agronomy Communication Mlo Resistance to Powdery Mildew (Blumeria graminis f. sp. hordei) in Barley Landraces Collected in Yemen Jerzy H. Czembor * and El ˙ zbieta Czembor   Citation: Czembor, J.H.; Czembor, E. Mlo Resistance to Powdery Mildew (Blumeria graminis f. sp. hordei) in Barley Landraces Collected in Yemen. Agronomy 2021, 11, 1582. https:// doi.org/10.3390/agronomy11081582 Academic Editor: Karen Barry Received: 5 July 2021 Accepted: 5 August 2021 Published: 9 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Plant Breeding and Acclimatization Institute—National Research Institute (IHAR-PIB), Radzikow, 05-870 Blonie, Poland; e.czembor@ihar.edu.pl * Correspondence: j.h.czembor@ihar.edu.pl Abstract: Barley (Hordeum vulgare L.) is one of the most important cereal crops in the world. Powdery mildew on barley, which is caused by the pathogen Blumeria graminis f. sp. hordei, occurs world-wide and can result in severe yield loss. Thousands of barley accessions are stored in national gene banks, and their characterization for breeding purposes is needed. This study was conducted to determine the resistance to powdery mildew in 33 barley landraces from Yemen, which were obtained from the ICARDA gene bank. Twenty differential isolates of barley powdery mildew were used. Nine single plant lines were selected from five landraces, based on tests that were performed with 30 plants per landrace, after inoculation with the most avirulent isolate of barley powdery mildew available. Two of these landraces originated from the Al Bayda province in Yemen, and three others originated from Dhamar, Sanaa, and Taizz, respectively. Next, single plant lines were tested using a set of 20 differential isolates ofpowdery mildew. Two lines that were selected from landrace from the Al Bayda province in Yemen, showed disease reaction designated as 0(4), which is specific for the presence of Mlo resistance. The new source of highly effective Mlo powdery mildew resistance that is described in this study could be used in barley breeding programs. Keywords: Hordeum vulgare; Blumeria graminis; barley landraces; Mlo resistance; resistance genes; powdery mildew 1. Introduction Barley (Hordeum vulgare L.) is the fourth most important cereal crop in the world. In many regions, barley is often grown in marginal agricultural areas with low annual precipitation (often less than 220 mm). Landraces in these areas are important, as they are often the only rain-fed crop possible and they are cultivated on mountain slopes, at elevations higher than other cereals. They are often grown not only for grain, but also for straw [1]. More than 485,000 accessions of the genus Hordeum are stored at more than 200 institutions worldwide [2]. These collections include new and old cultivars, landraces, mutants, breeding lines, and research and mapping plant materials of H. vulgare ssp. vulgare (299,165 accessions), wild barley H. vulgare ssp. spontaneum (32,385 accessions), and wild species of Hordeum (4681 accessions) [3]. Because of quarantine issues and safety reasons, many accessions are duplicated in gene banks. These genetic resources are of great value for breeding new cultivars that are well adapted to changing climate and weather anomalies, or more resistant to abiotic and biotic stresses [4–6]. The fungus Blumeria graminis (DC.) Golovin ex Speer f. sp. hordei Em. Marchal is considered to be one of the most destructive foliar pathogens of barley in many regions of the world. In countries where mildew is a problem, yield losses in experimental tests usually exceed 25%. However, the average annual losses in barley production are lower, and in Central Europe they are estimated to be about 10% [7]. The best means of controlling powdery mildew was using resistance genes. However, the resistance that is conferred by most of these genes has not been maintained for more than a few years (5–10 years), Agronomy 2021, 11, 1582. https://doi.org/10.3390/agronomy11081582 https://www.mdpi.com/journal/agronomy