Bing, D., Gan, Y. and Warkentin, T. 2011. Yields in mixtures of resistant and susceptible field pea cultivars infested with powdery mildew - defining thresholds for a possible strategy for preserving resistance. Can. J. Plant Sci. 91: 873-880. Powdery mildew (caused by Erysiphe pisi var. pisi DC.) resistance of field pea (Pisum sativum L.) cultivars is dominated by the single gene er1. Monoculture of the er1 gene may encourage pathogen evolution for more virulent race(s) and breakdown of the resistance. We promote the use of cultivar mixtures made up of resistant and susceptible cultivars to limit the pathogen evolution and to preserve the resistant gene in current cultivars. The objective of this study was to evaluate if such cultivar mixtures could cause significant yield losses and to identify the proper ratio of resistant and susceptible cultivars in a mixture. Three powdery mildew resistant cultivars were mixed with 0, 10, 20, or 30% of a susceptible cultivar, and gown in replicated trials at four locations over 2 yr in western Canada. The results show that severe powdery mildew infection caused yield reduction of 21-24%. The threshold of susceptible cultivars in a cultivar mixture depends on yield potential and performance of component cultivars, the severity of powdery mildew, and environments under which a cultivar mixture is grown. When disease severity was high and the resistant cultivar yielded well, 10 to 30% of the susceptible cultivar could be mixed with the resistant cultivars without significant yield reduction compared with the resistant cultivars in pure stand. The study suggests that such cultivar mixtures may be used in field pea production by providing more substrate to the pathogen so that the breakdown of resistant gene er1 may be delayed.