Two isogenic lines, normal and spotted-leaf, of Awnless Atlas barley (Hordeum vulgare L.) were grown at four seeding rates for 3 yr at Beaverlodge in northern Alberta. Rate and year caused highly significant differences in yield and yield components. However, seeding rate had little effect on reductions in yield and yield components caused by the leaf-spotting gene. Although leaf spotting appeared to cause little reduction in the photosynthetic area before heading, the reduction was enough to decrease head, plant and leaf size. The heavy leaf spotting that developed after heading contributed to a 22% yield reduction. Compared with a similar study at Davis, California, the results of the present study suggest that the longer photoperiod and higher temperature at Beaverlodge prior to heading depressed yield potential and increased the adverse effect of the leaf-spotting gene. The results suggest that leaf area, both before and after anthesis, is important for yield development and expression.