Leaf rust of barley is caused by the macrocyclic, heteroecious rust pathogen Puccinia hordei, with aecia reported from selected species of the genera Ornithogalum, Leopoldia, and Dipcadi, and uredinia and telia occurring on Hordeum vulgare, H. vulgare ssp. spontaneum, Hordeum bulbosum, and Hordeum murinum, on which distinct parasitic specialization occurs. Although Puccinia hordei is sporadic in its occurrence, it is probably the most common and widely distributed rust disease of barley. Leaf rust has increased in importance in recent decades in temperate barley-growing regions, presumably because of more intensive agricultural practices. Although total crop loss does not occur, under epidemic conditions yield reductions of up to 62% have been reported in susceptible varieties. Leaf rust is primarily controlled by the use of resistant cultivars, and, to date, 21 seedling resistance genes and two adult plant resistance (APR) genes have been identified. Virulence has been detected for most seedling resistance genes but is unknown for the APR genes Rph20 and Rph23. Other potentially new sources of APR have been reported, and additivity has been described for some of these resistances. Approaches to achieving durable resistance to leaf rust in barley are discussed.

Molecular genetic marker development in perennial ryegrass has largely been dependent on anonymous sequence variation. The availability of a large-scale EST resource permits the development of functionally-associated genetic markers based on SNP variation in candidate genes. Genic SNP loci and associated haplotypes are suitable for implementation in molecular breeding of outbreeding forage species. Strategies for in vitro SNP discovery through amplicon cloning and sequencing have been designed and implemented. Putative SNPs were identified within and between the parents of the F(1)(NA(6) x AU(6)) genetic mapping family and were validated among progeny individuals. Proof-of-concept for the process was obtained using the drought tolerance-associated LpASRa2 gene. SNP haplotype structures were determined and correlated with predicted amino acid changes. Gene-length LD was evaluated across diverse germplasm collections. A survey of SNP variation across 100 candidate genes revealed a high frequency of SNP incidence (c. 1 per 54 bp), with similar proportions in exons and introns. A proportion (c. 50%) of the validated genic SNPs were assigned to the F(1)(NA(6) x AU(6)) genetic map, showing high levels of coincidence with previously mapped RFLP loci. The perennial ryegrass SNP resource will enable genetic map integration, detailed LD studies and selection of superior allele content during varietal development.

Complementary genes for resistance to wheat stripe rust in an Avocet selection mapped to chromosome arms 3DL and 5BL. Susceptible Avocet selections lacked the 5BL gene due to a chromosomal deletion. This study reports the inheritance and genetic mapping of the YrA (temporary name of convenience to describe the specificity) seedling resistance to wheat stripe rust (caused by Puccinia striiformis f. sp. tritici; Pst) in a resistant selection of the Australian cv. Avocet [Avocet R (AvR)-AUS 90660]. Genetic analysis was performed on F2 populations and F3 generation families from crosses between wheats that carried and lacked the YrA resistance. Greenhouse seedling tests with two avirulent Pst pathotypes (104 E137 A- and 108 E141 A-) confirmed that the YrA resistance was inherited as two complementary dominant genes. Ninety-two doubled haploid (DH) lines from a cross between the Australian cv. Teal (Pst susceptible) and AvR were used for DArT-Seq genotypic analysis to map the seedling resistance. Marker-trait association analysis using 9035 DArT-Seq loci mapped the genes to the long arms of chromosomes 3D (3DL) and 5B (5BL), respectively. F2 populations from crosses between susceptible DH lines that carried either the 3DL or 5BL marker genotypes confirmed the complementary gene model. Fluorescence in situ hybridization (FISH) analysis determined that Teal carries a reciprocal T5B-7B translocation. FISH analysis also identified a 5BL chromosomal deletion in Avocet S relative to AvR that further validated the complementary gene model and possibly explained the heterogeneity of closely related wheats carrying the YrA resistance. The individual loci of the complementary YrA resistance were designated Yr73 (3DL) and Yr74 (5BL). Candidate single gene reference stocks will be permanently accessioned following cytological analysis to avoid the T5B-7B translocation.

We report on a new adult plant resistance (APR) gene Rph23 conferring resistance to leaf rust in barley. The gene was identified and characterized from a doubled haploid population derived from an intercross between the Australian barley varieties Yerong (Y) and Franklin (F). Genetic analysis of adult plant field leaf rust scores of the Y/F population collected over three successive years indicated involvement of two highly additive genes controlling APR, one of which was named Rph23. The gene was mapped to chromosome 7HS positioned at a genetic distance 36.6 cM. Rph23 is closely linked to marker Ebmac0603, which is flanked by markers bPb-8660 and bPb-9601 with linkage distances of 0.8 and 5.1 cM, respectively. A PCR-based marker was optimized for marker-assisted selection of Rph23, and on the basis of this marker, the gene was postulated as being common in Australian and global barley germplasm. Pedigree and molecular marker analyses indicated that the six-rowed black Russian landrace 'LV-Taganrog' is the likely origin of Rph23.