The procedure is described for developing 10 pairs of barley (Hordeum vulgate L.) lines, each isogenic except for a specific pair of alleles conditioning resistance and suspecptibility to culture CR3 of Erysiphe graminis (DC.) Merat hordei Em. Marchal. Each pair of lines was selected after four backcrosses to the cultivar ‘Manchuria,’ followed by 12 to 15 selfed generations. Four pairs of lines, developed from cultivars ‘Goldfoil,’ ‘Hanna,’ ‘Kwan,’ and ‘Psaknon’ differ by the resistance genes Ml‐g, Ml.h, Ml‐k, and MI‐p and their recessive alleles, respectively. Six pairs of lines differ by genes at the complex Ml‐a locus. The fines developed from cultivars ‘Algerian,’ ‘Franger,’ and ‘Durani’ differ by the resistance genes Ml‐a, Ml‐a6, and Ml‐a10 and their recessive alleles, respectively; and those developed from cultivars ‘Multan,’ ‘Long Glumes,’ and ‘Rupee’ each differ by one resistance gene and its recessive allele.

The reactions of 233 Triticum dicoccoides acessions, collected at 10 sites in Israel and elsewhere, to infection with cultures of Erysiphe graminis tritici, were determined . The reactions indicated that the number of sources of resistance to E. graminis tritici which can be obtained from T. dicoccoides plants growing wild in Israel and elsewhere is almost unlimited . One hundred and fourteen or 49% of the accessions were resistant, and 137 or 59% of the accessions were resistant or moderately resistant to infection with four cultures of E. graminis tritici which possess the virulence genes corresponding to most of the identified resistance genes in wheat . Accessions collected at sites with marginal habitats where T. dicoccoides grows poorly and has lower grain weight, were more susceptible than were accessions collected at sites with an optimal habitat for growth of T. dicoccoides . The results agreed with those in a previous study with Hordeum spontaneum, and indicate that to obtain H. spontaneum or T. dicoccoides accessions with the highest level of resistance to the powdery mildew pathogens, plants should be collected at sites in ecological and geographic regions where those two species occupy optimum habitats and are exposed to the powdery mildew pathogens .

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