POWDERY MILDEW RESISTANCE IN BARLEY LANDRACES FROM WEST ASIA AND NORTH AFRICA JH CZEMBOR Department of Plant Pathology, Plant Breeding and Acclimatization Institute - IHAR, Radzikow, 05 870 Bionie, Poland. Background and objectives Powdery mildew is one of the most important diseases of barley and is of great economic importance. Breeding for resistance is considered to be the most effective and economically feasible means of powdery mildew control in barley. West Asia and North Africa are the primary centers of diversity of barley. In marginal areas of these regions farmers still rely on landraces of barley. These landraces are often collections of seeds that have been exchanged for other goods among nomad tribes or from the seeds that were handed down from generation to generation. These landraces are characteristically genetically heterogeneous. This is manifested by obvious diversity in architecture of the plants. This diversity extends to characters which cannot be readily seen, such as genes for resistance to powdery mildew. In the landrace farming system, powdery mildew rarely develops to levels that significantly damage the yield. This is due to the stabilizing effect of the genetic heterogeneity and the presence of a certain level of resistance within the barley landraces. This indicates that barley landraces may be useful as a source of genes for resistance to powdery mildew [1]. The objective of this study was to characterize barley landraces from West Asia and North Africa in terms of the genes for resistance to powdery mildew. Materials and methods This investigation was done during 1996-1998 at IHAR Radzikow, Poland. Barley landraces which were used were provided kindly by International Center for Agricultural Research in the Dry Areas - ICARDA. The total of 897 barley landraces were screened for resistance to powdery mildew of barley. About 30 plants per accession were used for seedling preliminary test. This test was done in greenhouse under controlled conditions with mixture of powdery mildew isolates with all viruiences known in Europe. The inoculation was carried out when plants were 10-1 2 days old by shaking or brushing conidia from diseased plants. These isolates were purified by single pustule isolation, maintained, and increased on young seedlings of the variety Manchuria. The Pailas isolines differential set for powdery mildew of barley [2] was used to determine the virulence spectrum of all isolates used. Results and conclusions Plants with resistant reactions were found in the 364 accessions. From 1 to 5 resistant plants per each accession were grown in greenhouse to obtain their seeds. In this manner it was created about 800 single plant lines. These lines were multiplied in the field. The following year single plant lines from 121 landraces were chosen for further investigations. These lines were tested with 1 0 isolates of powdery mildew with known virulence genes and a number of different resistance genes were detected. This investigation identified new sources of resistance to powdery mildew of barley in selections from barley landraces from West Asia and North Africa. The newly identified sources of resistance to powdery mildew may be used in barley breeding for disease resistance and may significantly contribute to genetic control of powdery mildew. References 1. Czembor JH, 1996. PhD thesis, Department of Plant Pathology, Montana State University. 2. Kolster P, Munk L, Stolen 0, Lohde J, 1986. Crop Sci., 26:903-907.