating successive Australian barley breeding populations for stable resistance to leaf rust.

KEY MESSAGE: Genome-wide association studies of barley breeding populations identified candidate minor genes for pairing with the adult plant resistance gene Rph20 to provide stable leaf rust resistance across environments. Stable resistance to barley leaf rust (BLR, caused by Puccinia hordei) was evaluated across environments in barley breeding populations (BPs). To identify genomic regions that can be combined with Rph20 to improve adult plant resistance (APR), two BPs genotyped with the Diversity Arrays Technology genotyping-by-sequencing platform (DArT-seq) were examined for reaction to BLR at both seedling and adult growth stages in Australian environments. An integrated consensus map comprising both first- and second-generation DArT platforms was used to integrate QTL information across two additional BPs, providing a total of four interrelated BPs and 15 phenotypic data sets. This enabled identification of key loci underpinning BLR resistance. The APR gene Rph20 was the only active resistance region consistently detected across BPs. Of the QTL identified, RphQ27 on chromosome 6HL was considered the best candidate for pairing with Rph20. RphQ27 did not align or share proximity with known genes and was detected in three of the four BPs. The combination of RphQ27 and Rph20 was of low frequency in the breeding material; however, strong resistance responses were observed for the lines carrying this pairing. This suggests that the candidate minor gene RphQ27 can interact additively with Rph20 to provide stable resistance to BLR across diverse environments.

Isolate Specificity and Polygenic Inheritance of Resistance in Barley to the Heterologous Rust Pathogen Puccinia graminis f. sp. avenae.

Barley is a near-nonhost to numerous heterologous (nonadapted) rust pathogens because a small proportion of genotypes are somewhat susceptible. We assessed 66 barley accessions and three mapping populations (Vada × SusPtrit, Cebada Capa × SusPtrit, and SusPtrit × Golden Promise) for response to three Swedish oat stem rust (Puccinia graminis f. sp. avenae) fungal isolates and determined that barley is a near-nonhost to P. graminis f. sp. avenae and that resistance was polygenically inherited. The parental genotypes Vada and Golden Promise were immune to all three isolates, whereas Cebada Capa was immune to two isolates and moderately resistant to the third. Phenotypic data from the Vada × SusPtrit mapping population and the barley accessions tested also demonstrated isolate-specific resistance. In particular, the SusPtrit parent and several other accessions allowed sporulation by isolate Ingeberga but were resistant to isolate Evertsholm. Nine chromosomal regions carried quantitative trait loci (QTL) (Rpgaq1 to Rpgaq9) of varying effect, most of which colocated to previously identified QTL for resistance to other heterologous rust pathogens. Rpgaq1 on chromosome 1H (Vada and Golden Promise) was effective toward all isolates tested. Microscopic examination indicated that resistance was prehaustorial in Vada whereas, in SusPtrit, both pre- and posthaustorial mechanisms play a role.

Mapping of seedling resistance in barley to Puccinia striiformis f. sp. pseudohordei.

The barley grass stripe rust (BGYR) pathogen Puccinia striiformis f. sp. pseudohordei was first detected in Australia in 1997. While studies have established that it is virulent on wild barley grass, and can infect several barley cultivars, the basis of genetic resistance to this pathogen in barley is largely unknown. Understanding the genetic basis of host resistance and ensuring the selection of germplasm with multiple resistance genes are important to mitigate the potential impact of BGYR in barley production. Genetic analysis of seedling resistance to BGYR in two barley doubled haploid populations, Amaji Nijo/WI2585 (AN/WI) and Galleon/Haruna Nijo (GL/HN), indicated that resistance is governed by several genes. Marker regression analysis of the seedling resistance data from the AN/WI population detected a major QTL, BGYR_WI1 (resistance contributed by WI2585 with the closest marker explaining 52 % of the total phenotypic effect) on chromosome 1HS, flanked by the loci Xabg59 and Xabc310b at map positions 0.0 and 6.9 cM, respectively. Similarly, a major QTL, BGYR_HN1, (resistance contributed by Haruna Nijo with the closest marker explaining 70 % of the total phenotypic effect) was detected in the GL/HN population and was mapped to 1HS, flanked by the loci Xbcd135 and XHOR1 at map positions 12.8 and 24.5 cM, respectively. In addition, several minor loci that provided resistance against BGYR were detected in both populations. While defined QTL intervals were large, the analysis nonetheless provides new information on sources of major QTL controlling resistance to BGYR.

Genetic analysis of seedling resistance to crown rust in five diploid oat (Avena strigosa) accessions.

