Identification and mapping of a resistance gene to barley leaf rust(Puccinia hordei G. Otth)

Barley leaf rust (Puccinia hordei G. Otth) has been the cause of numerous and often devastating disease epidemics since the beginning of agriculture. Leaf rust is one of the most important diseases that affect barley (Hordeum vulgare L.) throughout the world. The pathogen Puccinia hordei is an obligate parasite. Symptoms of barley leaf rust may range from small chlorotic flecks to large pustules containing spores. Leaf rust epidemics reduce yields and grain quality.

Deployment of resistant cultivars is one of the most effective and economical means of controlling barley leaf rust. Identification and incorporation of new and effective sources of resistance are crucial to the success of barley breeding programs. Two types of resistance have been identified. They are race-specific resistance and partial resistance. A hypersensitive reaction by the host to infection of P. hordei isolates lacking corresponding virulence genes is indicative of race-specific resistance that is controlled by major genes. Sixteen race-specific genes (Rph1 to Rph16) have been identified. Partial resistance is generally polygenic and is often more durable that race-specific resistance.

The purpose of this research is to determine the inheritance of resistance to leaf rust in the barley experimental line VA 92-42-46, to identify the gene(s) conferring resistance, identify putative resistance related markers, and to map the gene(s) to one or more barley chromosomes using molecular markers. The Virginia barley line 92-42-46 was selected for this research project because it possesses resistance to P. hordei race 30, which has overcome resistance conferred by Rph7. Crosses were made between VA 92-42-46 and Moore, a susceptible cultivar to leaf rust. Inheritance studies were performed by screening F₂ progeny and F_2:3 families against race 8 and race 30 to determine the number of leaf rust resistance genes in VA 92-42-46. Allelism tests were performed to determine gene identity. A single dominant gene at the Rph5 locus or a tightly linked gene confers the resistance to P. hordei in VA 92-42-46.

Two populations, 'Moore' X VA 92-42-46 and 'Bowman' X 'Magnif', were used in this study for mapping molecular markers to provide comparison and confirmation of results. 'Magnif' possesses the resistance gene Rph5. Bulked segregant analysis was used to identify polymorphic RFLP and SSR markers that were used for mapping in each population. Linkage analysis revealed that the Rph5 gene maps to barley chromosome 3 (3H) above the centromeric region in the 'Moore' X VA 92-42-46 population. These findings agree with previous research that identified linkage between Rph5 and Rph7 on chromosome 3. The results obtained in this study do not support previous research that had reported the resistance gene Rph5 was located on barley chromosome 7 (5H). Further research should be conducted to verify the results of this study using the 'Bowman' X 'Magnif' population. The markers screened in the region above the centromere region of barley chromosome 3 were monomorphic for the 'Bowman' X 'Magnif' population except for the marker MWG561. Therefore, additional markers above the centromere of barley chromosome 3 should be screened.