Evaluation of barley landraces for resistance to leaf rust disease (Puccinia hordei)

Introduction
Leaf rust, caused by the fungal pathogen Puccinia hordei, is a major disease in barley that can cause significant damage to barley yield and quality. The use of genetic resistance is the most sustainable strategy for controlling this disease. Genetic resistance to barley leaf rust is divided into two general categories: seedling resistance or all-stage resistance (ASR) and adult plant resistance (APR). Due to the dynamics of the pathogen population, most ASR genes have become ineffective, and there is limited diversity in APR genes. This highlights the importance of searching for and identifying new sources of resistance to this disease. Barley landraces are considered a rich source of disease resistance genes. In this regard, the present study was conducted to screen local germplasm in the barley collection of the National Plant Gene Bank of Iran and to find sources of resistance to barley leaf rust disease.

Materials and methods
A total of 220 accessions from the barley collection of the National Plant Gene Bank of Iran, originating from different regions of the country, were evaluated under natural infection conditions for three consecutive cropping years 2021-2022, 2022-2023 and 2023-2024 in the field of Iraqi-Mahalleh Research Station of Gorgan as the hotspot of leaf rust disease. Traits of infection type, disease severity, and infection coefficient were used for evaluation. The relationship between resistance components was examined through correlation analysis. A significant difference in the variance of the infection coefficient was observed among the ten defined groups. The studied accessions were separated in a biplot based on their resistance rank and stability of response rank. Classification of the acccessions based on the infection coefficient and stability of resistance response was performed using K-means clustering with K set to 5. The resulting clusters were distinguished from one another in a multidimensional scaling (MDS) plot. All statistical analyses and visualizations were conducted using custom scripts in R software, version 4.3.2, within the RStudio environment, version 2023.9.1.494.

Research findings
The results of investigating the frequency of infection type in the evaluated germplasm showed that in the first year, seven accessions (3.18%) showed infection type O (immunity reaction), six accessions (2.73%) showed infection type R, and 11 accessions (5%) showed infection type MR. In the second year, 22 accessions (10%) had an immunity reaction and 10 (4.55%) and 22 (10%) accessions had infection types R and MR, respectively. In the third year, three (1.36%) and eight (3.46%) accessions showed infection types R and MR, respectively. The average disease severity in the third year was significantly higher than in the first and second years. The accessions located at the two extreme limits of the coefficient of infection had more stable reactions than the accessions with an intermediate coefficient of infection. The studied germplasm were differentiated into five groups based on the amount and changes in resistance reaction, and the results of statistical analyzes confirmed their proper separation and differentiation.

Conclusion
The results of different methods of data analysis of the research showed evidence of changes in pathogenicity in different years, which indicates the dynamics of the pathogen population in this disease hotspot and the need for continuous search to identify sources with effective resistance. The accessions KC18607, KC70044, KC18653, KC20636, KC19610, KC20924, KC18761, KC18394, KC20560, KC70441 and KC20806 showed stable resistance. Identifying resistant accessions in these landraces indicates their valuable capacity as sources of resistance to leaf rust disease. This resistant germplasm can be used in breeding programs as well as in genetic studies to identify resistance genes and develop markers.