Crown rust, caused by Puccinia coronata Corda f. sp. avenae Eriks., is a serious menace in oats, for which resistance is an effective means of control. Wild diploid oat accessions are a source of novel resistances that first need to be characterised prior to introgression into locally adapted oat cultivars. A genetic analysis of resistance to crown rust was carried out in three diverse diploid oat accessions (CIav6956, CIav9020, PI292226) and two cultivars (Saia and Glabrota) of A. strigosa. A single major gene conditioning resistance to Australian crown rust pathotype (Pt) 0000-2 was identified in each of the three accessions. Allelism tests suggested that these genes are either the same, allelic, or tightly linked with less than 1 % recombination. Similarly, a single gene was identified in Glabrota, and possibly two genes in Saia; both cultivars previously reported to carry two and three crown rust resistance genes, respectively. The identified seedling resistance genes could be deployed in combination with other resistance gene(s) to enhance durability of resistance to crown rust in hexaploid oat. Current diploid and hexaploid linkage maps and molecular anchor markers (simple sequence repeat [SSR] and diversity array technology [DArT] markers) should facilitate their mapping and introgression into hexaploid oat.

Characterisation and mapping of gene Lr73 conferring seedling resistance to Puccinia triticina in common wheat.

KEY MESSAGE: A gene conferring seedling resistance to Puccinia triticina was mapped to chromosome 2BS in the wheat Morocco. The gene was shown to be distinct and was therefore designated Lr73. The wheat genotype Morocco, widely susceptible to isolates of Puccinia triticina, was resistant to an Australian isolate of this pathogen collected in 2004. Genetic studies established that the resistance in Morocco was also present the Australian wheat genotypes Avocet, Halberd, Harrier, Tincurrin and a selection of cultivar Warigal lacking the resistance gene Lr20. Genetic studies based on a cross with Halberd showed that the gene is dominant and located on chromosome 2BS (XwPt8760-4 cM-Lr73-1.4 cM-XwPt8235). The gene was genetically independent of the Lr13, Lr16 and Lr23 loci, also located on chromosome 2BS, indicating that it is distinct. The locus designation Lr73 was therefore assigned. On the basis of multi-pathotype tests, it is likely Lr73 is also present in the Australian wheat cultivars Clearfield STL, Federation (with Lr10), Gatcher (with Lr10 and Lr27+Lr31), Marombi (with Lr1 and Lr37), Pugsley (with Lr1 and Lr37), Spear (with Lr1), Stiletto and Tarsa (with Lr1). Gene Lr73 is unlikely to be of value in resistance breeding. However, recognising Lr73 is important to avoid its inadvertent selection in breeding programmes. Furthermore, the apparent rarity of avirulence for genes like Lr73, sometimes referred to as "fossil" resistance genes, makes them of interest in terms of the evolution of disease resistance in host plants and of virulence in the respective rust pathogens.

Analysis of Stem Rust Resistance in Australian Barley Cultivars.

Eighty-two Australian and five exotic barley cultivars were evaluated at the seedling stage for resistance to the Australian stem rust pathotype 98-1,2,3,5,6. Although most of these cultivars exhibited mesothetic (mixed infection type) reactions that were associated with a high level of chlorosis, two ('O'Connor' and 'Pacific Ranger') were highly resistant. Marker analysis indicated that four Australian cultivars ('Empress', 'Vlamingh', Pacific Ranger, and 'Yerong') possess the stem rust resistance gene Rpg1. Tests conducted using North American Puccinia graminis f. sp. tritici pathotypes MCCJ and QCCJ supported marker results and indicated that 'Pacific Ranger' and 'Vlamingh' likely carry additional stem rust resistance genes. Based on pedigree information and results from multipathotype tests, these genes are believed to be uncharacterized and, therefore, new. The resistance in Australian barley 'Franklin' conferred resistance against all pathotypes tested in this study. Studies of inheritance to MCCJ revealed that it possessed an unknown seedling resistance, which was independent of and displayed additivity to Rpg1.

Permanent genetic resources added to Molecular Ecology Resources Database 1 February 2013-31 March 2013.

This article documents the addition of 142 microsatellite marker loci to the Molecular Ecology Resources database. Loci were developed for the following species: Agriophyllum squarrosum, Amazilia cyanocephala, Batillaria attramentaria, Fungal strain CTeY1 (Ascomycota), Gadopsis marmoratus, Juniperus phoenicea subsp. turbinata, Liriomyza sativae, Lupinus polyphyllus, Metschnikowia reukaufii, Puccinia striiformis and Xylocopa grisescens. These loci were cross-tested on the following species: Amazilia beryllina, Amazilia candida, Amazilia rutila, Amazilia tzacatl, Amazilia violiceps, Amazilia yucatanensis, Campylopterus curvipennis, Cynanthus sordidus, Hylocharis leucotis, Juniperus brevifolia, Juniperus cedrus, Juniperus osteosperma, Juniperus oxycedrus, Juniperus thurifera, Liriomyza bryoniae, Liriomyza chinensis, Liriomyza huidobrensis and Liriomyza trifolii.

