Phenotypic characterization of the Hordeum bulbosum derived leaf rust resistance genes Rph22 and Rph26 in barley

Virulence Dynamics of the Barley Leaf Rust Pathogen (Puccinia hordei) in the United States from 1989 to 2020

Barley leaf rust, caused by Puccinia hordei, is an important disease of barley worldwide. The pathogen can develop new races that overcome resistance genes, emphasizing the need for monitoring its virulence. This study characterized 519 P. hordei isolates collected in the United States from the 1989 to 2000 and 2010 to 2020 survey periods on 15 Rph (Reaction to Puccinia hordei) genes. We analyzed linearized infection type data to detect virulence patterns across the United States and in five geographical regions: Pacific/West (PW), Southwest (SW), Midwest (MW), Northeast (NE), and Southeast (SE). Over 32 years, we observed high mean infection scores for Rph1.a, Rph4.d, and Rph8.h; intermediate scores for Rph2.b, Rph9.i, Rph10.o, Rph11.p, and Rph13.x; and low scores for Rph3.c, Rph5.e, Rph5.f, Rph7.g, Rph9.z, Rph14.ab, and Rph15.ad. Virulence for Rph2.b, Rph3.c, Rph5.e, Rph9.z, Rph10.o, Rph11.p, and Rph13.x significantly differed between the two survey periods. From 1989 to 2020, regional patterns of virulence were found for Rph5.e, Rph5.f, Rph7.g, and Rph14.ab, while regionalities of virulence for Rph3.c, Rph9.i, Rph9.z were only observed in the 2010 to 2020 survey period. Virulence associations were also detected in the P. hordei population. Notably, isolates that were virulent to Rph5.e and Rph6.f were more likely to be avirulent to Rph7.g and Rph13.x, and vice versa. In decreasing order of effectiveness, Rph15.ad, Rph5.e, Rph3.c, Rph9.z, Rph7.g, Rph5.f, and Rph14.ab were the most effective Rph genes in the United States from 1989 to 2020. Pyramiding Rph15.ad with other widely effective Rph and adult plant resistance genes may provide long-lasting resistance against P. hordei.

Exploring Wild Hordeum spontaneum and Hordeum marinum Accessions as Genetic Resources for Fungal Resistance

Crop Wild Relatives (CWRs), as potential sources of new genetic variants, are being extensively studied to identify genotypes that will be able to confer resistance to biotic stresses. In this study, a collection of barley wild relatives was assessed in the field, and their phenotypic variability was evaluated using a Barley Description List, reflecting the identified ecosites. Overall, the CWRs showed significant field resistance to various fungal diseases. To further investigate their resistance, greenhouse tests were performed, revealing that several CWRs exhibited resistance against Fusarium culmorum, Pyrenophora teres, and Puccinia hordei G.H. Otth. Additionally, to characterize the genetic diversity within the collection, DNA polymorphisms at 21 loci were examined. We successfully employed barley-specific SSR markers, confirming their suitability for identifying H. spontaneum and even H. marinum, i.e., perennial species. The SSR markers efficiently clustered the investigated collection according to species and ecotypes, similarly to the phenotypic assessment. Moreover, SSR markers associated with disease resistance revealed different alleles in comparison to those found in resistant barley cultivars. Overall, our findings highlight that this evaluated collection of CWRs represents a valuable reservoir of genetic variability and resistance genes that can be effectively utilized in breeding programs.