Yield and yield components of glasshouse- and field-grown mixtures (blends) of near isogenic, highly resistant (HR) and highly susceptible (HS) oat lines increased significantly more than increases attributable to increasing proportions of HR plants in the blends. Disease index (DI) and lodging index (LI) of field stands were highly positively correlated (r= 0.95), while both DI and LI also had high negative correlations with grain yield (r= -0.87, and -0.95, respectively). The percentage of diseased seedlings in greenhouse mixtures decreased significantly more than decreases due to decreasing proportions of HS plants. Glasshouse stands of HS seedlings developed significantly fewer roots than developed by HR seedlings, or by 1:1 mixture (plant/plant) of the two lines. Soil samples from the rhizosphere of field blends with 20%+ HR plants had significantly fewer Helminthosporium victoriae spores than those from samples with lower proportions. In both field and glasshouse experiments, alfalfa had no significant interaction with disease incidence or yield. The buffering of HS plants against Victoria blight incidence by HR plants in mixtures is suggested.