Inheritance and molecular mapping of a gene conferring seedling resistance against Puccinia hordei in the barley cultivar Ricardo.

Genetic studies were undertaken to determine the inheritance and genomic location of uncharacterised seedling resistance to leaf rust, caused by Puccinia hordei, in the barley cultivar Ricardo. The resistance was shown to be conferred by a single dominant gene, which was tentatively designated RphRic. Bulk segregant analysis (BSA) and genetic mapping of an F(3) mapping population using multiplex-ready SSR genotyping and Illumina GoldenGate SNP assay located RphRic in chromosome 4H. Given that this is the first gene for leaf rust resistance mapped on chromosome 4H, it was designated Rph21. The presence of an additional gene, Rph2, in Ricardo, was confirmed by the test of allelism. The seedling gene Rph21 has shown effectiveness against all Australian pathotypes of P. hordei tested since at least 1992 and hence represents a new and useful source of resistance to this pathogen.

Mapping Rph20: a gene conferring adult plant resistance to Puccinia hordei in barley.

A doubled haploid (DH) barley (Hordeum vulgare L.) population of 334 lines (ND24260 × Flagship) genotyped with DArT markers was used to map genes for adult plant resistance (APR) to leaf rust (Puccinia hordei Otth) under field conditions in Australia and Uruguay. The Australian barley cultivar Flagship carries an APR gene (qRphFlag) derived from the cultivar Vada. Association analysis and composite interval mapping identified two genes conferring APR in this DH population. qRphFlag was mapped to the short arm of chromosome 5H (5HS), accounting for 64-85% of the phenotypic variation across four field environments and 56% under controlled environmental conditions (CEC). A second quantitative trait locus (QTL) from ND24260 (qRphND) with smaller effect was mapped to chromosome 6HL. In the absence of qRphFlag, qRphND conferred only a low level of resistance. DH lines displaying the highest level of APR carried both genes. Sequence information for the critical DArT marker bPb-0837 (positioned at 21.2 cM on chromosome 5HS) was used to develop bPb-0837-PCR, a simple PCR-based marker for qRphFlag. The 245 bp fragment for bPb-0837-PCR was detected in a range of barley cultivars known to possess APR, which was consistent with previous tests of allelism, demonstrating that the qRphFlag resistant allele is common in leaf rust resistant cultivars derived from Vada and Emir. qRphFlag has been designated Rph20, the first gene conferring APR to P. hordei to be characterised in barley. The PCR marker will likely be effective in marker-assisted selection for Rph20.

Mapping genes Lr53 and Yr35 on the short arm of chromosome 6B of common wheat with microsatellite markers and studies of their association with Lr36.

The rust resistance genes Lr53 and Yr35, transferred to common wheat from Triticum dicoccoides, were reported previously to be completely linked on chromosome 6B. Four F (3) families were produced from a cross between a line carrying Lr53 and Yr35 (98M71) and the leaf rust and stripe rust susceptible genotype Avocet "S" and were rust tested using Puccinina triticina pathotype 53-1,(6),(7),10,11 and Puccinia striiformis f. sp. tritici pathotype 110 E143 A+. The homozygous resistant lines produced infection types of ";1-" and ";N" to these pathotypes, respectively. The Chi-squared tests indicated goodness-of-fit of the data for one leaf rust gene and one stripe rust gene segregation. Linkage analysis using this population demonstrated recombination of 3% between the genes. Microsatellite markers located on the short arm of chromosome 6B were used to map the genes, with the markers cfd1 and gwm508 being mapped approximately 1.1 and 4.5 cM, respectively, proximal to Lr53. Additional studies of the relationship between Lr36, also located on the short arm of chromosome 6B, and Lr53 indicated that the two genes were independent.

Detection of Wheat Stem Rust (Puccinia graminis f. sp. tritici) Race TTKSK (Ug99) in Iran.

