Virulence dynamics of the barley leaf rust pathogen (Puccinia hordei) in the United States from 1989 to 2020

Authors: NAZARENO, ERIC - University Of Minnesota; MATNY, OADI - University Of Minnesota; Jin, Yue; FETCH, THOMAS - Agriculture And Agri-Food Canada; Rouse, Matthew; STEFFENSON, BRIAN - University Of Minnesota

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2023
Publication Date: 12/12/2023
Citation: Nazareno, E.S., Matny, O., Jin, Y., Fetch, T.J., Rouse, M.N., Steffenson, B.J. 2023. Virulence dynamics of the barley leaf rust pathogen (Puccinia hordei) in the United States from 1989 to 2020. Plant Disease. 107:3952-3957. https://doi.org/10.1094/PDIS-03-23-0583-RE.
DOI: https://doi.org/10.1094/PDIS-03-23-0583-RE

Interpretive Summary: Barley leaf rust, caused by Puccinia hordei, is one of the most important diseases of barley worldwide. The pathogen can readily produce new races that overcome deployed barley leaf rust resistance genes (Rph genes), demonstrating the need to conduct surveys to monitor pathogen virulence. In this study, we characterized the virulence phenotypes of 519 barley leaf rust pathogen isolates collected in the United States over a span of 32 years on 15 Rph genes. P. hordei isolates exhibited higher average infection scores for three Rph genes in 2010-2020 compared to 1989-2000. Regional patterns of virulence and virulence associations were detected in the P. hordei population in both survey periods. The results of the survey can be used to inform the deployment of combinations of effective Rph genes in various regions of the United States in order to protect United States barley from leaf rust yield losses.

Technical Abstract: Barley leaf rust, caused by Puccinia hordei, is one of the most important diseases of barley worldwide. The pathogen can readily produce new races that overcome deployed resistance genes (i.e. Reaction to P. hordei or Rph genes) in barley, demonstrating the need to conduct surveys to monitor pathogen virulence. In this study, we characterized the virulence phenotypes of 519 P. hordei isolates collected in the United States over a span of 32 years on 15 Rph genes (Rph1.a through Rph15.ad). To analyze virulence patterns, we converted the raw infection type data to a linearized scale of 0-9 and divided the country into five geographical regions, namely: Pacific/West (PW), Southwest (SW), Midwest (MW), Northeast (NE), and Southeast (SE). Over the 32-year period, high mean infection scores (7.1-7.8) were observed for Rph1.a, Rph4.d, and Rph8.h; intermediate scores (4.2-7) for Rph2.b, Rph9.i, Rph10.o, Rph11.p, and Rph13.x and low scores (1-4) for Rph3.c, Rph5.e, Rph5.f, Rph7.g, Rph9.z, Rph14.ab, and Rph15.ad. Virulence for Rph2.b, Rph3.c, Rph5.e, Rph10.o, Rph11.p, Rph9.z, and Rph13.x significantly differed between the two survey periods. In 2010-2020, P. hordei isolates exhibited higher virulence for non-environmentally labile genes including Rph3.c, Rph5.e, and Rph9.z. Regional patterns of virulence were also found in both survey periods. In the 2010-2020 surveys, isolates from the SW, NE, and SE regions had higher infection scores for Rph3.c (3.1-5.2) than those from the PW and MW (1.3-2.1). The opposite was true for Rph5.e and Rph5.f, where PW and MW isolates had much higher infection scores (3.8-4.5) than those from the SW, NE, and SE (1-2.2). The infection scores for Rph7.g in isolates from PW and SW were lower (1-1.7) than those collected in the Midwest and the east coast (2.2-5.1). For Rph9.i and Rph9.z, isolates from the SW and NE regions had the highest infection scores (5.4 and 4.2, respectively), followed by SE isolates (3.5-4.9), and then by PW and MW isolates (2.8-3.7 and 2.5-3.5). In the SW region, however, Rph9.i virulence scores were higher (5.4) compared to Rph9.z (3.2). Isolates from the PW, MW, NE, and SE regions had higher infection scores for Rph14.ab (3.5-4.6) than those from the SW region (2.6). In the 1989-2000 surveys, similar regional virulence patterns were observed for Rph5.e, Rph5.f, Rph7.g, and Rph14.ab. Virulence associations were detected in the P. hordei population in both survey periods. Notably, isolates that were virulent to both Rph5.e and Rph6.f were more likely to be avirulent to Rph7.g and Rph13.x, while isolates that were virulent to Rph7.g were more likely to be avirulent to Rph5.e and Rph5.f, and to be virulent to Rph13.x. In decreasing order of effectiveness, Rph15.ad (mean infection score of 1), Rph5.e (2), Rph3.c (2.1), Rph9.z (2.3), Rph7.g (2.6), Rph5.f (2.7), Rph14.ab (3.9) were the most effective Rph genes in the United States from 1989-2020. Pyramiding Rph15.ad with other widely effective Rph and adult plant resistance genes may provide long-lasting resistance against P. hordei.