Resistance to powdery mildew in selections from Tunisian landraces of barley

Two-hundred and thirty-two accessions of barley landraces collected from Tu nisia were screened for resistance to powdery mildew. A number of race-spec ific genes were detected using the detached leaf technique. Among the 232 a ccessions tested, 169 were susceptible to powdery mildew, 20 were resistant and 43 showed differential reactions to the three isolates of powdery mild ew used. An attempt was made to determine the number of genes, the types of gene, the types of gene action and the gene loci in 20 resistant accession s. Three types of cross were made: (1) the accessions were crossed to the s usceptible variety 'Pallas', (2) the accessions were crossed with 'Pallas' isolines, and (3) accessions with identical powdery mildew reaction pattern s were intercrossed. Three isolates of Erysiphe graminis f. sp. hordei were used: Bzm-1, KM 18-75, R13C. A number of different resistance genes were d etected among the 20 resistant accessions. Surprisingly, segregation indica ting single genes only were detected with the isolates used. Some of these genes could be associated with loci already known. In 19 cases a dominant a nd in one a recessive mode of inheritance was detected. The recessive gene was not located at the mlo locus. This investigation represents the first s ystematic study of race-specific genes for powdery mildew resistance in Tun isian landraces. The newly identified sources of resistance may be used in many strategies of breeding for disease resistance.