In 2007, new reports of stem rust caused by Puccinia graminis Pers. f. sp. tritici Eriks. in Lorestan and Hamadan provinces of Iran were considered unusual because stem rust had not been recorded previously in the Hamadan area where winter habit wheat cultivars are grown. Detailed investigations in these areas showed significant levels of stem rust in experimental plots and occasionally in farmers' fields, some that showed moderate to high levels of infection. Race analysis of four stem rust samples collected from Borujerd, Hamadan, and Poldokhtar (southwest) and Kelardasht (north) in 2007 was conducted using a modified North American Pgt differential set representing the resistance genes Sr5, 6, 7b, 8a, 9a, 9b, 9d, 9e, 9g, 10, 11, 17, 21, 24, 30, 31, 36, 38, Tmp, and McN, commercial cultivars, and genotypes known to carry the 1B.1R translocation. A race collected from Borujerd in 1997 was also included for comparison. Tests were carried out under standard controlled conditions (1,2). Two isolates from samples collected from Borujerd and Hamadan in 2007 showed high infection types (ITs 33+ to 4) on differential lines carrying resistance genes Sr5, 6, 7b, 8a, 9a, 9b, 9d, 9e, 9g, 10, 11, 17, 21, 30, 31, 38, and McN, and low ITs of ;C1= to 2=, ;C to ;N1=, and 2+ on lines carrying Sr24, Sr36, and SrTmp, respectively. On the basis of the high/low ITs on the 20 differentials in the modified Pgt differential set of North America, the two isolates of Pgt collected from Borujerd and Hamadan in 2007 were identified as race TTKSK. The two isolates from samples collected from Poldokhtar and Kelardasht in 2007 and the isolate collected from Borujerd in 1997 were identified as races TRFSC, TTJQC, and RRHSC, respectively. Race TTKSK identified in the current study produced high ITs of 3+ to 4 on the wheat genotypes Line E*4/Kavkaz, Fed.*4/Kavkaz, Clement, and Mildress and commercial cultivars Falat (Seri 82), Shiroodi (CIMMYT name Attila and Indian name PBW343), Atrak (Kauz), and MV17, all carrying the 1BL.1RS translocation and further confirming virulence for Sr31. The spread of Ug99 to Kenya (1999 to 2002), Ethiopia (2003), and Yemen (2006) suggests progressive migration from Uganda, following the pattern believed to have occurred for the spread of wheat stripe rust pathogen from East Africa in 1986 to India in 1998 (3). Our results are consistent with the TTKSK race identified in Iran migrating from the new African population. Seedling evaluation of Iranian wheat cultivars and advanced lines to isolates of TTKSK from Iran confirmed full susceptibility. These results reinforce the serious threat of race TTKSK to wheat production in Iran. In conclusion, the occurrence of race TTKSK in Iran, the susceptibility of Iranian wheat cultivars to this race, the presence of environmental conditions conducive to disease epidemics in different parts of the country, and the occurrence of the alternate host barberry in many of the mountainous areas of Iran, indicate a new and serious threat to wheat production in Iran and a potentially serious threat to neighboring countries. References: (1) Y. Jin et al. Plant Dis. 91:1096, 2007. (2) Z. A. Pretorius et al. Plant Dis. 84:203, 2000. (3) R. P. Singh et al. CAB Rev. 1 (No. 054), 2006.

Genetic Diversity in Australian Populations of Puccinia graminis f. sp. avenae.

ABSTRACT Sequence-tagged microsatellite profiling was used to develop 110 microsatellites for Puccinia graminis f. sp. tritici (causal agent of wheat stem rust). Low microsatellite polymorphism was exhibited among 10 pathogenically diverse P. graminis f. sp. tritici isolates collected from Australian cereal growing regions over a period of at least 70 years, with two polymorphic loci detected, each revealing two alleles. Limited cross-species amplification was observed for the wheat rust pathogens, P. triticina (leaf rust) and P. striiformis f. sp. tritici (stripe rust). However, very high transferability was revealed with P. graminis f. sp. avenae (causal agent of oat stem rust) isolates. A genetic diversity study of 47 P. graminis f. sp. avenae isolates collected from an Australia-wide survey in 1999, and a historical group of 16 isolates collected from Australian cereal growing regions from 1971 to 1996, revealed six polymorphic microsatellite loci with a total of 15 alleles. Genetic analysis revealed the presence of several clonal lineages and subpopulations in the pathogen population, and wide dispersal of identical races and genotypes throughout Australian cereal-growing regions. These findings demonstrated the dynamic population structure of this pathogen in Australia and concur with the patterns of diversity observed in pathogenicity studies.

Pathogenic Specialization and Pathotype Distribution of Puccinia hordei in Australia, 1992 to 2001.

Annual surveys of pathogenic variability in the leaf rust pathogen of barley, Puccinia hordei, from 1992 to 2001 revealed a significant shift in the composition of populations across Austra-lia. Virulence for the resistance gene Rph12, first detected in a single pathotype, 4610P+, in Tasmania in 1991, was subsequently detected in 1993 in South Australia, Victoria, and southern New South Wales. By the end of 2001, eight pathotypes with virulence for Rph12 had been isolated, and virulence for this gene was present in all Australian barley growing regions. Virulence was not detected for the resistance genes Rph3, Rph7, Rph11, or Rph14. The distribution and spread of the pathotypes detected, their possible origins, pathogenicity on several uncharacterized seedling resistance sources, and implications for resistance breeding are discussed.