The present investigation was conducted using twelve flax genotypes namely, Giza 9, Giza 10, Sakha 3, Sakha 4 (local varieties), Jiteka, Blenika, Ilona, Escalina (introduced varieties) and strains 16, 22 and 402//21/10/9 (local strains) grown under field conditions at Tag El-Ezz, El-Dakahlia Governorate during 2010/2011 and 2011/2012 growing seasons. Disease incidence (DI) on the tested genotypes ranged from 44.82 to 90.13% in the first season and from 38.86 to 81.42% in the second one. However, disease severity (DS) ranged from 48.83 to 92.25% in the first season and from 43.83 to 85.85% in the second season. In general, the local genotypes were more resistant to powdery mildew than the introductions due to their adaptation to the local populations of Oidium lini. Some of the differences between the genotypes in disease incidence and disease severity were inconsistent from season to season, which may indicate the occurrence of genotype x environment interaction. Ilona followed by Giza 9 exceeded the other tested genotypes in straw yield traits. However, Strain 22 followed by Strain 16 outyielded other tested genotypes in seed yield traits. Moreover, other tested genotypes showed intermediate estimates for all straw and seed traits in both seasons. All the significant correlation coefficients between disease intensity variables and agronomic traits were negative in each season, which implies that selection for powdery mildew resistance would necessarily lead to an improvement in agronomic traits and vice versa. On the other hand, many significant correlation coefficients were observed among agronomic traits in each season. Cluster analysis of the twelve genotypes based on dissimilarity in agronomic traits and susceptibility to powdery mildew, placed the fiber type genotypes and the oil type genotypes in two distinct unrelated groups. The genetic background of the fiber type genotypes (dissimilarity level = 15) was more diverse that that of the oil type genotypes (dissimilarity level = almost zero). The diversity of the fiber type genotypes may facilitate the selection process for better agronomic traits and more resistance to powdery mildew.